Thousands of years ago, tomatoes were common in South America and Mexico and were used in cooking more than eaten raw. They were first brought to Europe by the Spanish conquistador Cortés, in the 16th century.
Tomatoes are in the Solanaceae or nightshade family, and this caused them to be viewed with suspicion in Europe for some time. Unripe fruits and leaves contain tomatine, which would be poisonous if eaten in large quantity, but ripe fruits contain no tomatine.
In the UK, one of the first people to grow a tomato plant was John Gerard the surgeon, who wrote in his Herbal of 1597 that the fruits are unfit for human consumption. It was another 200 years before tomatoes were in widespread use.
- Days from seed to first harvest: 90, often more.
Tomato seeds sownTomatoes ready to harvestEarly spring under coverFrom midsummer until first frost
- Best climate has warm summer days of 22–35 °C/72–95 ° F, and at least four months between last and first frost dates.
Why grow them
Great flavour, colour, abundant fruiting over many months, a range of plant and fruit sizes – there is much to like about tomatoes. Tomato plants grow well in warmth but also tolerate a certain level of coolness, more, for example, than aubergines and peppers.
- They can be grown in many ways and you can fit them into almost any growing situation.
You can choose from a huge range of varieties, for the fruit size, colour and flavour you like. They will compare favourably to most tomatoes of commerce!
Tomatoes with much less flavour happened from the 1950s, after a phenotype (genetic trait) was discovered that enabled breeders to ensure uniform ripening. The tomatoes were of even colour all around, with no green top (greenback), but unfortunately also had reduced sugar and taste levels.
- Well-grown tomatoes are a source of umami flavour – savoury. The other six main tastes are sweet, salty, sour, bitter. astringent and pungent.
- Use of salt increases umami tastes, hence the increased flavour of tomatoes with salt.
Fruit or vegetable?
Botanically, tomatoes (also cucumbers, peppers etc.) are fruits or berries. However, in general usage we call them vegetables because their sugar content is much lower than that of fruits.
- Another way to clarify this difference is to ask whether you eat a food as part of the main course or as a desert. Tomatoes, cucumbers etc. are eaten with the main course – they have a savoury rather than sweet flavour and therefore are vegetables.
Pattern of growth
Different varieties of tomato plants grow small or large, and tall or bush. Results vary according to whether you grow them outside or under cover. This lesson concentrates on the latter because it is based around my experience of growing tomatoes in cool summers, where there is often not enough time for plants to give a worthwhile harvest of ripe tomatoes outside.
- Plants grow for three or four months before they produce worthwhile amounts of fruit.
- During summer, tomato plants put on a phenomenal amount of growth: 1) creating new stem and leaf, 2) developing trusses of new fruits, 3) ripening tomatoes. They are doing all three of these things, each of which needs a lot of energy, at the same time!
- Plants are killed by the first frost of the autumn, and often before that their growth has slowed in the darker conditions.
- Bush plants are called determinate and are annuals – they ‘crop then stop’, without trailing stems.
- Cordon plants are called indeterminate and are perennials, as long as there is no frost in winter. Their natural habit is trailing. We grow them as annuals, vertically upwards with support.
- Cherry tomatoes are usually sweet and small, although often larger than a cherry in size, with a corresponding reduction in weight of harvest compared to beef tomatoes. They ripen quite early, especially Sungold and some determinate varieties.
- Beef tomatoes are the largest in size and have fewer cavities in the fruit, therefore contain more flesh and proportionately less skin. The texture is often dense (meaty, hence the name) and full of flavour, but they ripen later than cherry tomatoes.
- Medium-sized tomatoes with firm skins are most common in stores and supermarkets, often of a variety selected for yield ahead of flavour. Mostly they are grown hydroponically for maximum harvest, to pay the bills – the price of tomatoes relative to other goods has declined steadily in the last 60 years.
You can grow hydroponically at home, but this requires a fair amount of kit. I do not recommend it unless you have sufficient time to monitor all the variables. It needs more knowledge and investment compared to growing in soil or containers. Your harvest will look good, but is missing important nutrition from soil microbes.
Brix, a measure of sugar
A way to measure sweetness is called brix, and uses a refractometer. Brewers and winemakers keep this device handy to check the sugar content of their must, which tells them how much alcohol there will be after fermentation.
