Seed trays, modules and pots

In the section on basic seed-sowing, I talked about the features of suitable seed-sowing containers. They need to hold a suitable volume of compost, retain water and yet allow excess water to drain away. They should also be clean – using clean containers and clean water helps prevent disease from attacking your seedlings. A nice wash in hot, soapy water should do the trick, followed by a thorough rinse. I tend to do mine in the kitchen sink – pity the poor Victorian pot boy who had to use cold water in a pot shed with no walls in the middle of winter.

Seed trays

Seed trays

The traditional choice for seed-sowing is seed trays (or ‘flats’). A shallow tray, with drainage holes in the bottom, they will hold a shallow layer of soil with a large surface area. They’re ideal for sowing large volumes of seed quickly, but you do have to pay attention to spacing to avoid over-crowding.

Seed trays are cheap to buy (especially the very flimsy plastic versions, but they won’t last long), but you can also make your own out of recycled food containers. Add some drainage holes in the bottom so that they don’t become waterlogged. If you can find food containers with clear lids (or clear containers that are a matching size) they you have a ready-made seed tray and propagator. The other advantage of using recycled containers is that you can match the size of the tray to the number of seeds you want to sow, although it is also possible to buy seed trays in half sizes or to fit onto the window sill.

One disadvantages of seed trays is that your seedlings are all in one batch, which means that if one succumbs to damping off disease then the whole tray is at risk if you don’t take action quickly (for more about damping off you’ll have to wait for the next chapter, which is on seedlings). If seedlings are left in trays for too long, they not only run out of nutrients and compete for space and water, but their roots tend to tangle as well. At some point, you will have to prick out each seedling into a more spacious home – a task which is either zen-like in all its repetitive glory, or a real chore, depending on your mindset and/ or stress levels.

Modules

Peanuts in Rootrainers

The solution to some of these problems is to sow in modules – essentially a seed tray divided into separate compartments. This allows each seedling to have its own root space, and you won’t need to prick them out. Damping off is less likely to spread if it sets in. If you buy seedlings mail order or from a nursery or garden centre then they’re very likely to come in modules – or possibly as ‘plugs’ which were grown in modules but have been removed for sale.

If you have bought seedlings in modules then once you plant them out you can wash the modules and use them again. They tend to be plastic or polystyrene, with a hole in the bottom which allows drainage and means you can poke the plug out when it’s time for them to leave.

You can also buy modules of varying kinds and sizes. Rootrainers are an extension of modules, with hinged ‘books’ that have four compartments. They come in two sizes, one short, one longer, and are ridged to encourage roots to grow downwards. Books are supported in plastic holders that maintain an air gap at the bottom, which stop roots from endlessly spiralling downwards and tangling up. The end result is a well-grown root system which can be planted out with minimal root disturbance.

And that’s the main advantage of modules – by giving each plant its own space, and not having to prick them out, they develop good roots. The plug of compost should (but doesn’t always) come out in one go, allowing you to pot up or plant out without damaging the roots – allowing the plant to romp away more quickly and avoid a ‘check to growth’.

The obvious disadvantage to modules is that they are more expensive in terms of compost volume and space.

You can use any small containers as impromptu modules – lots of people use yoghurt pots and small empty food containers. It’s cheap, and easy, and a good start for a single plant.

Biodegradable pots

Empty coir pots

Another step up the ‘no root disturbance’ ladder are biodegradable pots and pellets. They have the same advantages as modules, but when the plant is ready you simply plant them out, pot and all – nice and easy. The pot rots away as the plant grows.

The problem is that many biodegradable pots and pellets are made from peat. However, it is now possible to buy peat-free coir pots and pellets, which have the same advantages. (Check the label to make sure you’re buying coir, not peat, pots and pellets.)

Pellets are made from coir compressed and encased in a net. Each pellet is rehydrated in water for 10 minutes or so until it is soaked through, then placed in a tray for sowing. Each pellet is used for one seed (or enough seed to ensure one strong seedling later, with any excess culled).

The advantages of pellets are that they’re quick and easy to use, and create no mess. They do, however, make seed sowing expensive and as they hold a large amount of water they’re not ideal for seeds that rot easily. It’s also useful to note that once your seedling has grown, Sarah Raven recommends removing the net before you plant the pellets out – not doing so can retard root growth.

It’s also possible to make your own biodegradable pots. Some people use the cardboard inner tubes from toilet rolls, which are nice and tall and offer a good start to beans. You’ll need to stand them in a tray and disturb them as little as possible to avoid the compost falling out of the bottom.

Paper pots

And you can also make pots from newspaper, which makes good use of a waste resource and allows you to make your pots in whichever size you need (up to a point – newspaper isn’t strong enough to hold a large volume of compost when it’s wet). You can buy a ‘Paper Potter’ or similar mould to help you make the pots, or you can simply rely on your underused origami skills. There are plenty of internet sites with instructions on how to make paper pots, so look for a method that works for you. A small staple or a little bit of cellulose-based tape to hold a pot together is sometimes helpful, although they can be made with just strips of paper. It’s a nice job for keeping your hands busy on long winter nights in front of the television, or for little hands that need occupying in the school holidays.

Soil blocks

You can also sow seeds without using a container at all – using a compressed block of growing medium which is called a Soil Block. It’s not an entirely accurate name, as you don’t have to use soil in the mix. They’re good for raising seeds on a large scale quite cheaply, and you can buy a soil blocker that creates the blocks for you, or make a DIY setup to do the same thing.

I have no experience of soil blocks, but the basic idea is that you create a wet mix of your chosen growing medium, then press it into blocks. All of the recipes I have seen for the mix so far include peat, and there are suggestions that simply replacing peat with coir in this context is not entirely successful – something has to hold the blocks together and coir may not have the necessary properties. I am going to try some experiments, but in the meantime feel free to chime in if you have more experience.


In the next section I will be talking about peat-free seed composts – how to make your own, some of the brands on offer, and what to look out for.


This blog post was written by Emma Cooper and was published on The Unconventional Gardener website. If you're reading it elsewhere you may want to navigate away from plagiarised content.

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