It is important to ‘harden off’ or acclimatising greenhouse grown plants to outdoor conditions before finally planting them out. Move them straight from greenhouse to garden and they can suffer from shock, particularly in cooler weather; most recover but it checks growth, causes yellowing and some vegetables may ‘bolt’ or run to seed. ‘Hardening off’ takes two to three weeks, depending on the plant, the glasshouse temperatures and location of the garden, if you are in the north or south. Hardy plants acclimatise more rapidly than half-hardy or tender plants.
This is how you do it. Transfer plants from heated to cooler conditions on a cloudy day if you can, covering them with newspaper or fleece if it is sunny to stop them from scorching or wilting. If they have been raised in a heated glasshouse or propagator put them in a cold glasshouse for two weeks and from there into a well-ventilated cold frame for a final week, increasing ventilation gradually over time. Close the frame at night before finally removing the cover a few days before planting out. Cover the frame with carpet or hessian if late frosts are forecast. If you do not have a cold frame, put your plants in a sheltered spot like the base of a south facing wall or hedge. Cover them for the first week a couple of layers of fleece, reduce this to a single layer in the first part of week two. Bring plants indoors at night during week one. After about 10 days, depending on the weather, remove the fleece during the day and leave plants outdoors at night if it is mild. Leaving them uncovered towards the end of the third week, day and night before finally planting out.
Tender plants should not be planted out until after the last frost in your area. Listen to the weather forecasts and have temporary protection, such as cloches, fleece or newspaper to hand, should late frosts occur after planting out. It may sound like a lot of effort but it will be rewarded in the crops and quality of plants that follow.
It is also a good time to plant up hanging baskets, so they have a few weeks to establish, before putting them outside. Always buy your bedding plants from a reputable supplier; plants may cost more but the choice is generally greater, you ‘get what you pay for’. If they are a popular retailer plants do not spend too long on the benches and deteriorate. They should be compact, pest and disease free with undamaged green leaves and without masses of roots growing through the drainage holes. The compost should be moist.
Ventilate the greenhouse daily before temperatures get too high and provide shading from blinds on sunny days. Water using warmed rainwater on anything but seedlings (use tap water instead) and ‘grey’ water on ornamental plants.
Tie in the main stems of peppers, chilli’s and tomatoes to their canes while they are still soft and easy to train.
Check and control pests and diseases daily – look out for slugs and snails under pots.
Happy Gardening. Matt