I have used a refractometer over a few years, with the juice of different tomatoes grown in different ways – the table below gives an idea of their sweetness. Higher figures mean more sugar.
- The highest figure I secured was a fully ripe Victoria plum at 19.4. Here, beetroot and carrots are 8 or 9. The table offers clues about how to increase tomato sweetness: reduced watering is the main one – this is easier in pots or containers than in soil outside if it has rained. Choosing suitable varieties clearly influences flavour.
Tomato variety and where grown
Brix level in refractometer
Dorada soil outdoors
Primabella in pot, kept dry 24.9
Primabella in soil, same day 24.9
Rosada F1 greenhouse
Rosada F1 polytunnel
Sungold F1 greenhouse and polytunnel
Velocity F1 polytunnel
Katy apple (ripe)
Brix is sometimes claimed to represent nutritional quality, with higher numbers indicating more nutrition. This is not totally proven, but higher numbers certainly indicate reduced water content, and therefore you have denser food with more dry matter.
Inside a structure you have the possibility of growing indeterminate plants up strings – this increases harvests a lot. I refer to these plants as cordon tomatoes.
- In areas with cooler summers, growing under cover gives much more chance of worthwhile harvests, in summer as well as in autumn.
- When summers are hot and long enough, tomato stems have time to grow longer than a structure is high. You need a device to hold spare string, which enables repeat lowering of the now-empty lower stems down to soil level.
- The active flowering and fruiting zone of cordon tomato plants spans roughly 1.8 m/6 ft. Below that level are non-working leaves and finished trusses.
Plants grown under cover can be kept dry, which is a huge advantage in damp climates because it prevents late blight. Your key knowledge is that late blight cannot develop on dry leaves – see below.
There is more wind outside and indeterminate tomato plants need tying to strong stakes. There is a fair amount of time needed for securing them.
Bush plants are quicker and easier to grow than cordons outside, but take longer to harvest and fruits on the ground may be damaged by slugs; also, leaves may stay wet and suffer late blight.
Suitable for containers/shade?
Tomatoes are the container vegetable par excellence. The main criterion is a container large enough to grow the variety you choose – cherry tomatoes are most successful. Site in sun where possible, unless your summers are hot.
You need a compost with plenty of nutrition. The photos below illustrate what happens to plants when they run out of food. It’s not the end of the world and you still get a harvest, but it’s smaller and finishes sooner.
There are many proprietary foods available for tomato plants. You need a feed for container growing, compared to the little or no feeding needed for plants in soil. Also watering is much more frequent than for tomatoes growing in soil, where the root run is more extensive.
Make sure you have the time and means to water regularly. There are proprietary watering kits with pipes and timers, but that investment may well not be repaid in the value of your harvest, depending on how you calculate it.
Large containers can grow cordon tomato plants with stakes inserted in the container. Or you can grow special varieties for hanging baskets. Do try a few things!
My favourite varieties reflect the growing conditions here – summers warm but not hot, with the risk of late blight outside and five months between last and first frost.
In warmer climates, I would favour beef tomatoes over cherry tomatoes. In cooler climates, look for fast-ripening varieties.
- Sungold F1 ticks every box – it crops early and tastes amazing, plus can be grown outdoors in sheltered gardens. The yield is not heavy and the skin is prone to splitting.
- Sungella F1 is a larger version of Sungold, with more harvest and less flavour.
- Sakura F1, a red cherry, offers great flavour and larger fruits than many cherry varieties.
- Gardeners Delight was an early commercial cherry tomato variety in the 1970s, when it was smaller and sweeter than it is nowadays. The varietal maintenance has not been thorough because there is more money made from selling hybrid tomatoes. My last two attempts at growing Gardeners Delight resulted in medium-sized fruit with very average sweetness and tough skins.
Crimson Crush and Mountain Magic, both F1, resist late blight without being immune to it and have decent flavour. Crimson Crush is a medium-sized beef tomato.
I have had top results from open-pollinated cherry varieties offered by Culinaris in Germany. Resi, Primabella, Primavera and Dorada all have good flavour. They grow well in cooler conditions and have some resistance to late blight.
