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How to propagate strong seedlings

The skills you need are not difficult to achieve or complex to understand. I want your success rate to be high, because a lot of good things flow from having strong, healthy seedlings and plants, at their most favourable time.


I don’t like ‘stuff’, but some purchases or use of recycled materials will result in better plants.

Module trays are a key part of successful propagation. A lack of suitably sized trays, both easy to use and reusable, led me to design a new 60 cell module tray in 2020, the ‘CD60’. We have since split this into half and quarter size trays, with 30 and 15 cells respectively (all available from

The tray has smaller cells which need less compost. The cells have smooth sides and a large hole in the bottom of each – this gives good drainage, and allows you to push up the rootball of each plant without damaging roots.

  • After each planting, you can reuse the trays without any washing or cleaning, as with all propagation trays. Simply refill them, and resow or prick out again.
  • The trays are made of plastic; however, they are so strong that you can reuse them for decades.
Lettuce in a CD60 module tray, five weeks since sowing and no feed given; they need to go into the ground now
Peas for shoots from a CD60 – four weeks since sowing and ready to transplant
Strong growth of radish, multisown 17 days earlier in a CD60

You can also make your own pots from loo rolls and egg boxes, but these take more space and compost, and require more time than plastic trays. Try a few materials to see which you are most comfortable with. The most important thing is good drainage – nothing grows in soggy compost.

Choosing the right compost

The key qualities you need are:

  • Good drainage in limited volumes such as modules, which many proprietary composts do offer.
  • Good moisture retention.
  • A high level of nutrients in proportion to the volume of compost. This is way more important than for composts you spread on soil, which are more about biological stimulation.

I find that my homemade compost does not drain as well as most proprietary composts, and is lower in nutrients. Hence I am happy to buy potting compost, and to use my own on beds.

Finding a good compost to buy is not always easy, and the quality of some brands varies year on year. I am therefore reluctant to suggest any one name wholeheartedly, but make these general observations to help you choose:

  • Organic composts, with a symbol of approval / certification, are my first port of call, and most of them grow great plants but are expensive to buy. Worth the price in my experience, for avoidance of poisons and to support organic practice.
  • Beware of the description ‘organic’ without any symbol of approval, in which case ‘organic’ means absolutely nothing! Actually it suggests that the compost is not to be trusted, because of the misleading use of that word.
  • I find that potting compost is good to use even after 18 months, making it more worthwhile to buy a pallet (far cheaper) and share it among friends. Mail any company to enquire about prices; a pallet is usually just under a tonne, or perhaps 2000 litres / 440 gallons, in their proprietary sacks.

You can mix your own potting substrate based on homemade compost:

1.Put the compost through a 12 mm / 0.5 in sieve – this only works when the compost is reasonably dry.

2a. Add a quarter to a third by volume of vermiculite or perlite, to hold air and improve drainage.

2b. At the same time, improve the nutrient status with any of wood ash, vermicompost, volcanic rock dust, or seaweed of any kind, including seaweed meal.

Avoid too much undecomposed wood, which grabs nitrogen for its own decomposition and then stunts growth of seedlings. This can be a problem with many wood-based, peat-free composts – growth is slow and leaves are pale green, even yellow.


A way to avoid using module trays is to make blocks, which stand opensided. Simple! Except it’s often difficult, and there are some issues to be aware of:

  • You can’t use just potting compost, and it takes time to make the mix of blocking ‘soil’. It can also be messy, like mixing wet cement.
  • As well as a blocker, you need something to support the blocks, such as corrugated PVC, old mushroom or wooden boxes, or plastic seed trays.
  • Making tidy blocks that hold their shape is a skill to acquire.

On the plus side, seedlings mostly grow well in blocks and like the airy edges, where you often see many roots. Watering needs some extra care, especially in hot weather, because once blocks dry out it is not easy to get them moist again.

Blocking ‘soil’ is mostly compost, and there are many recipes out there of variable ingredients.

A simple recipe that I use is three quarters of multipurpose compost and one quarter of soil. The soil has many fewer nutrients, but helps to bind blocks together. You can also try adding a little wood ash, seaweed meal or vermicompost.

