Prunning Ornamental Shrubs

Prunning Ornamental Shrubs

Part of Ep. 203 Spring Planning

Prune when the knife is sharp!  Join  UW Arboretum naturalist Ken Wood to learn how to prepare your shrubs for the new growing season.

Premiere date: Feb 28, 1994

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley:
There are plenty of things to do to prepare for the new growing season, both indoors and out. We're at the University of Wisconsin-Madison Arboretum to do some spring pruning. With me is Ken Wood, Arboretum naturalist. Ken, there's snow on the ground. Why are we out here pruning?

Ken:
There's a little too much snow, actually, but this is the best time of year, for the next several weeks to do renewal pruning of our larger shrubs.

Shelley:
You mean late winter/early spring?

Ken:
Late winter, early spring.

Shelley:
Why?

Ken:
Well, the reserve of the plant is locked up in the roots and if we prune before the growth starts in the spring, that reserve is still there and can come back into new grow. Also, it's a good time to get a good clean cut. The bark is a little more brittle, frozen. And also there's likely to be much less problem with disease spread by insects or whatever.

Shelley:
Yeah, there's not a few out here, that I see.

Ken:
Not too many bugs today.

Shelley:
Alright, if late winter/early spring is the best time, is there a time that we should not be pruning?

Ken:
Well, there is a saying "Prune when the knife is sharp."

Shelley:
Do it when you can in other words?

Ken:
In other words, whenever you can get at it. But yes, it's true. There is a time-late July, August, early September-not good times to renew prune shrubs. And the reason is that once you do the pruning, the plant may try to put on a lot of new growth and that growth probably won't have a long enough time to harden off properly for the winter.

Shelley:
So they could actually die if we have a rough winter.

Ken:
Right,you could lose either the new growth itself or sometimes the whole shrub.

Shelley:
And so this time of year when we do it, we do it until we see buds forming basically?

Ken:
Right, until new growth starts forming on the twigs. At that point the plant is putting its reserves into the top part of the plant, better not to prune it too heavily at that time.

Shelley:
OK, if there are good and bad times to prune, are there good and bad ways to prune?

Ken:
Of course, the books always say yes, yes and no. A lot of us would look at a shrub like this and try to reduce its size by giving it a haircut, cutting everything off at a certain height. But, that is not a renewal process. It doesn't rejuvenate the plant. When you have broken twigs, or cut twigs, up at a height like this,new growth will start right up at the top where there's the most sunlight and these branches then get older and older and there's no vigorous new stuff coming up from the bottom of the plant.

Shelley:
And if all the vigorous growth is flat across here, you're cutting off the sunlight to the plant, too.

Ken:
That's right.

Shelley:
So are you cutting off flower buds? I mean are you just really--

Ken:
You're reducing the flowering, right. It just takes away the natural form.

Shelley:
Alright, then what is the right way to do this?

Ken:
Well, to renew a prune, the idea is to take out older stems and the Redosier Dogwood is a good one to demonstrate on because of the color. You can see that these older stems are getting corky bark on the branches.

Shelley:
Sure, so you're losing red.

Ken:
You'll lose the whole thing that you want the Redosier Dogwood for, right. But renewal pruning, if you're using loppers like this involves getting just as close to the ground as you possibly can, cutting off the stem, and then taking it out.

Shelley:
And so then we just pull these out like that. Now, so you do that with all of the older growth.

Ken:
Oh, on a shrub like this, probably half of the older stems and anyone near. It might take a couple of years to renewal prune a plant like this into two different winters. If the plant was really badly overgrown, you might even take three years.

Shelley:
OK, you handed this to me earlier. Would I use this at times rather than the loppers.

Ken:
If you've got a shrub that has really large stems, a pruning saw like this one with reverse blades on it, but it cuts when you pull back on it. So, with a narrow point that allows you to get into the center of the shrub and really clean things out without destroying all of the new growth at the same time.

Shelley:
Well, when you say something with thicker, larger stems, I think immediately of lilacs. Is that one that--

Ken:
Lilacs are a really good example of a plant that you probably want to use this kind of saw on, right.

Shelley:
So lilacs will benefit--

Ken:
Oh very much. In fact, an older lilac that's been left alone for many years, if you renewal prune it properly, you may be left with some spindly stocks that kind of flop. That's exactly what you want. The sunlight is getting down to the base of the plant. Your neighbor is going to look over the fence and say oh, you've just ruined that shrub, but you've probably done it exactly right.

Shelley:
So if my neighbor doesn't like it, I'm on the right track.

Ken:
That's a good guess, yes.

Shelley:
OK, well I have trouble visualizing it now. This one is so dense in there. I have trouble seeing what that's going to look like when you've taken out a half to a third of this.

Ken:
Well, we did one over here, Shelley, that was exactly the same shape and a week ago you wouldn't have been able to tell these two plants apart.

ShellY: These, wait a minute, these two were the same height, the same denseness?

Ken:
These two plants looked identical a week ago and yet by taking out half, maybe a little more of the biggest stems, we've rejuvenated the shrub. It now is a more manageable height, but it's still got its natural form.

Shelley:
And it's got the red color, too.

Ken:
Lots of red growth on it, right.

Shelley:
Well, OK, you mentioned it's at a more natural height. What if it doesn't quite fit? I mean what if it doesn't quite fit? I mean what if it's still too tall after I've done my renewal pruning?

Ken:
Well, I'd let you come in and snip out a few little twigs, but if its really still too tall, you probably have a shrub that's too big for the spot. Think about planting something that's a little smaller.

Shelley:
So, pick something appropriate to the site. OK, so far we've mentioned the dogwoods and you mentioned lilacs. What else benefits from renewal pruning?

Ken:
Forsythias, viburnums-- there's several other kinds of dogwoods. Any of these larger, multiple stemmed, vigorous growing shrubs.

Shelley:
I think a Spiraea, which has a great cascading shape.

Ken:
Spiraea's a good one, too. Right, that lovely vase shape.

Shelley:
And we keep that shape by doing renewal pruning rather than--

Ken:
Correct.

Shelley:
OK, is there anything else we should do this time of year once we're done with pruning?

Ken:
Early in the spring is the best time to fertilize shrubs, too. So, fertilizing is important.

Shelley:
So, let the snow melt a little bit, maybe.

Ken:
A little more than now.

Shelley:
OK, thanks, Ken.

Ken:
You're welcome.

Shelley:
And hang on to these prunings. Next up, we're going to show you how to make them bloom.

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