Beef tomatoes grow better under cover, in cooler climates.
- Black Russian has a rich flavour, soft skin and deep colour. It’s not really black, just purple. I tried Indigo Rose once, which is sold as a ‘black tomato’, and found it tough-skinned with little flavour.
- Brandywine tomatoes are large and taste wonderful – there are red, pink and yellow varieties.
- Super Marmande is an old variety from South West France, of variable quality according to who has been selecting the seeds. The flavour and texture are superb, fruits are medium-large. Possibly these were the origin of the huge beef tomatoes I found in a neighbour’s garden in South West France, which we called Mami tomatoes in honour of the old grandmother who grew them every year.
- Hybrids such as Gigantonomo, Big Boy, Country Taste and many others, have been bred for the exceptional size of their fruits. All of those that I have grown and eaten also taste good!
- Feo de Rio and Feo de Rigordo grow super tasty, red beef tomatoes of dense texture and top flavour.
Commercial tomato varieties are mostly red and of medium size. They have tough skins so that they transport well, plus have a long shelflife. Their water content is high because this results in a high yield for making a profit. Do look at the brix result of Velocity in the table.
- Orkada Fi tastes better at least.
- A medium-sized red fruit of decent flavour is Matina.
In the UK at least, there is something of a race to sow tomato seeds as early as possible. Although this may result in earlier harvests, it often also results in extra work and space needed, to look after plants until conditions are good for transplanting. As it happens, I am writing this on the 8th March when the temperature overnight here for two nights has been -5 °C/23 °F, and there are many posts on social media of people who have lost their early-sown tomato seedlings, in an unheated greenhouse for example.
- Seeds germinate in six to nine days.
- In cooler climates, germination is successful when you give warmth. It does not need light.
- Raising seedlings is most successful with full light; the temperature can be lower than for germination.
- Plants are killed by frost at any stage of their growth.
Light and warmth are easier to provide when sowing is not too early. Any time in early spring is good and my favourite time is mid-March, for eventual growing under cover.
For outdoor tomatoes, sow two weeks later. I sow early April under cover, to plant out in the third week of May, after our last frost date.
Sowing can either be in a small tray to prick out, or in small (say 3 cm/1 in) modules of multipurpose compost. For sowing in a tray, which drains less well than module cells, best use a seed compost (not John Innes, the quality has declined) or add 50% vermiculite/perlite to a potting compost. Some potting compost can work as seed compost – the main criterion is good drainage. A high level of nutrients does not inhibit germination.
Seed trays need less space than module trays, during the first two weeks. Hence it’s efficient to raise many seedlings in a small tray, for eventual pricking into modules. Every module cell then has a seedling, compared to the occasional gaps that result from sowing in modules and a few seeds not germinating.
- Tomato seeds need a temperature of around 15–20 °C/60–68 °F to germinate evenly. Therefore a warm room in your house is fine during the first week when it’s all about temperature, not light.
- Then give them a week or two on a windowsill, for light as well as warmth.
- After that they need to be in full light, to prevent stems from growing long and thin.
About two weeks from sowing, if your seedlings are in a seed tray, use a pencil or an equivalent tool to gently lift them out from below root level. Always hold them by the leaf, which risks less damage than holding them by the stem. Make a hole for each seedling in module cells of compost, and push the roots of seedlings into this compost, one per cell. It’s fine to bury the stems.
Nights may be too cold in any outdoor structure during early spring. You can avoid difficulties in various ways:
- Sow later, unless you can provide the necessary protections and extras.
- You probably need some heating for frosty nights in your full-light option outside, such as a greenhouse or polytunnel. An electric heat mat is useful for this.
- Or it may be sufficient to lay fleece over seedlings before any frosty night – double thickness.
- Grow lights are an option if you continue to grow seedlings in the house; you also need to have a watering method, such as a capillary mat, for bottom watering.
- Without a grow light, plants by a windowsill grow long, thin stems and then fall over. This is more likely when you sow too early.
To keep tomato plants sturdy and strong, don’t overwater them. There is no need to feed at this stage.
This is worthwhile for seedlings, because tomatoes are six to eight weeks old before transplanting. I find that this age of transplants works well here, where the spring is quite slow. In climates where spring is brief and then quickly hot, your transplants could be just four to five weeks old.
