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Hardening Off Chilli Plants

Up until recently this years crop of chillies have been living under lights down in my cellar. With the weather being so unpredictable this is by far the safest option as down there they avoid any cold nights in the greenhouse as well a constant supply of light from the florescent tubes. Recently, after moving house, the plants have been moved out from under the lights into a conservatory. The warmer environment has definitely helped their speed of growth.

Now that the weather has picked up a bit I have began the process of moving my plants outside. I have lost chilli plants in the past by moving them straight outside so i thought i’d explain the process of how I am hardening off my chilli plants now.

Why Harden Off Your Chilli Plants?

As Spring fully kicks in and the temperatures start to warm up, it is the time to start to think about moving your chilli plants out of the greenhouse or off the window sill and out into the big wild world. The main advantage to be gained by moving plants outside is the amount of sunlight they will receive outside should be significantly more than if you currently have them indoors on a windowsill.

In addition most gardeners (myself included) tend to be a bit over zealous with their planting in early spring and will be rapidly running out of room in their greenhouses as plants suddenly shoot up in the warm weather. Year after year I plant many more seeds than plants required and end up with minimal space both inside and in the greenhouse. Not only is space an issue but overcrowding in the greenhouse can promote fungal growth as a result of poor air circulation.

How To Harden Plants Off

In order to give your plants the best chance of success it is best to ‘harden them off’ prior to leaving them outside full time. This is the process of slowly getting them used to the cooler temperatures and windy conditions outside. In Spring temperatures can fluctuate wildly with hot days followed by very cold nights and strong gusts of wind an ever present threat to delicate stems. Chilli plant seedlings can easily be killed or severely damaged by these fluctuations if used to a constant indoor temperature. The need to harden plants off is not specific to chillies, but most types of veg.

How I Harden Off Chilli Plants

To help the plants adapt slowly it is best to first simply open all windows and doors on your hose/greenhouse to allow a breeze to sweep through during the day. This should help them develop stronger stems as well as improving air circulation. If you are growing your seedlings indoors, under gro lights you can use a small fan (an old computer fan is ideal) to help create airflow and strengthen stems.

After a week or so try moving the plants outside for a few hours during the middle of the day. Every 2 to 3 days leave them out a bit longer. Be sure to keep a careful check on the temperatures or wind forecast. If it looks like things may drastically change then play safe and leave the plants inside.

Assuming the weather holds you should be able to leave the plants out over night after 2-3 weeks. Again only do this once nighttime temperatures look to be consistently above 12 degrees. Until Summer arrives  be sure to keep one eye on the weather forecast in case there is a late frost. If the night time temperatures look too cold (single digits) then bring the plants back inside for the night or at very least cover them in fleece.

Hardening Off With A Cold Frame

If space is really short you could try using a cold frame to help get your plants outside earlier. A cold frame basically acts like a small greenhouse by providing a indoor like environment around the plants, sheltering them from the elements. If you do not have the budget or space for a full blown greenhouse then a cold frame is a great alternative.

Hardening Off With a Cold Frameimage by knitting iris

Hardening off chilli plants with a cold frame is much like I described above. One thing to watch out for is the potential to over heat your plants. The smaller size of cold frames means that on a hot day, if the lid is closed, temperatures inside can sore, high enough to damage your plants. During the day it can be a good idea to leave the lid open or slightly ajar to let the air circulate and keep the temperatures under control.


Hardening off you chilli plants is a key stage in their growth. Don’t do it and you could damage their growth or even worse lose you whole crop. Plants of all kinds hate sudden rapid environment changes so hardening off help them adapt to the transition of indoor to outdoor life.

Assuming your plants are fully hardened off and happy in their new home outside you’ll have a whole new set of things to worry about, mainly slugs & snails, aphids and various other pests that now have full access to your beloved chilli plants. The following articles may be of interest to you and be worth a read before disaster strikes:

3 comments… add one
  • Gary Thurston April 8, 2015, 3:11 pm

    I’ve been starting my chillies under lights but wanted to know if I need to harden them off if I wish to put them in my greenhouse. Any help would be greatful

    • The Chilli King May 5, 2015, 1:42 pm

      Hi Gary,
      If you greenhouse temp dips down under 10 degrees Celsius at night then you might want to start hardening them off my putting them in the in the evening for a few hours then back in the warm for the coldest part of the night.

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