Blockers of different sizes are often called ‘soil blockers’, but they need to be used with mostly potting compost, not topsoil
The smallest blocks you can make – these are 1.5 cm / 0.6 in mini blocks of 2 cm / 0.8 in depth, fiddly to sow but an efficient use of space
260 blocks of onion seedlings in a mushroom tray with not too many gaps, and lined with a little paper because I was germinating in the house
Onions growing in the little blocks in March – it’s now 18 days since they were multisown and they have been in house warmth

Sowing methods

How you sow seeds depends on their size, the plants’ hardiness, the time of year, and how many plants you want in each plant hole.

A golden rule:

  • Seeds and seedlings in a small container succeed better than the same number in a large container.

This has to do with two factors:

  1. Large volumes of compost and soil with few roots are more likely to be too wet and lacking air.
  2. Seedlings grow better when close to other seedlings, their companions.

This is why I recommend that you sow small seeds in, say, seed trays, then prick out and pot on. Small seeds make small roots and take a fair time to develop their root system. It is also one reason why multisowing is so successful – when several seedlings are together, they can quickly create a strong root system in the compost they are sown in.

The photo (below right) shows how the multisown basil in front grew more strongly in small modules than basil sown on the same day in seed trays and then pricked out (at the back of the image).

Two sowing methods: brassicas sown in a seed tray to prick out, and multisown in a module tray – choose your preferred option
This basil was multisown in 60 cell, 3 cm / 1.2 in module trays 24 days earlier, and is now ready to pot on

These photos illustrate how many plants can start life in a small area, in this case one seed tray. In the warmth of your house, you can sow seeds for a whole garden.

Mid-February – lettuce seeds of 14 varieties in a seed tray in the house
Just 9 days later, the seedlings are ready to prick out – 1500 of them!
All of these lettuce seedlings came from that one seed tray

How and when to prick out

Pricking out means transplanting tiny seedlings, usually from a crowded seed tray to a module tray. Tiny seedlings grow way better in a module tray than in larger pots, and occupy less room in the propagating area.

Many say that they have been taught to wait until a true leaf is visible before pricking out seedlings. However:

- Seedlings with a true leaf already have a fair number of roots, which take time to disentangle from other roots.

- It’s quicker to prick out seedlings with only their cotyledons, which are the first two (not true) leaves – you hold one leaf while lifting the seedling root with a pencil or a dibbing tool, which is like a fat pencil.

Points to keep in mind when pricking out:

  • Little seedlings can still have a fair size of root and sometimes it breaks a little. However, this rarely affects growth, as long as most of the root is transferred.
  • The stems of all seedlings need to be buried to make them more sturdy.
  • Their roots can simply coil into the hole made in each module, even upwards. Roots do not need to go straight down but can just be ‘bunged in’, though always carefully, and with a little firmness to ensure contact between roots and compost.
Brassica seedlings are ready to prick out when this size – these are tree cabbage and Romanesco cauliflower in June
Pricked out module trays of brassicas for winter – these seedlings would be massacred by insects if sown direct in the summer

Reasons to prick out and then transplant, rather than sow direct

Pricking out is a method for seedlings whose seed is small, meaning that they grow relatively slowly, compared to, say, peas, beans, cucurbits or sweetcorn.

Small seeds also make it harder to multisow in modules – celery and celeriac, for example.

There are numerous advantages to using this method:

  • Seed germination and seedling survival increase markedly when you give the best conditions, under glass or polythene.
  • Space is saved, as the germination phase requires less space for a seed tray than the equivalent number of sown module trays.
  • Less seed is needed, because germination is strong.
  • All modules are full after you prick out, with no gaps from uneven germination.
  • Growth is even, because when pricking out you select the strongest seedlings.
  • Growth is stronger when small seedlings are near to each other, rather than spaced out in beds.
  • Harvests can be bigger. In trials, I found that lettuce transplants grew more strongly than the neighbouring lettuce that I had sown direct on the same day.
Pricking out seedlings of chicory in July – we do this as soon as seedlings are large enough to get hold of one leaf
Sowing tomato seed into modules has resulted in gaps; the alternative would be to prick out seedlings from a tray to ensure all modules grow a plant

Why and when to pot on


Potting on is mostly for warmth-loving plants that need higher temperatures before being planted out, and for any plants running low on food supply and light / space, when their designated bed is not yet ready.


The simplest criterion is to pot on when seedlings are running out of space, which is drawing them upwards. on lengthening stems as they compete for light with close neighbours.

Another suitable time would be if plants are going yellowy-pale from lack of nutrients, in a rootball which is perhaps now too small.

