Hardening Off your Seedlings Ready for Planting Out

So here we are at the final stage before planting your seedling out into their final position in the garden or vegetable plot. As great as the urge is to get them straight in the ground, it’s important to take careful steps to harden off your seedlings as you could undo all of your hard work by leaving out this important step.

What is Hardening Off?

Basically, it is acclimatising your seedlings to the outside temperatures and conditions. You have kept them at a reasonably constant temperature within your home or greenhouse which has meant they have grown well but also could be shocked by a sudden change overnight or on a cloudy day, not being ready could mean a slowing down in growth or at worst the seedling dying off completely.

It causes a change in the plants structure, thickening its leaf structure and making the leaves more waxy and temperature tolerant. It also changes the way they grow, slowing it down but making the new growth much stronger than in its seedling stage.

It should be noted that it does not make frost sensitive plants into frost hardy ones, so your summer bedding plants will still need to be protected from unseasonally late frosts even if you have hardened them off.

How do I do it?

Once the seedlings are ready to plant outside and the risk of frost has gone, that is the time to start hardening off.

1. If possible, the first stage should be to go from your home/greenhouse into an outside unheated greenhouse or cold frame, do this on a dull/overcast day to prevent the seedlings from wilting in sunshine.

2. Starting with the door or lid open during the day and then closed at night for the first week or so.

3. Over the course of the next couple of weeks, the door/lid can be left further and further until it is fully open and at this stage then the seedlings are ready to plant in the ground.

If you don’t have a cold frame/unheated greenhouse it is still possible to carry out this step but is a bit more labour intensive.

1. During the day, place your trays in a sheltered location, in front of a south facing wall or hedge and cover with 2 layers of horticultural fleece to prevent the sun from scorching them and protecting them from temperature variations.

2. For the first week, bring them inside at night.

3. During the second week, remove one of the layers of fleece and start to leave the seedlings outside overnight but be wary of the weather and bring them inside if a heavy frost is forecast. If you are unable to bring them inside at this stage then cover with a thicker layer of fleece.

4. During the third week, remove the final layer of fleece completely to fully harden them off but still protect them if there is a heavy frost or indeed heavy rains.

5. As you have the seedlings outside, again be wary of pests and diseases. In particular slugs and snails but also birdlife like pigeons are a common issue for new plants so some sort of deterrent may be needed.

Your no ready to transplant your seedlings into their final planting position having them the best possible start and will soon be ready to pick handle of beautiful flowers or enjoy lots of tasty fruit or vegetable.

However, the cycle never ends and it will soon be time to plant the next batch so make sure you’ve cleaned off all your trays and tools ready to go again.