The autumn garden cut-back

To prune or not to prune? That is the question for autumn.

When it comes to perennials – the flowering border plants that come back year after year – there is no “right” answer.

Those that finish flowering in spring and summer are best cut back straight away as it often encourages regrowth and fresh blooms, and you don’t want grotty brown plants detracting from autumn displays!

But if they are running out of steam in autumn then they are very unlikely to do anything appealing again until next year.

So the best thing is to cut them right back? Well, it’s six of one and half-a-dozen of the other as to whether you cut in late autumn or early spring.

If you leave the old growth in place till spring then you are leaving habitat for wildlife to shelter in through the winter, anything from hedgehogs to beneficial insects. On the downside … you are leaving habitat for wildlife, anything from rodents to troublesome bugs and slugs.

(Consensus these days is to help all wildlife for a balanced ecosystem in the garden and the hopes the ‘good’ creatures eat the ‘bad’ ones!)

If you leave the old growth it can protect the plant’s root system from the worst of the snows and frosts, helping it to survive the winter. But if the plant had any sort of fungal disease, leaving the old growth will increase the chance of reinfection. And if the plant is turning to mush from winter wet, leaving the old growth will increase the chance of the rot reaching the roots.

Tall plants can get snapped, broken or even blown over in winter winds, which can harm the root system, and looks messy too. However, there are a number that have seedheads that birds can feed on through the lean winter months.

Verbena bonariensis is a good example of a plant that feeds the birds if you leave it, gives you free plants from self-seeding, but can suffer in the wind if not cut. So when it comes to taller plants, what you decide to do comes down to personal preference, the size/shape/sturdiness of the plant and to how sheltered a position it is growing in.

For other plants with bird-friendly seedheads click here.

Most perennials can get cut back hard to just a few inches, whether you decide to do it in autumn or spring.

Exceptions are the evergreen perennials – click here – and those that have a woody framework, which should usually only get a tidy-up.

The main cause of gardeners losing their (evergreen) Penstemon over winter is from over-enthusiastic pruning: taller plants will want the old flowering stems trimmed off to reduce problems in the wind, but the bulk of the plant should be left throughout the frosty season to protect the heart and root system of the plant.

Woody-stemmed Salvias such as ‘Hot Lips’ should get similar treatment, with only around the top half of growth cut down.

But if it’s a plant that regrows from the ground and doesn’t keep its leaves year-round you are likely safe to cut back hard.

If you decide to wait till spring it is a good idea to check up on your plants through the winter to make sure there isn't anything damaged, diseased or rotting that needs clearing away.

With Lavenders there are also arguments for and against trimming in either autumn or spring. But if left uncut they soon go straggly and bare at the base so they should definitely be done one time or the other each year! Cut about 1-2 inches below flowering stems.


Evergreen herbs such as sage, thyme and rosemary should also just get light trims to prevent them going 'leggy', preferably several times a year. Most other herbs, including mints, tarragon, parsley and marjoram can be cut hard in either autumn or spring, subject to the same considerations as perennials.


Most alpines are evergreen which means they should get no more than a trim. The majority are also spring flowerers and are best cut after flowering time. If you wait till spring you run the risk of new flowers being 'lost' amongst scruffy old growth.


Ornamental grasses often have seedheads for the birds, and can also look attractive in the winter frosts,  and this old growth should be left to protect the heart of the plant until spring.
 

The main pruning time for Roses is February/March when new shoots start to appear on the stems - click here for tips. If it’s not a variety that produces rosehips for winter there is no harm in giving it a tidy-up trim now if it is bothering you in some way.
 

Almost all shrubs are best pruned immediately after flowering time, if they need pruning at all. The main exceptions are those that will form fruits or berries after flowering – click here and here for examples.

So if you haven’t done your summer-flowering shrubs already, get out there!

For all plants, but particularly shrubs, which will generally have thicker stems, avoid any pruning when frost is forecast as it can cause damage to open wounds which can then spread into the rest of the plant.