Ready to pot on, these squash plants were sown just 12 days earlier and are growing very fast
Potting on cucumber plants from the modules I sowed them in, to 9 cm / 3.5 in pots
These squash plants were potted on 15 days earlier, and can be transplanted at any time over the next ten days

Two examples

Leeks can transplant as module-grown seedlings, but I am usually waiting for a spring planting of potatoes and early cabbages to finish. So we pot on leek modules to keep the plants growing, then transplant them in midsummer as larger plants which ensures a bigger final harvest.

Tomatoes grow like mad in April, once past the two-leaf stage. My sowing date of, say, 20th March causes me a little anxiety at first, because it takes three weeks from sowing to a decent true leaf. However, from mid-April, they grow like a rocket!

This module-sown leek plant is ready to go into a larger module or small pot, or to plant out
The same plants of multisown leeks, now in 7 cm / 3 in pots, and good for a month or more before transplanting
Aubergine and tomato plants in the greenhouse on 12th April; the tomato plants are ready to pot on
Potting on a month-old tomato plant, so it has more food, moisture and light

First sowings in cold weather

These need extra warmth, compared to the temperature you find outside in late winter and early spring.

The cheapest way to add warmth is to use what you already have – house warmth. At night time, especially in late winter and spring, ambient house temperatures are higher than in most garden spaces.

Vegetable seeds do not need light to germinate, except for celery and celeriac, so you can place sown trays in any warm spot indoors, even a dark cupboard, until the first leaves appear. The number of days this takes depends on seed and temperature – with celery and celeriac, for example, it’s eight to ten days.

Tips for using a windowsill and house warmth:

  • Before bringing trays and pots into the house, water them thoroughly and then allow them to drain; this reduces mess, because fully moist compost does not then need watering for a few days in the early season.
  • Once you see the first leaves, light is required. However, on windowsills, the leaves can access perhaps only one third of full light, resulting in long, weak stems as they draw upwards in search of it. Rotating trays 180° can straighten stems, but cannot shorten them.
  • Before stems are too long and weak, they need to be outside in full light. This is a reason for not sowing tomatoes in February, unless you have a frost-free space outside or use grow lights.

You can also use electric cable heaters on a greenhouse propagating bench, or even on a windowsill, under your module trays. For an outdoor space, this is an efficient way to use electricity compared to heating the whole space.

Grow lights

I cannot advise on these as I haven’t needed to use them, but mostly I hear good reports. LED grow lights are cheap to run, and look like Christmas in spring with their blue and red colours. You only need them for windowsills, not in a greenhouse where there is light all around.

Grow lights bring another level of management. For beginners, I recommend just sowing a little later, which should make them unnecessary because, by the spring equinox on 21st March, light levels are strong, even on windowsills, and plants can soon go outside.


If you have the space and resources, hotbeds are a way to provide warmth during spring propagation. Should you want to make a hotbed, the key points are:

  • The volume must be at least one cubic yard or metre, to generate and hold a worthwhile amount of heat.
  • The manure needs bedding as well, preferably straw, or fresh, small-piece wood chip.
  • Fresh horse manure gives more heat than other manures, and the straw adds cellulose food for the bacterial multiplication, which creates heat.
  • The manure and straw needs to be trodden down firmly, while watering if it’s dry.
  • Fresh manure needs adding to the sinking heap every month or so, to maintain heat and volume – we bring top-ups in mid-March and mid-April.
  • After manure has been added, there are ammonia gases for a few days that singe leaves – I always give extra ventilation in the greenhouse at these moments, both for a new heap and any top-ups.
  • Your heap needs watering perhaps more than you expect, in order to replace the moisture lost as steam, and to keep it working and warm.

Germinating seeds and small seedlings on the February-made hotbed – this is early March and it’s slightly cold outside, –3°C / 27°F

Spring sowings under cover

Spring into action as soon as you can, but not before about the middle of February, or early to mid-March in, say, Zones 4–5. A lovely maxim is this:

Later sowings can catch up in the spring, because every succeeding day is lighter, and tending to be warmer.

Don’t be overawed if your neighbour has sown before you. Later sowings meet better conditions, grow more evenly and can even overtake earlier

sowings. The dates I give are for first sowings and are as early as I recommend, unless you are in, say, Florida or North Africa. Check with local knowledge to see what others do.

