When should you plant perennials ?
The simple answer to this is that you can plant them at any time of the year, however there are some times that are better and some times that may not work so well.
The overriding rule is this – can the plant establish and is it going to get stressed ? To establish a perennial well you want the roots to establish into the soil, they are the plants foundations – its no good for the top to grow if the roots can’t supply enough water – this requires moisture and is aided by well prepared soil. This is why Autumn was always the traditional time to plant – the air is cool, controlling top growth and the soil is warm and wet so the roots get a good start. Then when Spring comes, the plant is all set up to go.
Spring is also an excellent time. You can plant in Spring and the roots and top can grow away together. The soil is still wet and the plant can establish well before the stresses of the Summer. The advantage of Spring over Summer is you get to see instant results !
Summer can perfectly fine so long as you take a little more care. Remember the soil will be drier and the demand for water will be greater because there are longer, possibly hotter !, days and the plant has more leaf to support. Just make sure you satisfy the plants water needs. Best to give the ground a good soaking before planting (ie half a bucket) and then keep an eye out. Remember it is water at the roots, not a damp surface that is important – a quick shower of rain might make you wet, but its often useless to the plants. Soak less frequently rather than sprinkle every day. (its a good general rule – if you wet the surface regularly you can draw the roots to the surface rather than keeping them deep, which is exactly what you don’t want.)
This is least favourite for planting. Tough perennials will probably be OK, but something more delicate may just sit wet and may rot. In general the soil will be too cold for root growth and the tops will be dormant so nothing much is going to happen. Just put you plants somewhere sheltered until Spring comes.
So in conclusion, you can plant anytime, but in order of preference Autumn, Spring, Summer then Winter.