Growing in Module Trays

, written by Benedict Vanheems gb flag

Sowing into module trays

If you’re growing vegetables in any quantity it pays to be well organised, both on and off the veg plot. Maintaining records of what is sown, where and when is a must and will help you to get the most from your patch but organising the raising of seedlings is vital too.

For me, module trays – those seed trays divided up into rows of individual cells – are a real godsend. Module trays are what plug plants are grown in and allow the ultimate in regimented planning. Alongside meticulous record keeping they’re what keep my vegetable plot shipshape and regular. If you’re unfamiliar with module trays or have never used them I’m hoping this piece might encourage you to start. Far from being an unnecessary middle step between sowing and planting out, module trays save time and, crucially, will allow maximum use of the land you have available.

Succession Sowing - One Out, One In

Of course, sowing into modules doesn’t suit all crops and in many cases would be impractical (the likes of long-rooted carrots or breakneck-speed radishes for example). But for leafy salads, legumes, veg following on from early or overwintered crops such as members of the cabbage family, and just about any vegetable that needs a reasonable amount of measured space between plants, they prove very handy indeed. You can be starting off one crop in the greenhouse or cold frame while another is still in the ground. That way when the first crop is harvested it’s a simple matter of grubbing out the old to drop in young plants of the replacement crop. No waiting – just out with the old and straight back in with the new.

Sowing peas into modules
Sowing peas into modules

Another advantage of cell growing is the absolute management of this nursery environment. Module trays raised up onto benching are less likely to be subjected to subversive slugs, pesky pigeons and malevolent mice! They can be given just the right amount of water, a perfectly tailored germination temperature (assuming suitable propagators) and a fulsome root environment designed to speed things healthily along.

Cell Sizes in Module Trays

Module trays are commonly available at standard and half-sized seed tray sizes, as well as narrower strips designed to fit a windowsill. The size of the cells within the trays varies, with those of around 2.5cm (1in) and 5cm (2in) wide most useful to the vegetable grower. Those with a 2.5cm (1in) cell size are ideal for salads and starting off most of the cabbage family. Larger cell sizes are just the ticket for sowing larger seeds (for example squash or beans) and those crops that can be multi-sown, where a small cluster of seeds is sown into each cell.

Seedlings in module trays
Seedlings in module trays

Seeds can be started off in a seed tray of seed compost before pricking out (transferring) into individual cells. Suitable veg for this treatment include lettuces, annual herbs and tomatoes. Others can be sown directly into the modules, either one seed per cell for bomb-proof germinators (the likes of sweetcorn, squash and zucchini) or three or four seeds per cell for planting out as a cluster – for example, beetroot, spring onions and peas.

In all cases it’s important to fill your trays properly by using quality compost pressed firmly into each cell. Don’t fret about over firming the compost as seedlings prefer a supportive root zone; a well-filled cell will hold more moisture and nutrients, allowing seedlings to remain happy in the cell for longer. To sow, simply make an indentation into the top of the cell and place your seed/s into it. Cover with fresh compost. Larger seeds may need poking down into the compost. Watering the compost thoroughly before sowing will avoid disturbing the seeds. Move the tray to a warm, bright place to commence germination.

Module grown peas
Module grown peas

Planting Out Module-Raised Seedlings

Planting out cell-raised seedlings is a remarkably satisfying – and surprisingly speedy – process. I’ve been known to go from blank canvass to super-charged salad bed in just half an hour! Before planting your plug young plants they will need to be hardened off to outdoor conditions, if they’re not already accustomed to them. Place the trays somewhere out of harm’s way and cover them with fleece at night when cold conditions are forecast. After a week like this they can be set out. For more advice see our article on Hardening Off

Use a dibber (essentially a thick, pointed stick) to make holes for your plugs then water into the holes one or more times until the water is slow to drain out. In this way you will know the ground is properly moistened for the young plants. Poke out each plug by its drainage hole using the blunt end of a pencil and pop the root ball into its hole, firming back the soil around it. Watering before planting saves a lot of effort, as the roots should have enough moisture in their immediate surrounds to break out and settle down with minimal intervention from you. That said, be on hand to water as necessary; think the opposite of little often – big, hearty thirst-quenching soaks once a week at most.

Brassicas in module trays
Brassicas in module trays


It is worth buying or acquiring the sturdiest module trays you can afford as they’ll last for many seasons. This throws up its own challenges, however. It is essential that trays are thoroughly cleaned after each use to reduce the risk of fungal diseases lurking about for the next crop. Slugs and snails can also test patience as they find their way into every nook and crevice of stacked trays. Remain eagle-eyed and ready to hoik them out!