A key understanding is this:

  • Seeds require more warmth to germinate than their seedlings need to grow.

In natural conditions, this prevents abortive and too-early germination, hence my emphasis on germinating in the house.

Starting vegetables early is not a ‘natural’ thing to do, but puts food on the table a month or two earlier than would happen otherwise.

Giving warmth to germinating seeds in late winter to early spring – module trays sown and stacked until seeds germinate, in the conservatory

Summer and autumn sowing

Propagation continues all through spring and summer, and with summer sowings it becomes important to be more precise with your dates. The key difference is this:

  • In spring, later sowings catch up. After midsummer, they don’t. This is thanks to declining light levels and then falling temperatures.

Keep this saying in the back of your mind:

  • One day’s growth in July would need two days in August, four in September, a week in October, half of November and the whole of December.

It’s a sobering thought, if not literally true. Make a note of summer sowing dates wherever you are most likely to be reminded, to ensure that you sow before it’s too late.

One lovely aspect of summer sowings is the speed of germination, and this highlights the importance of warmth to break seed dormancy.

I hear people worry about it being too warm for lettuce seeds to germinate, but I succeed here with greenhouse temperatures over 32°C / 90°F by day. I put trays out of direct sun, under the bench for four days or so, until leaves are visible.

June sowings emerge fast – it’s two weeks since I sowed these lettuce, beetroot, swede and spring onion
A summer sowing of spinach on 10th August – there are three seeds per cell and I want one to two plants
September – it’s only nine days since we multisowed these spring onions, with ten seeds per module
Mid-September propagation for transplanting outside, sown between 9 and 33 days earlier – spring onion, chard, land cress, chervil, Claytonia and spinach
28th September – mostly salads to transplant under cover, except for the corn salad / lamb’s lettuce and spring onions

Watering seedlings

This is a skill you need to practise, for even distribution of water at the best time. Take note of the weather forecast, and observe your results. Water, watch, and learn.

Beginners usually overwater, which causes more problems than underwatering. It’s fine for potting compost to look a little dry on top, and even for plants to wilt occasionally in hot sunshine – one clue that watering is needed!

There are two occasions when it’s good to water seedlings (and all pot plants) more, so that the compost is almost at full moisture:

  1. On the morning of a day that will be hot and sunny.
  2. When you are going away for a couple of days.

Water amounts increase as seedlings grow into plants: they need more water than before, but still not to capacity. Lift your trays and pots to check their weight, to get a feel of the water content. Heaviness suggests you hold off, while a moderate weight is good.

The water needs of seedlings in plastic trays and pots are different from when plants are larger and growing in soil, when they need a lot of moisture to match their rapid growth rate – roots are free to travel, and it’s difficult to overwater.

By contrast, it’s easy to overwater in winter. At this time of year, watering of seedlings can be minimal to none at all, even in mild weather. There is little bright sun, and the air is often damp. Plants do not lose much moisture in evapotranspiration during winter months.

Watering seedlings on the hotbed, always with the hose pointing downwards in order to be more precise with applying the water
Watering September trays in the greenhouse, using a fine rose

Damping off

Roots need air as much as moisture. During propagation, compost inside seed trays (in particular), plastic pots and modules can become airless or anaerobic. Roots then rot, plants suddenly go limp and then quickly die. It’s a painful thing for the gardener as much as the plants, and it’s called damping off.

This can be a dramatic problem, caused by fungal spores which are often present in humid conditions. Spores develop on, and may destroy, leaves that have water droplets on for sufficient time. Damage may result from any or all of the following:

  • Giving too much water, especially in dull weather when plants only grow slowly.
  • Watering in the evening, meaning that seedlings stay wet overnight.
  • Seedlings being too close together in a tray, which reduces evaporation and means they stay wet for quite a time after watering.
  • Seedlings growing in too large a pot, proportionate to the size of plant; the roots are then too tiny to pull moisture out, which would allow air in. (It’s easier to water correctly and safely when you start seeds in small cells, and pot them on as they grow.)
  • Dense compost, which holds too much water.
  • Insufficient ventilation.

Lessen the risk of damping off by doing the opposite of the points above!

Most at risk of overwatering at seedling stage are:

  • Cucumber, melon, basil and lettuce.
  • All small seedlings in trays and with touching leaves, which rarely dry out.
  • All seedlings in damp and humid weather – some days it’s best not to water, so watch your plants and be aware of the forecast.
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