How to Successfully Harden Off Seedlings

It has been said that being a good parent involves first learning to have your child around you 24/7 and then learning to let them go.  I think raising seedlings is very similar.  At first giving them all the attention they need to have the best possible start is the vital thing.  But soon it’s the letting them go that’s important, particularly when prize seedlings that have been nurtured for two months are about to face the outdoor world.  As the grand day approaches, the last thing you want is a late frost killing them off, so learning how to ‘let them go’ is crucial...

‘Hardening off’ is the vital process of getting seedlings ready for the big outdoors.  Without this vital step it is all too easy to lose precious plants or have them wilt from the sudden change in conditions between a warm windowsill and a draughty garden.  How best to harden plants off is the subject of much debate amongst experienced gardeners, so I thought I would round up the best advice available:

Seedlings need to be gradually introduced to the outdoors in stages over a period of at least a week and preferably two.  This generally involves taking them outside for just a couple of hours each day at first and then gradually lengthening that time until they are ready to spend a whole day and eventually nights out.  This time is required for them to adjust due to the:

• Greater range of temperatures outside
• Chill factor, moisture loss and buffeting of moving air
• Greater variation in water supply due to the above factors which increase evaporation from plants outside
• Direct strong sunlight which can scorch young tender leaves

To ease this transition, an intermediate stage can be introduced – either a greenhouse, cold frame or row cover.  Cold frames and greenhouses have the advantage that they can be opened up more and more over a few days, providing a further element of control.

Personally, I move my plants into my greenhouse for increasing amounts of time.  Over the last two weeks they have gone from a couple of hours in the weak morning sunshine to whole days out and now finally overnight when a frost doesn’t look likely.  As my greenhouse is unheated, the transition from there to outdoors will be easier and will probably just take a weekend.  I now have tomato plants, looking healthy and strong, surviving an overnight low of 40 degrees Fahrenheit (4 degrees Celsius) and ready for potting up or planting in the greenhouse bed.

Here are some tips I’ve found useful:

1. Watch the weather closely – the best time to harden plants off is on wet or overcast days under cover of a greenhouse or cold frame. Overcast days don’t stress the plants with high afternoon temperatures and are unlikely to produce hard frosts at night.
2. Once hardened off and planted out, still watch the weather – even if a late frost is forecast, plants can be covered with fleece or bubble-wrap and still survive
3. When taking plants outside, don’t place them on the ground where slugs can access them or birds can knock them over looking for grubs.
4. At first, limit the amount of direct sunlight they receive – choosing a place which is shaded in the afternoon.
5. I always ‘hedge my bets’ and keep a few plants back inside as an insurance policy, just in case an unexpected frost kills the main batch off

One tip I haven’t tried is ‘tickling’ tomato plants.  It sounds crazy but some people swear by it – just brushing the tops of tomato seedlings with your hand or a piece of paper a couple of times each day is said to toughen them up a bit before hardening them off properly.  Apparently there is some research to support this method, though it could all be down to the extra vigilance of regularly checking up on the plants while doing so!

Whatever happens, it’s rapidly approaching that time to let them go and let the real growing begin.  Roll on the warmer weather!