Pop out module seedlings when they are starting to crowd together in the module tray, before they develop stems that are too long and thin.
- Place each module rootball quite deeply in a 7 cm/3 in pot by mid-April, and then move the rootball from its 7 cm/3 in pot to a 10 cm/4 in pot two weeks after that.
- Or pop module plants into a 10 cm/4 in pot, to save one potting process. However, this uses a little more space in your propagating area.
Same soil? Rotation?
This is pertinent for those of us growing plants in the soil of an under cover space every summer. For how many years is it possible, without a damaging buildup of pest and disease? There are many variables to consider, which means that no simple answer is possible.
- No dig, and a surface mulch of high-quality compost, increases your chances of successful growth every year.
- Tomato pests increase in the soil, which may result in plants being smaller and less healthy. One remedy is to buy plants grafted onto rootstocks that resist soil pests.
- The main soil pest to worry about is root-knot nematode – see below.
- Diseases do not accumulate; it’s often thought, for example, that spores of late blight can somehow linger through winter and then establish on plants again in the spring, but they cannot – see ‘Diseases’ below.
Transplant size and time
The ideal size is 20–30 cm/8–12 in high plants, ready just after the last frost date, or perhaps a week before that for growing under cover. However, your plants may be ready before the ground is. Perhaps there is something still growing from the winter, or the weather may be unusually cold.
Plants kept in their pots for too long lose the green leaf colour, with lower leaves turning yellow and even brown. Stems elongate and toughen.
- Fortunately, you can still transplant such plants – tomatoes are incredibly strong.
- The main drawback is that you will have lost time; a second one is loss of propagating space.
- Make a note of the timings, because this suggests that you sowed too early.
Bury the lower part of tomato stems, both when pricking out and transplanting, to keep them sturdy. When setting in the ground, use a trowel to make the hole, a little wider than each plant’s rootball and around 5 cm/2 in deeper. Push the rootball in firmly and give a good watering at this point.
See ‘Support’ below for information on how you may want to pop a string in the hole before planting.
Inter- and companion plants, flowers especially
Carrots are famous for having an affinity with tomatoes. You might, for example, have a sowing of early carrots in a polytunnel, between which you plant tomatoes in gaps that increase each week as you harvest carrots. Then, within a month, the carrots are finished and you have tomatoes growing. See Lesson 15 for how I did this with cucumber plants, and Lesson 6 with Brussels sprouts – both lessons in Course 3A.
Alliums grow well with tomatoes. Either grow early spring onions, between which you transplant summer tomato plants, or transplant late spring onions between tomato plants that will finish in a month or so.
A lovely rule of thumb for companion planting is that vegetables grow well together as plants when their harvests taste good together, as long as the companion plants do not grow too large and are not maturing by early autumn, when tomatoes need full use of the soil. Hence onion, garlic, basil and parsley make great companions for tomato plants.
An excellent flower to grow, with cordon tomatoes especially, is any kind of dwarf marigold. The growth habit is compact and low to the ground which complements the taller tomato growth. Many marigolds secrete a substance called limonene, whose smell deters aphids.
Spacing both under cover and outside is 45–50 cm/18–20 in, no closer. This allows room for growth over a long period.
- There is air around all leaves to reduce the risk of blight.
- Space makes it easier to manage growth and pick the fruits.
If using a string to support cordon plants under cover, place one end of the string in the hole before planting. Best pre-tie a knot, on the end that is under the rootball, otherwise there is a small risk of the string sliding up and out of the hole once plants are bearing a lot of weight.
Do not use jute or any natural fibres for this, because they would rot and break. Use polypropylene, which can be reused the following year.
Outdoor plants to grow as cordons can have a stout bamboo placed close to the rootball. Make a first tie when about 30 cm/12 in high, and then tie the stem to your bamboo every 15–20 cm/6 –8 in.
How often to water
Tomato plants grow so fast that they need a lot of water, but for plants rooting in soil it does not have to be every day. We water every other day in hot sun and twice a week in cloudy weather, at 51 degrees latitude and in moist, temperate conditions. In hotter climates, daily watering is good.
For new plantings in dry weather, give water every day or two, just around their rootball. After a week, you can water less often, assuming there is moisture in the soil from winter.
