Welcome to my WTF Gardening series, where I take common gardening terms and explain them for those who are new to the world of plants. Find the entire WTF Gardening series here.

Thinking about growing cuttings or starting some seeds? You’ve probably seen or heard the term ‘hardening off’ somewhere, whether that’s in a video tutorial or on the back of a seed packet. Unfortunately, this kind of instructional content can be a little vague, leaving novice gardeners scratching their heads. However, it’s important to understand what hardening off means, as it’s a pretty important step in the growing process for seeds!

Hardening off means gradually transitioning seedlings from a controlled environment – such as an indoor windowsill – to the colder, harsher conditions of the outdoors, where they’ll eventually spend the rest of their days!

Below, I explain more about hardening off, including how to do it and why getting it right is essential for your plants’ health and growth.

What’s the point in hardening off?

When plants are started indoors, they’re generally sheltered from fluctuating temperatures, heavy rain, wind and direct sunlight. As a result, they’re unprepared for life out there in the great big world! If you were to pop your seedlings straight outside once they’ve grown an inch or two, you’d more than likely be saying goodbye to your plants fairly soonish, as they’d wither and die within a day or two.

By exposing seedlings to outdoor conditions gradually, usually over the course of one to two weeks, you allow them to adjust to the changes in temperature, light and wind. This gradual transition helps prevent shock and stress to the plants, ensuring they thrive once planted in the garden.

How to get the hardening off process right

The key word here is ‘gradual’. Don’t rush the hardening off process, or it could stunt your plants’ growth or even compromise their health. Here are some steps to help you get the process right:

1. The in-and-out: Begin by placing your seedlings outdoors in a sheltered spot for a few hours each day, gradually increasing the exposure time over the course of several days to a week. Just remember to bring them inside after they’ve had their timeout!
2. Keep an eye on the weather: Pay attention to weather forecasts and avoid exposing your seedlings to extreme conditions such as strong winds, heavy rain or unseasonably cold temperatures. If it’s snowing, definitely don’t put your plants out!
3. Protect them from sunburn: If your seedlings have been growing under artificial light indoors, they may be susceptible to sunburn when first exposed to direct sunlight. Place them in a shaded area, such as under a tree canopy, or use shade cloth to protect them until they adjust.
4. Water and feed appropriately: As your seedlings adapt to outdoor conditions, they may require more frequent watering to prevent them from drying out. Continue to monitor soil moisture levels and fertilise as needed to support healthy growth.
5. It’s time to go full-time: Over the course of two to three weeks, gradually increase the amount of time your seedlings spend outdoors, as well as their exposure to sunlight and wind. By the end of the hardening off period, they should be ready for transplanting into the garden.

Anything else I should know?

Here are some handy tips to keep in mind during this period:

• Frost-sensitive plants will not be made hardy by hardening off – avoid planting them out before the last frost (usually in late spring) at all costs.
• If your growing conditions were particularly warm, the hardening off process will take longer.
• You don’t need a cold-frame (a kind of mini greenhouse that lies close to the ground) but it can really help make the transition smoother. If you don’t have one, place your plants in a sheltered position by a south-facing wall, and use shade cloth to shelter them from the sun.

Want me to unpack a specific gardening subject as part of my WTF Gardening series? Let me know in the comments below.

Mr Plant Geek

Michael has been involved with gardening and plants since he was just five years old. He is a self-professed Plant Geek, and was listed in the Sunday Times top 20 most influential people in the gardening world, thanks to his plant hunter role at Thompson & Morgan.

Michael was responsible for new plant introductions such as the Egg and Chips plant and the FuchsiaBerry and keeps busy travelling the world in search of new plants as well as lecturing worldwide, including stints in Japan. He is very active on social media – so why not give him a follow at @mr_plantgeek or Facebook – and writes a plant-focused Substack called Grow This, Not That.