Perennials are plants which live for several years, and usually remain in the soil during their dormant season or period. They can be grown directly in the ground or in containers. To winterize your perennials a few simple steps will help to ensure that your plants remain in the best condition.
Most perennials die back once their flowering period has ended, prompted by a drop in the temperature. This can mean that the plant loses all its leaves and stalks so that nothing is visible above ground at all during the winter. It can equally mean that although the majority of the plant dies back, a clump of leaves, stalks or the base of the plant above ground remains. If you know that your particular perennial dies back completely over winter, you may find it helpful to mark its location in some way during the autumn with a label in the ground nearby.
Perennials grown in containers need a little more care than those grown in the open ground. The steps you take will depend on what kind of container you are using. For plants grown in ceramic or terracotta pots and planters, frost-proofing is an important winterizing task. This is because the container is made of a natural substance which absorbs water. If the temperature drops below freezing point, the pot will expand and contract with the soil and may crack. This could leave the roots open to attack from rodents, insects or general disease. To help prevent frost damage, either move the container indoors or into a frost-free location such as a coldframe, or wrap the whole plant and container in horticultural fleece or sacking, or layers of newspaper tied with string to provide some insulation.
For all perennials that are reduced to a clump above ground level, it is important to keep the clump free from garden debris. Remove any fallen leaves or branches that may cover the clump so that diseases do not have a chance to establish themselves. Unless the perennial has decorative seed pods or stalks, remove these at the end of the autumn too. If a prolonged period of bad weather, particularly snow or hard frost, is forecast you may need to protect the clump from the elements. To do this, take some horticultural fleece or sacking and cover the clump with the fleece, using stakes or tent pegs to hold it in place. You will need to remove the fleece every couple of days to remove the weight of any snow on top. This can help prevent the cold from "burning" the leaves will keep the temperature of the soil a little higher than the surrounding area.
For perennials that have grown too large for their position, dividing them should be carried out before the winter, or in the spring. To divide a large perennial, dig the whole plant out of the soil, taking care to keep an much of the roots structure in place as possible. You may want to gently wash the roots off with water so that you can see more clearly where the main and secondary roots are. Take 2 garden forks, and insert them both with their backs together, into the centre of the plant and root ball. Gently ease the forks apart so that the plant breaks into 2 sections. You can either replant one section in the original location (making sure you add some slow-release fertilizer such as blood and bone meal) or replant both sections in new locations.
Taking a few minutes to settle your perennials down for the winter could make all the difference to their survival, and save you money in the long run.