Lettuce

The three photos below illustrate the three main times of sowing, using the method of sowing in a tray to prick out seedlings into module cells.

Lettuce seeds do not germinate when the average temperature, night and day, is above about 26° C/80° F. This is sometimes claimed to prevent harvests of summer lettuce, because they won’t even start growing!

My experience is that seeds germinate all through summer in my greenhouse, where day temperatures can be 35° C/95° F, with 16 °C/61 °F by night. Germination may be hindered in hotter climates and is helped by shading new sowings to protect them from hot sunlight.

• Seeds germinate in two to six days.
• Fresh seed germinates more quickly.
• Compost for seedling growth needs to be free-draining.

Sowing time

In temperate climates, you can sow just four times a year, to have lettuce harvests for up to 36 weeks a year. This is with my ‘pick and come again’ method, not ‘cut and come again’.

To extend the harvest period through winter, use the last sowing date, for transplanting under cover in October or mid-autumn.

Sowing method

You can sow direct and thin to your desired number of plants, but that means bed space is occupied for longer between sowing and harvest. If you have under cover space and propagation materials, it’s quick to raise transplants. Plus you can achieve earlier harvests, from a first sowing in late winter.

• Lettuce are easiest to pick when single plants, not multisown.
• Sow two seeds per module cell, then thin to the strongest.
• Or sow in a tray, to prick out tiny seedlings when four to seven days old.

Use the freshest seed you can buy, or save your own, and accept that older seeds are slower to germinate but usually catch up in the end. As long as the compost is good!

Thicker sowings for cutting

This is a totally different way of growing and harvesting leaves. Either scatter seeds on a tray, or sow quite thickly in rows in a bed – rows 10–15 cm/4–6 in apart and 2.5 cm/1 in between plants.

Harvest by cutting, either after ten to twelve days for micro leaves or after three to four weeks for baby leaves. Be sure to cut above the level of the smallest new leaves, otherwise growth cannot continue. The next stage, after baby, is teen, which is what I suggest picking.

The differences between this method and picking outer leaves of plants more widely spaced are:

• With each cut, you have some yellow and mildewed leaves which need separating out.
• After three or four cuts, plants lose vigour or die. You need to have resown about a month before this, therefore any time saved by cutting, not picking, is offset by time spent clearing and resowing.
• You need more seed.

A key point is that lettuce sown thickly makes a rapid harvest of baby leaves to cut, but this is followed by diminishing second and third harvests with many yellow leaves.

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The season for transplanting starts a month after sowing. We make first plantings in beds from mid-March, and always with fleece sitting right on top of lettuce seedlings. This retains the maximum amount of warmth, without the fleece being damaged or blown off in high wind.

• After early May, fleece is too hot for lettuce – use mesh instead if you need to cover, or bird netting. Mesh serves for shade and for pest protection.

Transplant size and time

Small is beautiful. Much as lettuce seedlings look thin and fragile, they are surprisingly strong and willing to grow quickly. You can transplant lettuce any time you see weeds making new growth, because they grow so well in cold conditions.

Transplant method

Transplanting is quick with small plants, which need only small holes. The job is even easier when you plant into soft surface mulches of organic matter, which lettuce roots thrive in. A wooden dibber pushes easily into the surface, then just push the module rootball in firmly.

Spacing

Plant care, especially for salad leaves which one revisits for frequent harvests, is easier and quicker when spacings are uniform. The same 22 cm/9 in serves for all salad plants. There are four nice results for both us and plants, from this amount of space:

1. It affords enough moisture and food for plant roots to continue growing, and to support production of new leaves over many months.
2. Plants are close enough for communication and mutual support.
3. There is room for easy picking of decent-sized leaves.
4. After each pick there is some bare space around every plant, resulting in better airflow, less mildew and fewer slugs.

Lettuce for leaves or heads?

When your desired harvest is of heads, they require a month or more of extra growing time between sowing and harvest. This is possible from wider spacing of plants, at 30 cm/12 in for most lettuce. The exceptions are Little Gem types of small heads – plant them at 20 cm/8 in.

Perhaps you are growing lettuce for heads, then change your mind. No worries, because even when plants are close to heading, you can pick off outer leaves for a large initial harvest. This enables new growth and longer-lived plants, as shown in the photo sequence below.

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The mention of lettuce for heads leads to watering, because the larger the lettuce plant, the more water it requires to grow healthily. The photos below show the results of not giving enough water to larger plants, called tipburn.

• If you did not know, you might think it’s a fungal disease or something very serious.
• Trim the affected leaves and give plenty of water, then regrowth resumes healthily. For mature heads, harvest and trim – there will still be good leaves.

How often

Lettuce leaves are 95% water and therefore need plenty, except in damp weather when too much moisture on leaves for too much time results in downy mildew.

• To minimise mildew on leaves and reduce slug activity, water in the morning rather than in the evening.
• For plants of medium size or larger, water every day in dry summer weather.
• In autumn and spring, this can be twice a week.
• In winter, at the other extreme, we do not give any water to lettuce under cover, between the solstice and late January – a month! This results in a dry surface which decreases fungal problems and slug numbers.

How much

It’s more about frequency than quantity. New plantings, above all, need water every day in dry weather, so that new roots can find the moisture they need to grow fast. Otherwise plants grow very slowly.

Shading helps in hot sunshine.

Extra mulch to retain moisture?

This needs care because of slugs and woodlice, of which you do not want many near to lettuce. I would not mulch with any undecomposed materials around lettuce, except in arid climates. Shading is more appropriate.

