Tuesday, April 1, 2008

Cotinus, Hibiscus, and "Rejuvenation Cuts"

(an image of Cotinus coggygria 'Royal Purple' taken last summer, photo credit: Alex Feleppa)

I have 5 purple smoke bushes, two Rose O’ Sharon hibiscus and three Luna hibiscus. A friend advised me to prune them all to the ground in late winter/early spring in order to get them to branch out. So in late February, that’s what I did. I cut them all to the ground. Now it’s March 31st and there are a number of other shrubs in my yard (snowball, honeysuckle, hydrangea, kerria) that are leafing out, but there are NO signs of growth in my smoke bushes or hibiscus. Did I probably kill them?

Thanks for writing. Honestly, I doubt you killed any of those shrubs, especially because all three of those are some very tough plants. However, by pruning them back as heavily as you did you cannot expect them to grow and act quite the same way as they have in years past when you did nothing to them.

Allow me to give you a little botany/horticulture refresher. In the fall the shrubs have built up their reserves of energy, in the form of carbohydrates, and they store that energy so it can be used to help the shrub properly leaf out in spring. Deciduous shrubs leafing out requires the majority of that finite amount of stored energy, and once the new leaves are out, the shrub can begin a new year of photosynthesizing and rebuilding those reserves. Now, this is not to say that “rejuvenation cuts”, as I learned to phrase them, are a bad idea. Like you said, they promote new growth from the base and often can help you achieve a more full and bushy looking shrub. However, you do want to think about the fact that your shrubs now have to do “double duty” because of those cuts in relation to a finite supply of energy. What I mean by “double duty” is first the shrubs have to callus those cuts, then they have to push out new growth.

The cells right where you cut are going to callus in order to prevent insects and disease from infiltrating the stem and root tissue. This, of course, requires energy. Right below the bark on all woody plants is the cambium, the layer of xylem and phloem that carries water and nutrients vertically throughout the shrub. Because the shrubs are determined to stay alive, the cambium will force “dormant buds” right below those callused cuts to swell, leaf out, and grow into new shoots. We hope that the new shoots will grow large and healthy and provide you with a great new look in your garden. However, I hope my description has helped to clarify why that will take a lot longer than in years past when pre-formed buds simply had to leaf out with an energy supply they knew was not being used anywhere else.

For your own education, in the future this is what I would recommend. I would have advised you to make some rejuvenation cuts, but I might have recommended taking back half of the stems instead of the entire shrub. The following spring the dormant buds and new growth emerge from where you have pruned, but the shrub also has some older branches and buds in place to ease it through that stress. Then next winter you go back again and, assuming the first cuts led to new growth like you wanted, then you cut back the other half of your older branches. The process is elongated over two seasons, but by the second spring you have a shrub significantly less in size that you know will bounce back well and not die on you because of too much stress.

Now, again, I do not think you killed your shrubs. But, I do think that because of the need to callus and then leaf out your shrubs are going to be a bit behind the rest of your garden. For my gardens here in New York, Cotinus (smokebush) and hardy Hibiscus tend to leaf out and flower later in the summer than many other shrubs so I might guess it will be another month before you see much new growth. Another important thing to know now is that you want to provide proper care and not add additional stress to the situation right now. Keep the shrubs well watered, mulch them but do not bury the crowns of the plant where the buds are trying to emerge, and keep an eye out for potential pest issues. You do not need to fertilize or apply any extra nutrients, just let the shrubs bounce back as best they can on their own.

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