Look out for nurseries, garden centres or fellow gardening enthusiasts off-loading unwanted trays. These can be picked up for a fraction of the cost of buying new. Re-use and recycling websites are another good source. With a modest stack of module trays you’ll be ready to transform the organisation of your plot.

For more information see our articles on Raising Plug Plants and Comparing Different Types of Plant Pots.

By Benedict Vanheems.

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Show Comments


"how do u know when to plant out your seedlings i.e. is it size or visual of the roots or what-obviously depending what youre growing but as a general guide? "
mark on Saturday 28 April 2012
"Hi Mark - there are several factors: the most important is the weather - is it too cold (spring) or too hot and dry (summer). Then there's whether the plants are outgrowing their modules - if the roots are pushing out through the bottom then you need to consider hardening them off (see link above in the article) and planting them out. Also, after about 6 weeks plants often need new nutrients so should be either transferred outside or potted up into larger containers..."
Jeremy Dore on Friday 4 May 2012
"I accept the reasoning in this article (meticulous record keeping! mud stained scraps of seed containers with pencilled scrawl and vows to write it up!) But I've not been able to find sturdy trays - probably not looked hard enough - after one season mine broke into pieces whereas on 4th season with seed trays."
Nick Dibben on Saturday 5 May 2012
"I did this for the first time. What I got was long gangly stems and good roots. I did have the trays in a very sunny window and turned them every day. I fertilized them after 4 wks. Do you have any suggestions to achieve more stocky hartier plants? "
Gillian Getz on Saturday 5 May 2012
"Hi Gillian. Apologies for taking so long to reply to your question. To keep plants more stocky it's best to keep them on the cool side (if you can). Warm, moist conditions are great for getting seedlings started off, but once they have grown on you ideally need to harden them up a bit by gradually acclimatising them to conditions they'd experience outside. Evenly bright, fresher conditions will lead to sturdier growth and better-adjusted young plants."
Benedict Vanheems on Sunday 27 May 2012
"Hi, Please can you tell me what is the mesh type material that is found in shop bought plugs? Thank"
Kay on Thursday 4 August 2016
"Hi Kay. Some plugs have a fine biodegradable mesh bag around the potting soil, which can be planted also. I'm not entirely sure what it's made from though!"
Ben Vanheems on Thursday 4 August 2016
"Hi Ben, Thank you for your quick reply, you've been a great help. Kay "
Kay on Thursday 4 August 2016
" what number of plants can you get in your trays"
michael corrigan on Tuesday 3 January 2017
" what amount of plants can I get in your plant modules "
michael corrigan on Tuesday 3 January 2017
"Hi Michael. It really depends on the size of the modules and what you are looking to start off in them. So in a larger module you might have, say, a couple of lettuce plants, or a couple of pea plants, or just one squash seedling. In smaller modules you'd tend to have just one seedling per module, with the seedling planted out as soon as the roots fill the potting soil in the module."
Ben Vanheems on Tuesday 3 January 2017
"hi again what I wanted to know I can buy the modules with about 40 spaces I want to start my onion set in them as a friend on gardeners world says he gets better bulbs to plant on do you sell any larger modules thanks Michael "
michael corrigan on Wednesday 4 January 2017
"Unfortunately we don't sell module trays. I'd try a gardening retailer for these - module trays are widely available, so you shouldn't have any problems finding some."
Ben Vanheems on Wednesday 4 January 2017
"Can I use the app to record seeds planted in seed trays i.e. not already positioned in my garden?"
Michelle Tanner on Monday 23 July 2018
"Hi Michelle. The Garden Plan Pro app will help you lay out your garden and record plants in the ground. To record seed planted in seed trays etc. you may like to sign up for the free Garden Journal, which is web-based and available (for free!) . Simply click on the 'Journal' tab at the top of this web page then scroll to bottom of page to sign up for the Journal."
Ben Vanheems on Tuesday 24 July 2018
"Ben, I have a quick question about module sown peas (shelling or snap). How bug is your cell size and how long do you keep the seedlings before transplanting. Mine are a week old and 1inch long roots are already protruding from the module. I am at least three weeks away from planting them outside and might just use them for shoots and sow another batch for planting outside. Any help or guidance is welcome. Thank you. 🌱"
Prunella on Sunday 20 March 2022

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