Always prepare the ground intended for herbaceous perennials well in advance. You can, in theory, plant container-grown herbaceous subjects at any time of the year when the weather is suitable, but even so there are still two optimum periods – in early and mid-autumn (September to the middle of October), when the soil is still warm enough to help re-establishment, and in March to early April when the hardest weather is (with a bit of luck) over. Spring is the best time for perennials which have a tendency to be slightly tender. Bare-rooted perennials should always be planted during these periods.
A month or two before you want to plant, dig in some well rotted manure, compost or similar, and if the soil is at all on the heavy side, you should try to improve the texture with sand, gypsum or perlite, and plenty of humus-forming material. If you are planting in the autumn, add some bone-meal or other slow-release, high-phosphate fertilizer to encourage good root formation without stimulating the top into premature sappy growth. For spring planting, a top dressing of a balanced fertilizer is more suitable.
Dig a hole large enough to take the roots comfortably and break up the bottom so they can grow out of it properly. Set the plant at about the same depth as it was in the container or open ground from which it was lifted, or only about half an inch below so you can work a little soil into the crown. Some plants, such as peonies, will not flower if they are replanted too low, and others will not if the crown is deeply covered, so take some trouble over the job. Firm the perennials and fork over the whole area lightly to tidy the bed and remove the unavoidable consolidation which occurred during planting. A mulch of organic material, or even polythene or mulching sheet, at this stage is beneficial.
Planting mixtures of peat, or peat substitute, sand and fertilizers are now obtainable. If you have a lot of planting to do and you are fairly happy with the soil the stock is going into, you might find this an unnecessary additional expense, if, on the other hand, you have never planted anything before, you have only a little planting to do, and/or you are not sure whether your soil is as good as it ought to be, these mixtures would give the new things a sporting chance of getting away well if you follow the instructions properly.