- It’s good when the soil or compost surface is dry between each watering.
- Container plants need watering daily, even twice daily once plants are large, and in hot conditions.
Watering is a skill to learn, because overwatering can depress growth by causing too little air around roots. Also, you are wasting time and water.
- A rule of thumb is that larger plants and hot conditions mean more water is needed.
- By late summer, reduce both frequency and amount, to discourage new growth.
- During early autumn (September here), for plants both under cover and outside, you can almost stop watering plants in soil. Continue to water containers, but less.
Under cover, a final watering can be about mid-September. This helps plant metabolism to switch from growing stem and leaf towards ripening of fruit.
- With reduced watering at the stage that many fruits are ripening, their sweetness increases.
Extra mulch to retain moisture?
Any mulch of organic matter helps to reduce watering, but plants keep sucking moisture whatever mulch you use. A thick mulch does not mean you never need to water and it could soak up the moisture from a summer shower, making it less easy for plant roots to access new rainfall.
In my 2020 polytunnel, I noticed little difference in growth and need for water between plants whose roots were mulched with compost only, compared to those with miscanthus grass and seaweed on top of the compost mulch.
You also need to consider the succeeding vegetable plantings.
- In the polytunnel, we transplant winter salads and leaf crops in October, within a few days of removing tomato plants.
- If there was a mulch of undecomposed organic matter, we would need to remove this before planting salads. Otherwise, it would encourage slugs and would make leaf picking and cleaning more difficult, with bits of mulch sticking to the leaves.
How much feed, or none at all?
There are many answers to this question, partly depending on what you want from your plants. For example, I am not used to growing with fertilisers and expect my plant leaves to carry a few imperfections, which some might call ‘deficiencies’. Tomato leaves are not always a lush and dark green colour.
Using feeds should increase the amount of harvest, but there are also risks of making plant growth more attractive to insects such as aphids. Also, you can reduce flavour, sweetness and trace element content of fruits.
Plus you need to consider how long the feeding will take you, the cost of any feed, and how you work out which feed is needed in what amount, and when.
- Diagnosing a need for nutrients is difficult, even for professionals. See ‘Potential problems’ below for the example of blossom end rot, which many books advise is caused by a shortage of calcium. This is correct, however the shortage of calcium in plants (not soil) is itself caused by a lack of water, which means that plants cannot access the calcium that is present around their roots.
- The simple remedy for blossom end rot is to water more, not to give calcium feed.
I suggest that as long as leaves look a reasonable colour and fruits are developing with some abundance, you do not need to feed plants growing in soil. However, plants in containers almost certainly will need feeding.
For this, as opposed to a proprietary feed of synthetic chemicals, I cannot give a precise formula or amount because organic feeds vary so much. To increase trace elements, I recommend some feeding with seaweed, or a scattering of basalt rock dust on the soil before planting. Or add dry seaweed with basalt rock dust to the compost in your container before planting.
- A homemade option is to feed with a liquid from leaves of stinging nettles and comfrey. This is not smelly if you make it by stuffing any large bucket or pot as full as possible with the green leaves. There must be a small hole at the bottom to allow a resulting black liquid to drain into a container below – any bucket large enough to support the container or bucket above.
- Dilute at approximately 20 to 1 when watering plants.
How to judge readiness
It’s your call at which stage to pick fruit. Anything from half coloured if you like them tart and firm, to fully coloured for softer and sweeter fruits. Leaving tomatoes on plants after they have reached full colour sees their texture become a little fragile, and there is less ‘bite’ to the flavour as the acid diminishes.
How to pick
Handle tomatoes gently, bearing in mind that many varieties you grow have softer skins than ones you may buy. Lift a tomato upwards with your thumb on the calyx (the green, star-shaped stalk) to keep it attached to the tomato after it has been picked. They keep for longer when no skin is broken, which would happen if you detached the calyx.
In exceptional cases, you may have a whole truss with tomatoes all ripening at a similar time. You can then cut a whole truss, which is beautiful to behold and can be stored for a while.
When to pick and how often
In hot weather, from midsummer, picking can be every day for a few tomatoes, or twice weekly for plenty each time. Every time you go to pick some fruit you see other things that need doing, such as sideshooting, tying in, or perhaps extra watering. During high summer, and for best results, tomato plants need almost daily attention.
