Sunday, 18 November 2018

Physocarpus - taming a wild one

I had a question recently from Corine ("Hi, Corine!") about pruning of a particular shrub. The question was:

"We have a very overgrown Physocarpus ‘Dart’s Gold’ that I am rather fed up with:  I am tempted to cut it right down to the base but I don’t want to kill it as it gives us privacy."

If you're not familiar with Physocarpus, take a moment to look it up - go on, you're already on the internet - as it's a lovely shrub, and rather under-used. It doesn't have a useful common name: the nearest is "Nine-bark" but I've never heard anyone call it that.

'Dart's Gold" is not one that I particularly like myself: when it comes to Physocarpus, I like Physocarpus opulifolius 'Diablo' (or 'Diabolo') because it has fabulous dark purple foliage. I always think of the "opulifolius" part as referring to the foliage - the "folius" part - as being "opulent" , which it is, and as for the Diabolo bit: maybe there's a connection to diabolical, the devil, dark, who knows?

Whatever the basis of the naming, it's a tough, hardy, shrub, which grows in pretty much any soil or sun conditions: it has nice upright stems (if you prune it according to the following regime, that is!) and in spring, it has sprigs of creamy white flowers, which are particularly nice against the dark foliage of the Diabolo cultivar.

So, the pruning: Corine's question is a fairly standard one, and my normal answer is to describe the old "cut out 1 in 3" pruning regime. If you're not familiar with this regime, here's the details: it's a really, really useful thing to learn as it can be applied to pretty much all garden shrubs.

All you have to do is cut out one third of the main stems each year, from as low down as you possibly can. Ground level would be ideal, but sometimes - if they are very old and congested - it's not possible to get that close to the base, so you might end up having to cut them at ankle height, or even at shin height.

A pruning saw is usually the best tool for the job, or a pair of loppers if you can get them in amongst the stems: and yes, it will involve crawling around on your hands and knees, so wear gloves, kneepads, and protective eyewear.

Why do this? The easy way, non-gardeners think, is to take the hedgetrimmers to it, and just round it down to whatever height you want it.

Ah, but this ruins the shape, the "form" of the plant.

When you cut a shrub, it will tend to sprout two or more new shoots from the cut end, so by using the hedgetrimmers, you will gradually create a hedge: a free-standing one,  which is round rather than long, but still, basically, a hedge. If you do it repeatedly, it will get bigger, and thicker and thicker on top, while getting thinner and thinner at the base, until you get a top-heavy "lollipop" which looks ugly, and is still too large.

Furthermore, if it's a flowering shrub, then you will probably never have seen flowers on it, from the day you started using the hedgetrimmers.

Instead, start an annual regime of cutting out one third of the main stems every year.

Year one: cut out a third of them. Any third, it doesn't matter, you might find it easier to cut out the ones on the outside, but you'll get the best effect if you can spread them out throughout the shrub. Cut them out as low as you can. If you really, really need to reduce the height, then go ahead and chop the tops off what's left.

Year two: you'll have the two-thirds of the original stems (now sprouting lots of lanky bits high up where you trimmed them) plus a whole batch of new, straight, upright ones which have sprung from the base. Carefully avoid these new ones, and cut out  half of the old ones, right down low again. Again, if what's left is too high at the top, reduce the height a little.

Year three: aha, now it gets to be fun: cut out the last third of the old stems. Right down low.  Now you have got rid of all the old, congested, wood: you have a year's worth of new straight stems from this year, along with a year's worth from last year. So your shrub is now clean, and clear, comprising straight, upright stems which can arch naturally, instead of fighting each other and having their heads chopped off by the hedgetrimmers.

Year four onwards: each year, cut out the oldest, tallest stems, right down low. Not necessarily a third of them: if you want more bulk, leave it for another year: if it's getting too tall, take out the tallest stems, but always cut them right down low.

This regime produces a shrub on which nothing is more than three or four years old: at least two-thirds of it will be bearing flowers, and keeping it under control will now be quick and simple, and nothing like the wrestling matches you had for the first three years.

When to do this? Whenever you have time. If it's a deciduous shrub, then doing it in winter means you can see the stems more clearly without all the leaves getting in the way. Doing it in spring means that regrowth will start quickly, covering up any gaps you might have inadvertently made.

Why do it in instalments?  It's kinder to the plant: the parts which you leave can continue to photosynthesise and support the plant, while the new shoots are growing. Too much of a shock can sometimes kill a plant, especially an old, well-established one.

Although having said that, if it's driving you mad, I would always say "give it a go" on the grounds that you are already fed up with it, so if it dies, well, it's no great loss, is it?

So, getting back to Corine's original question, if it's driving you mad but you want to retain privacy, then do half this year, and do the other half next year. I'd suggest doing the "back" half first, on the grounds that you won't then have to look at the bare cut ends for a couple of months. I'm assuming that if privacy is an issue, then it's on a boundary somewhere...  then, next summer, when it's regrown some new stems at the back, cut out the other half of the old ones, the "front" ones.

Thus, you retain some privacy, and give the plant time to recover between prunings.

Good luck, Corine, let me know how it goes!