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Last week I wrote about pruning but didn’t have enough space to address all the aspects of pruning that gardeners need to know about. This week we will finish the discussion about pruning. Shrubs are another category of pruning that takes a little more time to write about. There are shrubs that bloom in the spring and shrubs that bloom in the fall. There are shrubs that we can treat like a perennial and shrubs that reach the same heights as small trees. They all have different needs.

The first thing I would like to discuss is renewal pruning. Renewal pruning is a pruning that can be done to any multi-stemmed shrub. An example of a multi-stemmed shrub would be forsythia or common lilac. What renewal pruning entails is the removal of 2-3 of the largest heaviest tallest branches at the ground level. Every time we make a cut on a plant we are changing the hormone level of that branch. Let me explain. Every branch (trees and shrubs) have a hormone at the tip of the branch called an auxzin. This auxzin controls the growth of the branches behind that branch. If we remove the tip of the branch we remove the auxzin and the rest of the leaves and buds behind that tip are released to grow at any rate they want, and they usually pick fast. The result is that you get a witches broom effect. A witches broom is a growth that has many branches coming out of one point. It can be caused by salt damage or by insect feeding or in this case pruning. Eventually you get a plant that has very little growth at the bottom and massive amounts of growth at the top. The shrub becomes top heavy. It always looks like a little girl lifting her dress up to me. When we renewal prune by removing the growth at the bottom, we have the same hormonal affect, but it happens at the bottom of the shrub. We get a plant that maintains its natural form, stays full at the bottom, is reduced in width and height, and is healthier because we are removing the old growth which is more susceptible to diseases. Another quick way to prune multistemmed shrubs that have gotten out of control is what we call basal pruning. You get out your chainsaw and cut the whole plant down to about 8″. The root system is well established so the new shoots will come up thick the next spring and you’ll have a lovely shrub within 2 seasons. Pruning of this nature can be done successfully to spirea, potentilla, dwarf lilac. This type of pruning should be done in the fall regardless if the shrub is a spring flowering plant or a late summer/fall bloomer.

Now let’s talk about timing of pruning. If you have a spring flowering plant you will want to prune right after the plant is done flowering. Spring flowering plants form their buds for the next spring right after they are done blooming. Pruning later than spring will remove flower buds for the next year which will diminish the showyness of the plant. Fall blooming shrubs are ok to prune while they are dormant. Its easier to see the branch structure in the dormant season because there are no leaves on the branches. They are also less susceptible to disease because everything is dormant.

Single stemmed shrubs like burning bush should be pruned similarly to a tree. Always go back to a node. Try to prune from within and not just on the tips of the branches. This will help you avoid that witches broom affect we spoke about earlier.

My tree fruits professor at the University of Wisconsin Madison used to say, “prune when your saw is sharp, Prune so nobody knows you’ve done any pruning, except on fruit trees which you prune until you can throw a cat through it. But we’ll save that discussion for another day. Happy pruning.