From acclimatising your indoor plants to survive outdoors, to dealing with leggy seedlings, follow Alys’ top tips on how to care for your seedlings.
How to harden off your seedlings
If you’ve been growing seedlings on a windowsill, indoors or in greenhouse it is essential that you harden them off properly before you plant them out. Hardening off usually take two weeks, after that as long as there’s no threat of frost you can plant your seedlings into their final position.
Seedlings can do very well inside, its warm, they (hopefully) get watered often, there are no slugs to tend with; it’s all very cosseted. Life outside is a little more tricky, but eventually all seedlings have there to fulfill their adult life. Hardening off then, is the process by which, they acclimatize from the warmth of indoors to the harsh realities of rain, wind and pests. If you take a seedlings from a windowsill straight outdoors to plant you’ll find they suffer from transplant shock or show signs of cold damage even though it might not be that cold. Often this manifests as flopping over, a sudden very pale colour to the leaves or the edges of the leaves whither and scorch.
Hardening off is an interim stage which allows the seedlings to adapt. Hardening off traditionally happens in a cold frame, but not everyone has one of those, though it easy to fashion something equally suitable. Old brick and an old window, a fishmongers polystyrene box, a sturdy cardboard box or if nothing else, a piece of fleece will do. The idea is that you move your seedlings outside but to a protected space.
You can fashion together a temporary cold frame from stacked brick to make the base and a pane of glass or old window to go on top or you could use a sturdy box and piece of glass (toughened glass or old glass shelves from fridges are perfect). On the first day you just put the seedlings out for the day but bring them back indoors for the night, over the next few days as long as there’s no hard frost you start to leave the seedlings out permanently.
After a week, you can remove the windowpane and expose them to both day and night temperature, eventually in the last few days leaving them outside without any protection. After two weeks, they are ready to be planted out. If you are using just fleece I would keep it permanently on for a week and after a week I would expose the seedlings in day time only for a few days and then eventually taking the fleece off in the night as well.
There’s a few things to remember, bright sunny days in spring, might not be hot, but they can still scorch tender young things. It’s worth on sunny days covering newly ousted seedlings with netting to act as a little shade (net curtains work well for this). Also, slugs quickly find out what you’re up to and love to munch off your precious babies. Check under pots, trays and modules in the day because slugs love to hide here. Beer traps, a simple shallow bowl filled with cheep beer, are excellent at catching wandering slugs too.
If there is a late threat of frost and you’ve planted out your seedlings in the ground, drape fleece over them, even covering them gently with twigs or a single sheet of newspaper will keep an air frost off till morning.