At some point you will experience a glut of more tomatoes then you can eat, or even give away perhaps. It is better to pick them when ready, instead of leaving them to go soft on the plants. The next step is to work on storage of this wonderful bounty.
By mid-autumn, there is as little sunlight as in late winter. Over many years I have compared the ripening of tomatoes in an unheated greenhouse or polytunnel, with tomatoes picked green* and stored in the house. The latter ripen well, and with little loss of flavour from having picked them early. Or, more correctly, there is little increase of flavour from leaving them on the plant.
- We have a saying that on the 10th of October, the devil spits on the hedgerow blackberries. They no longer sweeten while changing colour, because the days are now too short and dark for much photosynthesis to happen, which is needed for plants to create sugars and the volatile compounds in ripe fruit. It is similar for tomatoes.
* By ‘picked green’, I mean fruits that are fully developed and showing perhaps a hint of colour. You cannot ripen undeveloped fruits, which need to grow more – they cannot do that once picked.
In your house, picked tomatoes keep better on the counter than in the fridge. 10–15 °C/50–59 °F is a good temperature for keeping tomatoes in nice condition, without them ripening too fast and with little loss of flavour. When in a fridge, the low temperature can slow down decomposition but may reduce sweetness and flavour. It’s not necessary to keep tomatoes in a fridge for long-term storage.
Unripe but fully grown tomatoes store for at least a month in ambient house warmth while slowly ripening. Trials suggest that ripening them out of sunlight, even in a drawer, is better than in full sun or next to a banana. Christmas tomatoes look the part, but have unremarkable taste and sweetness.
- Cherry tomatoes can be frozen whole in polythene bags.
- Tomatoes are excellent for canning/sterilising in glass jars in a water bath at near boiling temperature.
At Homeacres I have an electric dehydrator and find that drying tomatoes is an excellent way to preserve flavour. Slice into 6 mm/0.25 in strips for dehydrating, over 12–18 hours at 50 °C/122 °F. Or use a slow oven, preferably with a fan and a half-open door. Sunlight in autumn is rarely strong enough to dry fruits.
- The result is thin and dense fruit strips, highly aromatic, which store for over a year in jars.
- During dehydration, they lose 95% of their weight.
- Best results come from beef tomatoes because they have less moisture to start with.
Be sure to save seed only from open-pollinated varieties, not hybrids. To check this out, I saved seed from one Sungold hybrid tomato and grew ten plants the following summer. Each plant grew differently, with quite large fruit of totally unremarkable flavour.
- Tomatoes do not cross-pollinate. This allows us to save seeds from tomatoes of two different varieties, growing right next to each other.
If growing more than one plant of a variety, save seed from fruit growing on the plant that has the nicest growth habit and harvest.
- Cut your tomato, as if preparing it to eat, and scoop out any seeds using a spoon.
- Place seeds, and any tomato fruit sticking to them, in a cup with some water and leave them in the kitchen to ferment. This breaks down germination inhibitors on tomato seed, through fungal action.
- After five to seven days you will probably notice a black mould of decaying fruit pulp on top of the water, which you can scoop off – the seeds are at the bottom, now much cleaner.
- Give the seeds a rinse and then place, say, on cardboard, in a dry place for a few days. Then pop in an envelope or old seed packet, clearly labelled.
- Tomato seeds store for up to ten years in any dry environment. Dry and ambient temperature is better than cold and damp.
Aphids are a common problem in spring. Their numbers build up quickly but should also diminish quickly when predators arrive.
When soil fertility is good, plant roots have sufficient moisture and transplanting was at the correct time, aphids may be present but should not cause any serious problem. By midsummer, the chances are you hardly notice them any more.
1.Greenfly/rose or peach aphid (Macrosiphum rosae)
There are several species and colours of greenfly, from yellow to light brown in colour. When they are numerous, you see leaf distortions as well as black mould on the undersides of leaves. The mould grows on the aphids’ honeydew excretions, which are enjoyed by ants.
- Spay a moderate jet of water on aphids to displace them, for a while at least.
