Hardening Off Seedlings: When & How to Do It Right
Whilst hardening off your plants is a relatively simple process, it is essential to making sure your seedlings grow well when you transplant them outside. Plants that have grown indoors need time to adjust to outdoor conditions properly in order to grow successfully.
If you’ve never done this before or maybe need a refresher, we’ve put together what you need to do and how to make the process a success.
What is hardening off?
The phrase ‘hardening off’ is used for the process of gradually getting your seedlings used to outside conditions before you plant them out. We do this by slowly introducing them to lower temperatures and more sunshine so that they are ready to stay outside. This period will help your plants to develop thicker leaves which will make it easier for them to withstand outdoor conditions.
How long to harden off plants for?
Whilst there’s a lot of debate about how long the hardening off process takes, there are no strict rules and it is often down to personal preference. Generally, it will take 1-2 weeks, but more tender plants like tomatoes, cucumbers, and chillies may need longer as they will be more sensitive to the cold.
We prefer to take our time when it comes to hardening off to give our plants the best chance of thriving. Take things slow and steady to give your seedlings plenty of time to adjust before the big move. If you shorten the length of time, your plants might not be properly adjusted and therefore may grow poorly or not at all.
How do you harden off your seedlings?
There are a couple of different approaches to hardening off your seedlings.
Harden Off Using a Cold Frame
If you’re lucky enough to have a cold frame, these are the perfect environment to harden off your plants. They will protect them from wind which will avoid them drying out and they also let in plenty of sunlight which will help your seedlings adjust.
Hardening off in this way is a much simpler process. Place your plants in the cold frame and leave the lid open during the day, closing it at night for the first week.
In the second week, open the lid a little more each night until it is fully open just before you’re ready to plant.
Harden Off Outdoors
To harden off outdoors, find a sheltered spot in your garden that will offer your plants protection from the elements.
To start with, leave your seedlings outside for a couple of hours before bringing them inside again. Dot this on a reasonably mild day where it isn’t too hot or cold, and it isn’t raining or snowing to give them a gentle start.
If you have a more extreme weather day during the process, keep your plants indoors until the problem weather passes to keep your seedlings safe.
Over a period of about 2 weeks, gradually increase the time you leave them outside until you are leaving them out all day.
After the first week, you could leave your plants outside overnight using some horticultural fleece to keep them warm. This is also a great thing to do even after they’ve been transplanted if the weather takes a cold turn as it will offer protection to help them continue growing successfully.
Keep an eye on your plants and if you see any signs of stress like wilting, yellowing, or stunted growth, bring them back inside and reduce their exposure time until they’ve recovered.
Planting Out Your Seedlings
Once your plants can withstand the warm days and colder nights outside, they’re ready to move to their new home. Make sure your soil is well-prepared before moving them and be careful with your seedlings when transplanting to avoid any damage.
