Blog: Plants under glass grow soft – toughen them up to cope with the outdoors…
We’ve had a real blip in this warm spring – or the weather went back to normal – take your pick! This is why it’s vital to know how to harden off plants properly to avoid casualties. One of the things you dread as a gardener in North-East England is the wind blowing from the North or East in winter or spring, bringing with it low temperatures and extremes like The Beast from the East.
And last week we had it – biting winds coupled with a night frost. Like a scout, you have to be prepared, as we’ve hit the time when plants need to start hardening off. You’re looking at things like (more details for all of the highlighted plants):
- Half-hardy annuals
- Hardy annuals and vegetables grown in modules/pots under glass, such as runner beans and courgettes
- Half-hardy/tender perennials which have overwintered in cold greenhouses, exotics such as cannas and geraniums (Pelargoniums)
In my garden, this is the second wave of plants to go out – first were the almost hardy succulents. This week is for those that have been in the unheated greenhouse – geraniums, cannas, scented-leaved geraniums, agapanthus and dahlias. The door has been left open, so they’re well on their way.
Frost-free nights
According to the week’s weather forecast, we’re frost-free with mild nights, which looks pretty perfect. Last (in June) will be runner beans, courgettes, tomatillos, bananas, ginger and coleus. Everything is grown in pots, as my soil is heavy clay. Container gardening also means you can arrange them to produce different effects (like hiding the car, which is permanently on the drive now).
A good tip is to keep pots grouped. It makes watering much easier, and if there’s a late frost, just pop a large sheet of horticultural fleece or net over the top. Even hardy plants get used to regular watering, still air, and stable temperatures. Putting them outside to survive in widely fluctuating temperatures, much stronger sunlight, and winds will lead to a check in growth, even death if they are caught by frost.
The effect of hardening off is to thicken and alter the plant’s leaf structure and increase leaf waxiness. It ensures new growth is sturdy, although much slower. You can read more about growing plants from seed.
How to harden off plants… do it gradually
You need to harden off gradually, over a couple of weeks, depending on the weather. On a mild day, start with 2-3 hours of sun in a sheltered location. Protect seedlings from strong sun, wind, hard rain and cool temperatures. Hardy plants acclimatise faster than tender kinds.
If you don’t have a cold frame, place plants in a sheltered position in front of a south-facing wall or hedge and cover with fleece to prevent sun scorch and temperature shock. For the first week, leave it outside during the day, but bring it in at night. In the second week, leave outside at night, but keep covered (unless there’s a frost forecast).
Towards the end of the fortnight, remove the fleece during the day. If the weather is suitable, leave the plants outside at night but ensure they are covered. After this, leave them uncovered before planting out. Covering with an old curtain or extra fleece can protect from sudden, sharp night frosts.
