Hardening off vegetable seedlings before planting outside

It's important to give young plants sown indoors time to adjust to life outside. To survive in the allotment, vegetable patch or kitchen garden, seedlings need time to harden off, gradually getting used to the more varied conditions they'll experience outdoors before being planted out permanently.

We've had some lovely warm spring days lately, but the nights are still quite chilly and there's a chance that there'll be frost around on colder mornings. 

Young seedlings grown indoors are vulnerable and tender so it's important to gradually acclimatise them to being outside. 

Putting them outside without helping them adjust gradually could cause damage or even kill them off.

Having been mollycoddled indoors since being sown, heavy rain, strong winds or even hot sunshine will be quite a shock to young plants. 

I get mine used to life outside by moving them into a cold frame for at least a week before I plant them in the ground on the allotment.  

I have a few batches of broad bean seedlings which are ready to move outside, and I've sown some peas, lettuce, beetroot, carrots and spinach in modules and seed trays.

See also: Start growing your own by sowing seeds

See also: How to sow seeds - A Beginner's Guide

Broad bean seedlings ready to move outside...gradually

Once they've grown into healthy seedlings, I can move them into the cold frame. 

I built my cold frame myself from bits of wood recycled from an old fitted wardrobe that we dismantled. I'm not a very good carpenter by any means, which shows it is not difficult to make a cold frame if you need to. 

It's really just a wooden box with a hinged lid. The lid needs to allow light in. Mine is an old piece of clear plastic sheet I found. I've seen old window frames or glass doors recycled for the same purpose. 

My cold frame full of seedlings

If you have an unheated greenhouse then you can move seedlings to there and leave the doors and vents open during the day. 

I prop the lid of my cold frame open during the day and close it at night. 

If you have neither a cold frame or a greenhouse, I would recommend moving your seedlings outside to a sheltered spot every morning and bringing them back indoors in the evening.

In the cold frame, young plants get to experience some of the weather they'll need to cope with outside, particularly the wider range of temperatures, but are protected from the extremes, especially frost. I also close the lid during particularly heavy rain or stormy conditions.

Once they've had at least a week to get used to life outside, I'll take them up to the allotment and plant them in the ground as soon as conditions are suitable.

I get an average of about 80 minutes a week to grow fruit and vegetables on my allotment. If you've also got limited time to grow your own, then come and visit me regularly on the 80 Minute Allotment.

Look after your seedlings and happy growing,
Paul

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