June is a lovely time of the year and an especially busy one in the gardening growing calendar. Greenhouses and cold frames are filled with young seedlings desperate to be planted outside. But before you get started, find out about hardening off plants.

The days are getting longer and often brighter. Traditionally the end of May is the last time before summer that we are likely to experience a frost.

So, it’s very tempting to plant out those young plants straight from their protected environment. But, there is one more stage to go through before doing this – and that is to harden off the seedings.


Hardening off is basically reducing stress on young plants by gradually getting them used to a harsher environment than the one they’ve been growing in. This applies to hardy plants as well as tender plants, vegetables, fruit, and flowering plants.

The easiest way to do this is to put your seedlings outside during the day and bring them in overnight for at least seven days. Then, they can be planted in their permanent growing position for the year, ideally in a slightly shady spot sheltered from excessive wind.

If you have cold frames, you can also put the seedlings into these for seven to fourteen days. Simply close the lid at night until the seedlings are ready to be planted out.

Why do we do it?


Toughening up young stems.

Young stems are soft and very flexible, especially when they’ve started their life in a protected environment. This is because there’s been no need for them to toughen up.

Placing them outside into a harsher environment on a gradual basis slowly hardens up the stems. Not only does this make them less susceptible to breaking in the wind but also less appealing to slugs and snails that adore tender young stems.

Avoiding damage caused by extreme temperature fluctuations.

Some seedlings, like tomato plants, are very susceptible to leaf damage caused by moving the plants from a warm environment to a much colder one. Plants can also quickly become dehydrated as the cuticle on the leaves has yet to harden which prevents excessive water loss.

Leaf damage interrupts the efficiency of photosynthesis and can result in weaker plants and an inferior crop.

Reducing stress.

Hardening off seedlings gradually means that the plants are exposed to wind, sun, and rain which helps prevent transplant shock. This is an expression used for seedlings that become stunted, don’t grow well, or die from the sudden changes in temperature and exposure to sunlight.


This month, make the transition to outdoor growing by hardening off your plants to get them acclimatised to their final growing position.

Spring is here, see my post on spring pollinators:

Or check out my Pinterest board for more ideas: