Grow veg

Seedlings – growing up and moving on

The follow-up stages in the life of your vegetable seedlings

Tomato seedlings ready to prick out

So you got busy sowing seeds a few weeks ago, and maybe, like me, you got a bit carried away despite all your good intentions to keep things under control this year. Somehow you found yourself sowing three varieties of leeks instead of one, six different types of winter squash, more brassicas than you can shake a stick at, and then you decided that you needed four different types of tomatoes rather than the two you were definitely going to stick to.

If any of that sounds familiar, you will now have quite a few little plastic punnets and small seed trays brimming with seedlings, and if you haven’t already started to move things on, you certainly need to be doing it now.

It’s really a simple and satisfying process to prick out seedlings for the next stage of growing, but there are one or two things worth knowing about when and how to do it.

This advice is a broad rule of thumb to keep things simple. It is easy to follow and works for everything you’re likely to be growing.

The first leaves that emerge when a seed germinates are called the seed leaves (cotyledons) because they developed in the embryo that was inside the seed. Most of the plants you’ll grow have two seed leaves, although bulbs (onions and leeks), have one seed leave that’s usually long and thin. The seed leaves look different from the ‘true’ leaves, which grow as the plant starts to develop and they will shrivel up and drop off once the plant is growing.

Cotyledons, or seed leaves, are the first leaves that appear when a seed germinates. They don’t look the same as the ‘true’ leaves of the plant.

When you see the first true leaves, that’s usually a good indication that the roots have developed enough to cope with being moved. Any earlier and there might not be enough growth on them to cope with being moved and if you wait much longer the roots will get more established and possibly more entangled with the roots of the other seedlings in the tray which makes it harder to get them out without causing damage. You want to do as little damage as possible to the delicate roots so they recover and start growing again quickly.

First, prepare your larger pots or modules. Fill them with compost and tap them to settle it. Using a dibber, a pen, pencil, or your finger, dibble a hole large enough to place the roots into easily.

Hold one of the seed leaves so you don’t damage the real leaves or the stem. Push your dibber into the soil, a little way away from the base of the stem and work it down to the bottom of the container. Wiggle it around gently so you get underneath the roots and carefully tease them free of the compost, then lift the plant by the seed leaf.

Dangle the roots over the hole you made so they go in straight. Lower the seedling. You can bury the stem as well, putting the seedling in so the soil level is just under the seed leaves. It doesn’t do any harm, and if the seedlings have become leggy on your windowsill it’s a good idea plant them quite deeply so you get a sturdier plant. With your dibber or a finger, push the soil back around the seedling lightly, but don’t firm it too much.  Repeat this with all your other seedlings.

When you’ve moved all the seedlings you should water the pots well and this will settle the soil around the roots.

What size of pots should you pot on into?

You are probably familiar with bonsai trees. The principle with bonsai trees is that restricting the area for root growth will restrict the size of the top growth as well. Of course there’s a bit (actually quite a lot) more to it than that, but you get the picture. If you restrict the room for the roots, you will usually restrict the amount of top growth you get as well. When you’re growing vegetables you don’t want to do that, you want them to grow in a controlled but uninterrupted way until they reach their mature size and there are a couple of ways you can make sure that happens.

Potting on in two stages

This is generally for small seeds that will eventually become quite large plants, such as tomatoes, aubergines, peppers, cabbage, kale, broccoli, calabrese, Brussel sprouts etc and lettuces. These will be moved on initially into individual modules or pots of around 9cm diameter, and then when the roots have filled that, they can be planted into the ground, a larger pot (10-15 litres) or a growbag where they’ll grow on to full size.

Potting on in one stage

Plants with larger seeds like cucumbers, courgettes, squash, runner beans, dwarf French beans, peas, broad beans and sweetcorn, are best started off in either individual pots or modules and then just planted out once the roots have developed to fill the initial pot, or moved on to larger pots (10-15 litres) when they’re ready (be careful with all of the tender veg which shouldn’t be planted or moved outside until the risk of frost has passed). Some of these can also be sown directly into the ground if that’s easier, see earlier blog posts for more on that.

Not potting on at all

You might want to grow cut and come again salad leaves or some root vegetables in pots, so you can just sow these directly into a larger pot where they can grow on, avoiding all root disturbance.

A thought about plant roots and plant responses to being moved

You see the stems and leaves of plants developing above the ground so you know what progress they’re making. Under the soil the roots will also be developing; branching and spreading through the soil. The plant will strike a balance in the amount of energy it expends on developing roots in proportion to the energy it is putting into the development of the stem and leaves. It’s difficult to move a seedling without doing some damage to the roots, so pricking them out slows down the growth process temporarily because the roots will usually need to recover and get established again. You might notice that any seedlings left in the tray you sowed them into will grow faster for a while than the ones you’ve pricked out. Over time the roots of the seedlings you moved will recover and the top growth will start to catch up, and in the long run they’ll do better because you’ve given the roots more space and they’re growing without the competition from the other seedlings.

A well developed, branching root system on a lettuce plant.

Some plants famously hate root disturbance, especially anything with a tap root like a carrot or parsnip and some flowering plants like zinnia and annual poppies. Sowing them directly where they’re going to flower is often said to be the best way to grow them. In practice I’ve often sown annual poppies and zinnia in modules, a few seeds to a module. You need to thin them down to one plant per module, but then there’s very little root disturbance when you plant them out. I’ve always found it works, avoiding the uncertainty of sowing seeds directly in the ground outside.

Zinnia dislike having their roots disturbed but are easy to grow from seeds sown into modules and then planted out with little disburbance

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