Hardening off seedlings - a question

Here's an email I had neglected to answer for far too long, from Wossername:

I have planted some aubergine, chilli, pepper and marigold seeds in a heated propogator near the windowsill in my kitchen.  Most of the seeds have germinated quickly and have grown quite nicely.  I opened the vents on the propogator to allow some humidity out and all was well.  I have found however that my aubergine seedlings have fallen over slightly and are leaning to one side.  My question is, is this supposed to happen, or have I done something wrong?

This is an interesting one. What tends to happen with plants grown in propagators is the seeds germinate in balmy conditions - usually around 21C with very high humidity provided by the sealed environment of the plastic lid. This creates "softie" seedlings, so that when the high temperature and humidity are removed, either when the heat is turned off, the vents on the lid opened wide, or the lid removed altogether, they often wilt: the cell structure can't cope with the changed conditions.

The solution to the problem is a gradual rather than sudden change in conditions so the seedlings can toughen up - or harden off, as it's known. Turn off the heat and leave the lid on; then open a vent part way a couple of days later; then take the lid off for an hour or two, putting it back on immediately if the seedlings start to wilt.

But ...

There's always a but, isn't there? My tomato seedlings have been getting some tough love this year. Normally I'd gradually harden them off in the way I've described above, and then gradually introducing them to the outside temperatures.

This year it just hasn't happened this way - they started off in an electric propagator which I turned off as soon as the seeds had germinated as they only really need the extra heat for germination. Then as the seedlings grew up against the top of the clear plastic lid I removed that and left them on the windowsill for a few more days to adapt to life in a slightly less humid environment.

I selected the strongest seedlings (how to tell? thickest stem, healthy leaves, not necessarily the tallest specimens) and planted them in individual pots. They've gone straight into the cold frame out in the garden, cold turkey. I think I've been lucky with the weather, which has been unseasonably warm, but so far the seedlings are fine. The cold frame will be shielding the plants from the extremes of temperature - they'll stay in there and with regular watering they should be ready to plant outside in June. But had the weather been less favourable, I could easily have lost them all: it was a gamble that paid off when I have lots of other things on my mind and little time for cossetting tomatoes.

So to go back to Wossername's query - if you can, gradually harden off your seedlings. When time is short, take some shortcuts - although be prepared to act quickly if the plants show signs of distress.

Anyone got tips for Wossername?

Comments

[Bob Ewing]

I have to agree that the seedlings need to be hardened off, aubergine can be tricky in its youthful stages.

[Wossername]

Jane, thanks. The aubergines seem to be coming along alright now. They are in a growbag in the greenhouse and being fed regularly with tomato feed. It seems to be doing the trick.

[Frankie]

I'm rather hoping hardening off seedlings is avoidable in most cases. I don't have a cold frame (yet) or a back door (living in flats).

What I did do with a few things, like the leeks (probably still too small to go outside but we needed the room inside) was to cover as many as I could with a bottle cloche for a week or so. They seem to have coped (so far at least).