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The secret to success when planting tender young bedding plants is to plant them at the right stage of development into well-prepared soil so that they don't suffer a check in growth. Always plant tender bedding after the last expected frost date in your area and make sure they are hardened-off properly. Protect new plants from slugs and snails and keep well watered until they are established.
Move plants to a coldframe about two or three weeks before you want to plant them out. Hardy plants, such as winter and spring bedding, can be moved out to the coldframe as soon as they are large enough. Tougher summer bedding plants, such as Antirrhinums, can be moved out a month before the last frost date in your area. Tender plants such as Begonias, Impatiens and Pelargoniums should not be placed into the coldframe before the last expected frost date.
Harden-off the plants by gradually getting them acclimatised to the weather conditions outside. To start with ventilate the coldframe by opening the lid slightly on warm, still days only, closing it at night. After a few days, gradually increase the amount of ventilation each day until you remove the lid completely. Replace the lid on cold nights for the following week or so, but then remove it permanently.
If you do not have a coldframe, you can harden-off plants successfully in a sheltered spot at the base of a south-facing wall or hedge. To start with bring the plants in at night. After a week, leave them outside on all but the coldest nights, but cover with a double-layer of garden fleece on other nights.