How to stop seedlings becoming leggy

The last year has seen so many of us growing plants from seed, but the question I’m asked time and again is how to stop seedlings becoming leggy. There seem to be many instances where both new and experienced gardeners alike can get seeds to germinate, but then they grow tall and, in some cases, collapse.

There are a range of different reasons why this happens and some useful tips that will help prevent it happening as well as some techniques to overcome leggy seedlings if it the problem has already occurred. Ultimately it all comes down to trying to promote steady germination and growth indoors – you may notice that it generally isn’t problem when sowing directly into the ground or even in pots outside. So what is going on and how can we overcome such legginess.

Why do seedling grow tall

Indoor sowing is used to bring seeds into growth when conditions outdoors are too cold to guarantee germination. We tend to think that all seeds will benefit from plenty of warmth to bring them on, but in many cases they do best in cooler temperatures. generally 10-12C is fine for seeds of hardy plants; 15-18C for half hardy and many tender plants; while above 18C is only really necessary for the seed of some tropical species.

Think about the temperature that you keep your home at, particularly by day and few of people have the thermostat set at 18C or below! That means that if you are trying to germinate the great majority of veg and ornamental plants that we raise for the summer, then the temperatures indoors will generally be too high. Such excess warmth causes stems to grow and stretch, fuelled as they are by the stored sugars in the seed.

The imperative of most seedlings is to get their leaves in the light so that they can start to photosynthesis – the process by which they make their own sugars to fuel further growth. They are designed in part to be able to push their first set of leaves up as high as possible to get them into the sun. Lots of warmth coupled with the low light levels indoors encourages the seedling to extend and extend. And this is why they become leggy.

Preventing leggy growth

While both warmth and moisture are need to kick start the swelling of the seeds and the emergence of baby roots and leaves, once the seedlings appear it is best to give plenty of light. This is usually coupled with reducing the temperature around them so that growth is slow and stocky. Ideal conditions can often be found in a spare bedroom or less-used living room which is kept cooler than the rest of the house. You may also find that a well-ventilated porch or conservatory can be more easily kept at a suitably cool temperature.

Many gardeners also use heated propagators for germination and these often encourage legginess too. It is crucial that seedlings are removed from such additional warmth as soon as they appear. Even the lid of an unheated propagator can trap too much heat and drastically reduce the light levels inside for the seedlings. This stage in there life is crucial and while they don’t want it too warm, they don’t need to go to the other extreme which could cause a cold shock.

How to deal with tall seedlings

Most seedling raised indoors will benefit from being moved quickly into their own small volume of compost – shortly after their first set of seedling leaves are fully opened and signs of the next ‘true’ leaves start to appear. This process is know as ‘pricking out’ and involves carefully lifting the seedlings from the compost in which they germinated and setting them in freshly filled trays or small pots of compost. The benefit of doing this while the seedlings are still very small is that the roots won’t be too far reaching and entangled with each other which means that they’ll suffer less damage.

It may seem counterintuitive to deal with such tiny plants, but they are tougher than you might think. Gently handle the seedlings by their seed leaves, taking care not to crush them, and you’ll be amazed at how easily they transplant and re-establish. Leave them too long and you may well set them back in their growth.

But the biggest benefit of pricking out seedlings is that you can bury part of the stem below the first set of ‘seed’ leaves – particularly useful if that stem has become stretched or leggy. If you needed to, you could bury the whole length of the seedling stems so that the finished level of the compost is just below the seed leaves. In reality just burying half of it makes a big difference to the stability of the seedlings. And, while young, that stem has the ability to turn into root tissue – something which is not so easy when the seedling become older.

How to prick out seedlings

The overall process of pricking out is relatively easy and straightforward, although it does take delicacy and practice. The aim is to move the seedlings causing as little damage as possible, so to start make sure that the compost that they germinated in is moist. This will allow the roots to part company with compost easily.

You can prick the seedlings into another tray of compost – simply spacing them them more widely apart. Alternatively you could choose cellular or divided tray inserts so that each seedling has it own self-contained pocket of compost. Or you could pop them into individual pots – the smallest you can find. Don’t use large pots to prick out into or the seedlings won’t be able to grow a robust and supportive root system in such a large volume of compost.

Fill your chosen trays or pots with fresh, sieved compost, tapping them gently to consolidate, but not firm, it in place. Next use a pencil or garden ‘dibber’ to make deep holes at 50mm spacings in the surface of a tray of compost, or at the centre of each individual division or pot. These holes should do deep and be made wide enough to accommodate the young root system of the seedling. You can better gauge this with practice.

Use the pencil or dibber to push down into the compost, alongside a seedling and carefully lever it out by the roots. Hold the seedling by the tip of the seed leaf and lift it over the hole in the fresh compost so that you can lower it down into position. Aim to get all the roots down to the bottom of the hole so that at least half the stem under the seed leaves is below the surface  of the compost too. Still holding the seed leaf, use the pencil or dibber to lightly press the compost in from the side to fill the hole and hold the seedling upright. Take care not to be heavy handed.

Once you’ve repeated the process with all the seedlings, water them in with a fine rose on a watering can. Put the transplanted seedlings in a warm (12-15C) place with plenty of light to grow on. So don’t suffer from leggy seedlings – learn some new techniques and give them a fighting chance for the best start in life.

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