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Pricking Out, Potting and Hardening Off Greenhouse Plants

By: Dr Gareth Evans - Updated: 26 Sep 2012 | comments*Discuss
Transplanting Seedlings Germination

Growing a range of plants from seed is one of the great opportunities which a greenhouse opens up to the gardener but a few techniques need to be mastered before the full benefits can be enjoyed. Getting seeds to germinate at all can be an achievement in itself – especially if the plants are particularly exotic or unusual – but this is, of course, only the beginning, with pricking out, potting and hardening off still to come, before they can take their rightful place in the garden.

Pricking Out

The term “pricking out” refers to the transfer of seedlings into more spacious growing conditions, where they can begin to develop into young plants. As soon as germinated seeds are about to develop their first true leaves – as opposed to their initial “seed-leaves” – they should be lifted individually from their seed tray and then gently planted in a hole prepared in the compost of another tray. The seedlings should be planted in straight rows around 5cm (2 inches) apart, giving them room to grow without having to compete with their neighbours for space or light.

Once each tray is full, it should be watered carefully, using a fine rose on the watering can, to settle the compost around the young roots without disturbing them – soaking the compost thoroughly. In the first few days after pricking out, the seedlings will need regular watering and extra shading to protect them while their roots grow. Once they have become established, unless they are of a variety which is prone to scorching, such as begonias for instance, the shading should be removed to give them the best light.


Although the idea behind this is self-evident, it is worth remembering the old-time gardener’s maxim that the aim is to grow a large plant in a small pot and not the other way about – an approach which encourages the growth of a good root network. The real trick behind successfully potting up is to ensure that plants are never removed from dry soil for potting, so soak the seed tray or pot thoroughly and leave it for an hour or two before you make the transfer, making sure that you keep the root-ball intact.

With older style clay pots, it was always a good idea to place a few stones or pieces of broken crock in the bottom to improve drainage before filling with compost, but plastic pots have more holes, making this practice largely unnecessary, though many gardeners still follow it. The transplant should then be centred on a layer of firmed-in compost and the gap between the root-ball and the sides carefully made up with more compost, which is also firmed-in. How firmly this should be packed down depends on the plant being potted. Some, such as calceolarias, winter-flowering begonias, gloxinias and streptocarpus do best with light compaction, the likes of pelargomiums, fuchsias and hydrangeas prefer a more firm base, while the majority of woody shrubs need to be potted very firmly indeed.

Hardening Off

“Hardening off” is the process of acclimatising plants brought-on in the greenhouse to conditions outdoors – something which must be done gradually if they are not to be shocked or killed. A cold frame in the garden is often the best way to achieving this, softening the transition from greenhouse to garden, the plant’s exposure to the elements being slowly increased from none at all at first, to full, over a period of perhaps ten days or a fortnight. If space in the frame is at something of a premium, it is possible to speed the whole thing up a little, provided a sheltered area of the garden is available to which they can be transferred after about a week or so, to finish off their acclimatisation. Obviously even hardened half-hardy or tender plants should only be planted out in their final position once all danger of frost has passed.

Mastering these three techniques is essential if the gardener is going to be able to take full advantage of the plant propagation opportunities that the greenhouse offers and the really good news is that they can all be learnt very swiftly. All that remains is to put them into practice and then enjoy growing whatever takes your fancy.

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