How To: Prick Out Seedlings

Here’s a quick guide to something I’ll be doing a lot of over the next few weeks: pricking out and potting up seedlings.

I’m sure there are a lot of first-time gardeners out there, aiming to brighten their gardens with flowers, or boost their larders with food crops, who might not have ever encountered a tray of seedlings before and aren’t quite sure what to do with them.

Hopefully this handy how-to guide will help.

Preparation

For this how-to, you’ll need the following bits of kit:

• A tray of seedlings ready for pricking out.
• Pricking out tools: a mini fork or scoop (see below).
• A module tray, or appropriately sized pots, not-quite filled with good-quality peat-free compost or soil/compost mix that’s suitable for the type of plant you’re growing i.e. acid-loving plants need an acid (‘ericaceous’) compost mix; do some research before you begin.
• A dibber, for making a small hole in the compost.
• A gravel tray, to stand the pricked-out seedlings in (best option), or a mini watering can with the rose / sprinkler head removed (also good), or a spray mister (not ideal, but it’ll do…).

Anatomy of a Seedling

First off, what is a seedling, and how does it work?

When the majority of domestic crop and/or flower seeds germinate. they have just two jobs: to produce a root and a shoot. The root should grow downwards and begin to draw up minerals and water from the soil or growing medium, The shoot should grow upwards and produce either one or two ‘seed leaves’ (technically: ‘cotyledons’).

Seed leaves are the first leaves to develop and are often very simply shaped, and not necessarily the same shape as the final, ‘adult’ leaves (which is why it can be tricky to identify seedlings when they first germinate, and why labelling seed trays is so important). Their one job is to start the process of photosynthesis, providing energy to the seedling so it can develop its roots – either sub-roots from the main tap root, or a network of finer roots – and its adult, or ‘true’ leaves.

Here’s a celeriac seedling from the batch shown above, showing the fine roots, slender stem, the seed leaves – one of which I’m using to hold the seedling – and the first true leaves at the top of the stem. (Apologies for the blurring, as usual my phone camera decided the tray in the background was a much better focal-point…)

Timing: Try to Catch Them Early…

Generally, the gardening books will advise you to prick out seedlings just after the first pair of true leaves has developed. Some growers prefer to catch them even earlier, at the seed leaf stage, in order to minimise the risk of damage to the incredibly delicate roots, shoots and leaves, but that can make them very tricky to handle.

The tray of celeriac seedlings shown at the top of the post is probably three or four days past ideal pricking out stage. The seedlings are starting to get a bit long and tangled; left much longer they’d be extremely difficult to separate without damaging them in the process. But all is not lost, with a bit of care and a steady hand, they’ll be fine.

Here’s the method to follow:

1. Separate and Lift…

The important thing to remember at this stage is that seedlings are incredibly fragile things. Snap the stem at this stage and it’s game over for that particular plantlet; the chances of successful re-growth are so close to nil it’s probably not worth even trying.

So: never try to pull a tiny seedling out of its growing medium (I’ll use ‘compost’ from here on to save time). Instead, try to lift and lever up the seedling from below if possible – I like to use an old dinner fork, which is less likely to slice through roots than a spoon or scoop – and allow the compost to fall away from the roots naturally.

Once you’ve loosened a section of seedlings in the tray, it’s time to move them on. As shown in the previous photo, you should always hold seedlings by one of their seed leaves, never by a true leaf or stem. The seed leaves will usually shrivel up and disappear at some point anyhow, so if one of them is damaged at this stage it’s not such a great loss as one of the true leaves.

2. Dib, dib, dib…

As per the ‘preparation’ section, you should have your pots or modules of growing-on compost ready to go. With your dibber – which can be anything from a small plastic or wooden dibbing tool, to a lolly stick, to a plant label, whatever you have to-hand – make a hole in the compost that you’re going to plant the seedling into. If you’re potting up a whole tray of seedlings, you can try to make all the holes in advance, but I usually find at least half of them manage to fill themselves back in before I get to them, so I tend to dib as I go along.

Still holding the seedling by the seed leaf, gently drop it into the dibbed hole. Make sure the roots are all contained within the hole – sometimes they can catch on the edge, or fold back on themselves – and use your dibber to nudge them into place if necessary. Then gently firm the compost around the seedling, taking care to make sure it’s upright once you’ve finished.

A quick pro-tip to bear in mind: be ruthless! It’s very tempting, especially if you’re a novice gardener, to try to save and pot up every single seedling. But one thing you learn after a few years of growing is that sentimentality can often be unproductive.

Weaker seedlings won’t grow half as well as their stronger counterparts and are more likely to fail along the way. But they still take up a module or a pot’s worth of fresh compost – which is a waste of both a pot and the compost – and they still need to be monitored, watered, and moved around – which is a waste of your time and attention – for little or no eventual return.

So: only select as many of the strongest seedlings as you have pots for. The rest can be used as micro-herbs if you’re that way inclined, or consigned to the compost bin.

Repeat until you’ve filled all your tray modules or pots, as per: (this pic is a little over-exposed – thanks again, phone camera – but you get the general idea…)

As you can see, I’ve used a plastic module tray for these celeriac. They should grow on nicely until they reach a size that’s suitable for planting out, but if I need to I can always pot them up into larger, individual pots. I’ve also remembered to label the module tray.

If I was pricking out into individual pots I’d label each one, especially if I was growing several varieties of a single type of plant, or plants that have similarly-shaped leaves – celeriac, celery and flat-leaved parsley, for instance – that could be easily mixed up.

3. Water Well, and Carefully…

The final stage of the pricking out and potting-up the process is irrigation. Pricked out seedlings need to be able to access moisture quickly in order to prevent their tissues from drying out, which could kill them. But at the same time they don’t need to be drenched – roots that are saturated in watery compost can’t take up essential nutrients, which could also kill the seedling.

My preferred method is to stand the modules or pots of seedlings in a gravel tray of water for a few minutes. The water will soak into the compost from below, where it’s needed, without soaking the leaves and stem. After a few minutes, the modules or pots can be take out of the tray and left to drain, which prevents water-logging. At this stage I usually transfer them to another gravel tray, which I use for watering the young plants as they grow.

If you don’t have a gravel tray – or a washing-up bowl, or a bucket, or anything you can use as an outer water-container for a few minutes, then a small watering can, or a jug with a narrow pour-spout, is okay, as long as you remove the rose from the can and just allow the water to trickle from the spout, as close to the base of the seedling as possible, without wetting the leaves. Failing that, you can use a mister or spray bottle, but it’s not as easy to avoid leaf-dampening.

And Now We Wait…

Your successfully pricked-out seedlings should grow on happily in their fresh, new compost. Remember that they need to be kept watered, but not over-watered, and usually need access to plenty of light, but not too much strong, direct sunlight, which can scorch and damage the leaves.

Here’s that same tray of seedlings, snapped this morning, 12 days after they were pricked out, and growing well – as you can see, most of the seedlings have developed at least one more pair of true leaves, which generally means they’ve rooted successfully:

It’s going to be another few weeks before these celeriac are large enough to plant out at the allotment, so I’ll need to keep a daily eye on them to make sure they don’t dry out and wilt. If they start showing signs of nutrient deficiency – yellowing leaves is the classic symptom – then I might need to pot them up into fresh compost or, if time is short, give them a boost of nitrogen-rich plant food.

And then it’ll be another few months before they’re ready to harvest. Some crops are quicker, but growing your own food is a waiting game, patience is most definitely required.

Good luck with your pricking out and potting on! Let me know how you get on – and please feel free to post any follow-up questions – in the comments, below.