2.Greenhouse whitefly (Trialeurodes vaporariorum)
Their lifecycle is similar to that of greenfly. The greenhouse name comes from where they are most prevalent and noticeable. Susceptible plants include cucumbers and tomatoes, where one often sees them under cover.
Damage to plants happens from the tiny larval stage, when they are barely visible to our naked eyes.
- Best avoidance is from not planting too early.
- During late spring, allow the white- and greenfly populations to increase enough to feed predators, who arrive in early summer.
- You can buy Encarsia formosa or lacewing larvae to reduce aphids, from early summer to early autumn. They are expensive and not worthwhile unless you have, say, ten plants or more, and expect significant damage.
Root knot nematodes (Meloidogyne hapla)
These can be common in soil used for growing vegetables over several years. They are most active in warm soils, 21–30 C/70–86 °F, and reduce root function, with knots visible on affected roots.
Growth will be stunted or below average. If you suffer this, and unless you can allow, say, two years between each summer of growing tomatoes, it’s worth buying plants on grafted roots. Some sellers also offer resistant varieties.
Tomato hornworm (Manduca quinquemaculata)
This is common in warmer climates, and the bright green caterpillars make significant holes in fruits in particular. The moths can be diverted by interplanting with umbellifers that flower, such as dill. Such plants, and marigolds too, attract insect predators of the caterpillars.
Late blight (Phytopthora infestans)
This is the main disease threat for tomatoes and potatoes. It arrives during summer, when night temperatures stay above 10 °C/50 °F for 48 hours, and air humidity is above 90% for all that time.
This results in enough warmth and moisture for growth of newly arriving blight spores, which become prevalent everywhere in such weather and blow around in the wind. Leaves quickly turn translucent brown, followed by dark spots appearing on stems and infection of tomato fruits or potato tubers.
- In cooler and especially dry summers, blight is unlikely to be a problem.
- It’s often ‘called wrong’, and blight warnings are issued by manufacturers of fungicide when there is no need to worry.
In temperate and damp climates, blight can be maddening for outdoor tomatoes, arriving just as they finally ripen in late summer. Salvage what you can, which may be little in damp weather.
If you spot blight damage on tomato leaves, best cut them off immediately – likewise for infected fruits and trusses. However, once the blight is in plant sap, enough to infect fruits and trusses, you may not harvest any more healthy fruit. Tomatoes may look normal when green, then start to brown and soften as they ripen. A further disappointment is that tomatoes ripened on blight infected plants do not taste sweet.
- Dry leaves do not host blight spores, therefore water plants under cover at root level only. If you can keep leaves dry, all will be well, but it’s not possible for outdoor plants.
- For outdoor tomatoes in cooler climates, grow early-ripening varieties such as Sungold F1, and cherry rather than beef tomatoes.
- It is safe to put all blighted material onto compost heaps, because blight spores need living tissue to survive. Therefore they die as soon as decomposition happens, which occurs even in cool heaps, just more slowly than in hot ones.
- For this reason, there is no need to change the soil in a greenhouse or polytunnel where blight infected tomatoes were growing, before planting tomatoes the following year.
Early blight (Alternarium solani)
Far less common, and less troublesome compared to late blight. I mention it as a possibility – growth would be weak, with spindly stems. Remove infected plants.
Horse manure and hay are at a small risk of contamination by aminopyralid weedkiller, occasionally sprayed on grass for making hay. Increasingly, however, it’s being used to control broadleaf weeds in other fields and verges.
It’s the most common weedkiller to persist in poisonous form and it’s lethal to potatoes, tomatoes and legumes, whose growing tips become curled and twisted.
I only heard of this weedkiller after suffering its effects in 2014, a year after I had added some horse manure to bulk up a heap of homemade compost. Potatoes, beans, lettuce, chard, alliums and cucurbits grew with deformed and stunted leaves, and matched the online photographs of damage.
The poison has many points of entry to gardens, all invisible:
- Horse manure from stables where horses have eaten hay sprayed with weedkillers such as Banish, Forefront, Grazon, Pharaoh and Runway. Or if horses were bedded on rape straw sprayed with Astrokerb.
- Hay used by gardeners, as above.
- Horse manure used in commercial bagged compost for mulching, which may even be called ‘organic’. The word organic on bags of compost means nothing if there is no symbol of certification.
