Soil preparation and planting
To prepare the soil for planting you need to dig and improve the soil. In general, however, if a perennial has long, thick roots and only a few fibrous roots, it will be one which penetrates fairly deeply, so double digging will be strongly advised. Where you are planting a mainly fibrous-rooted plant, you can get away with single digging, but bear in mind that double digging gives better results for any subject in the long term.
If you are preparing a border or an island bed, try to do so in autumn even if you are not planting until spring, so that work will be as easy as possible after the dry weather of the summer. All types of soil, even clay kinds, will be more malleable at this time and there will be less and less pressure from other work as the autumn goes on. Although the general rules of application of organic matter apply, it is not advisable to supply fertilizers at this stage unless the soil is known to be very short of nutrients.
Time of planting
Perennials are usually all right throughout the winter, but they are best planted between October and November and again between February and April. If they are planted during the earlier period they have a chance of becoming reasonably established before bad weather sets in, and if they are planted during the later period, the worst wintry conditions will almost certainly have passed. This applies particularly to evergreens.
Perennials should be planted in groups of three to seven plants, avoiding straight lines. The groups should be placed well apart – 50 cm (20 in) – and should be made to drift into one another. The average density of plants should be about five per sq m (sq yd) – higher with shorter plants and lower with taller. Always plant groups of one variety to obtain the greatest effect. As a general rule, no plant should be taller than half the width of the border.
Method of planting
Always use a trowel for planting newly purchased plants and rooted divisions. Most plants have exposed and fibrous roots. Dig a hole large enough to allow them to be spread out in it and of such a depth that the crown of the plant is in the surface of the soil. Cover them with some soil and consolidate it between the roots with the back end of a trowel handle. Add soil until the hole is full and carefully firm it, leaving the surface soil round the plant level. Avoid trampling down the soil in the bed. It is a good plan to stand on a board. Remove any board or foot marks by lightly hoeing. A little grit or gravel on the soil round the plants and close to any top growth will put off foraging slugs and snails, and prevent rotting of leaves and stems.
Some perennials have root-balls. This is obvious with newly supplied plants, because they are carefully wrapped by the nurseryman. Place these in a hole a little larger than the root, carefully remove wrapper without any disturbance and firm soil into the hole.
If plants have tap-roots, as anchusa does, for example, dig a deeper hole and break up the soil below its base to allow their more easy penetration.
Care after planting
Although most perennials are hardy, even these will be slightly vulnerable, and a weather eye should be kept open for prolonged frost and prolonged rain, each leading to soil conditions unfavourable to the roots of such plants. Protection of the crown with mulches of leaves, straw, sacking, compost, bracken, etc., will keep off the cold, and gentle forking round the plants at intervals will help with drainage.