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These vary, according to whether you want heads or leaves, and how large you like either of them to be.

Heads are ready when they turn paler, have folded in and feel slightly firm. Don’t wait for them to be too solid, because by then there is often decay. Use a knife to cut through the stem, just above any damaged leaves at ground level.

The rest of this section is about leaf harvests – removing outer leaves by hand.

How to judge readiness

Harvests begin once outer leaves are almost touching. This shows strong root development which enables steady regrowth.

How to pick leaf lettuce

Best plant health and regrowth happen after no-knife harvests. Harvest with a push and twist motion, on older leaves at the base of each plant. Work from the bottom up – older leaves first, always leaving the small and youngest heart leaves.

• Small heart leaves do more photosynthesis per area than larger leaves, so removing larger leaves does little to reduce growth.
• No damage by a knife means the small leaves continue to power new growth.
• Outer leaves are less sweet than head leaves, but contain beneficial polyphenols and quercetins (Beth Marshall, ‘Grow Yourself Healthy‘).

When to pick and how often

How many leaves you harvest depends on when you want another harvest, and the time of year. Pick more leaves off plants in warm weather, to moderate growth and harvests, and do the opposite in winter.

This method ensures harvests every week for half a year, more if you can protect plants over winter. During summer months, it’s good to pick every week at least, to keep plants in productive mode for longer, growing new leaves of high quality.

Storing

Homegrown leaves are full of vitality and store well in a plastic bag, ideally at temperatures of 4–7 °C/39–45 °F, and with moisture on the leaves. Therefore you can harvest, say, twice a week, then wash leaves and shake out excess moisture, and keep cool or in a fridge for eating as needed.

• Moisture on leaves helps them to stay alive, so they store for longer.

Selling lettuce leaves

Packs of salad leaves are the most profitable vegetable for market growers. Three-quarters of what I sell by value is salad leaves, of which about two-thirds is lettuce, in varied proportions through a whole year.

An issue, however, is that you need moisture-retaining bags for shelf life when selling. We have tried many compostable bags which actually breathe, so the leaves soon wilt. Sometimes they degrade too fast and affect leaf flavour. To have compostable bags for wet ingredients is not easily solvable.

Saving seed

In my experience, lettuce varieties do not cross-pollinate. Many times I have saved seed from different varieties, planted right next to each other, and they grow true. The main issue in saving seed is that your season of growth must be long enough for seeds to mature, after flowering in late summer.

A heart or head is the midpoint in the life cycle of lettuce plants. Their next stage is growth of a stem inside the head, which erupts upwards to bear flowers and then seeds.

Lettuce flowers are barely noticeable, pale yellow before they morph into clusters of seeds, say 10–12 in what was each flower bud.

Little white tufts appear at the end of each cluster and look like seeds, however the latter are well protected inside an outer sheath. See the photos below for how we rub them out.

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Pests

• Slugs are the main pest and I recommend you keep your lettuce tidy, plus the area around them. This reduces slug habitat, but there are usually a few in damp climates. Transplants are more successful at growing away from some nibbles than the small seedlings of direct sowing, and morning rather than evening watering reduces slug activity at night because leaves and soil at the surface have dried during the day.
• Aphids often colonise lettuce leaves in late spring, both green and white ones. Control them with extra watering until the predators arrive, by June. Aphids look bad for a while but most plants then recover, so don’t panic and just water.

Lettuce root aphids (Pemphigus bursarius), also called Poplar-lettuce gall aphid

These cause more damage than aphids but are less common. They arrive in mid to late summer, especially where Lombardy poplar trees are nearby. They overwinter in the trees and breed in the spring, before flying to secondary hosts, preferably lettuce.

You rarely see them, but they eat roots and then your plants wilt rapidly. There is no remedy once you see this happen, but keeping summer lettuce well-watered increases plants’ ability to continue growing, even while aphids eat their roots.

At the end of summer, the aphids stop feeding and migrate back to the tree hosts. In warmer climates, there is some risk of them continuing to live on lettuce. I have never seen this in England, yet.

• Endive and chicory do not suffer damage from these root aphids.

Leatherjackets (Tipula spp.)

These are a common grub in damp temperate regions, especially near to fields and lawns of grass. They are larvae of the European Crane Fly or Daddy Long Legs, which flies and lays eggs in late summer to early autumn.

Mild winters favour their survival. Expect damage to spring plantings, if there were few temperatures below -5 °C/23 ° F in winter. The larvae are a dark and matt brown colour, 10–20 mm/0.4–0.8 in long, with a soft and leathery exterior. Their food is plant roots, such as grass, lettuce, spinach and beetroot. Other plants such as onions can suffer.

The best remedy is to react as soon as you see a plant suffering. It may be so damaged that it needs to be removed and replaced. Before replanting, use fingers or a trowel to make a small hole no deeper than 7 cm/3 in, just around where the damaged plant was growing. Often you will find the grub – cut or squash it, or throw it in the hedge.

Just before spring plantings, you can reduce larva numbers by covering a bed with black polythene overnight. Pull back the cover at dawn and collect the leatherjackets.

Mildew

Downy mildew (Bremia lactucae) is the main lettuce disease, especially in autumn.

Lettuce grown for heads will always have some mildew on the outermost leaves, and this is not a worry. You can remove them, and from late spring to early autumn the centre leaves will stay mostly healthy.

• Frequent picking of outer leaves prevents mildew developing.
• Water in the morning not evening, so that leaves dry quickly.
• Expect some mildew on autumn lettuce and also in winter. Minimise it by watering less frequently in winter, even for lettuce in containers.

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