- Potting composts that have used contaminated horse manure or green waste composts. Readers sent photos of problems in 2020 from Bord na Mona compost, J Arthur Bowers and various Westland products. They were a minority, but you can’t tell if the poison is in a bag until your plants suffer, and it’s shocking when it affects your garden.
Pyralid problems have also occurred from the following:
- Mushroom compost – this may be contaminated as it often contains horse manure.
- Green waste compost, if some of the grass mowings were from lawns sprayed with weedkiller containing clopyralid, which affects growth similarly to aminopyralid. My local composting operation do trials of most, though not all, batches, using pea and red clover seeds. However, this is expensive because it adds time to their process.
- Some cow manure.
- Gardens and allotment sites close to fields sprayed with weedkillers containing aminopyralid weedkiller.
There is no straightforward remedy. In cases of mild contamination, the simplest is to wait for soil microbes to dissipate the poison, often within a year. Or, if you have spread a lot of contaminated material, best remove it, but to where?
We have been handed a difficult problem, and in 2021 a gardener who suffered this, Anna, set up a Twitter group where you can report damage. The hope is to find out how bad it is – if there is a high frequency of problems, we have grounds to push for the poison to be banned.
Under cover tomatoes may suffer this if ventilation is inadequate. For example, the leaves roll in hot weather in my 12.8 m/42 ft long polytunnel, especially the leaves of cherry tomatoes plants, much less on beef tomato plants. I have no remedy except to wait for cooler weather. My polytunnel has doors at both ends but no side vents, because in winter I do not want too much cool breeze blowing in. The winter harvests are more valuable than the summer harvests.
- Leaves rolling look as though the plants are short of water, but watering does not reduce leaf roll.
- Ventilation of polytunnels from the doors at each end should be adequate, up to a length of about 9.5 m/30 ft.
Blossom end rot
As I explained above, this happens when plants cannot find sufficient moisture in the soil to carry nutrients to developing fruits. Give plenty of water as soon as you see any such rot, which shows as a small and jet black spot where the flower was. If the spot is a large area, this suggests the plant needed a lot more water than it could find.
Tomato mosaic virus
Viruses show as bright yellowing/curling/veining of leaves, and mottled fruits with light streaks and patches. They can affect celery, legumes, lettuce, cucurbits and solanums, plus many flowers.
This one is not common and I have never suffered it. If it does happen, best remove plants to the bin.
Verticillium and Fusarium wilt
Carried by a soil-borne fungus, whose spores can fill the veins of infected plants, to the point of preventing water and nutrient transmission. Lower leaves wilt and may turn yellow, then drop off.
The spores survive in most conditions, but can only multiply sufficiently to cause disease in warm moisture, with soil temperatures over about 30 °C/86 °F. Most vegetable gardeners will not suffer these wilts. I would suspect leaf roll first.
- Beef tomatoes sometimes lose their growing point and then cannot continue to grow normally until a sideshoot becomes the new leader. This delays growth a lot and is caused mostly by transplanting too early, when there is insufficient warmth for healthy growth. It’s worthwhile keeping a few plants in reserve after transplanting, for use as replacements if needed.
- Irregular watering can cause skins to split. This only matters if you were hoping to store tomatoes for a long time. Otherwise it’s an indication of ripeness, because unripe fruit rarely split. Avoid split skins by watering consistently.
- Spots on leaves are nothing to worry about – they are mostly on all leaves that are in the early stages of decay.
Use a sharp trowel or knife to cut around surface roots, enabling plants to come free but leaving most of their roots in the ground. Cut stems to 10 cm/4 in lengths before adding to a compost heap.
It is safe to put any blighted material on the compost heap. Blight spores die once the plant tissue has decomposed, and even in cool heaps this will be before midwinter.
Tomatoes grown outdoors finish by mid-autumn, which means there is little time for new plantings, especially after bush tomatoes.
However, cordon tomatoes give many possibilities. Options are to underplant or sow winter and spring vegetables, starting with spinach in late summer, then salads in early autumn and spring onions in mid-autumn.
- Spread a little compost before undersowing or planting, because you may not find space to spread it in late autumn, between the new vegetables that are already growing.