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Pruning basics in the desert

Bypass pruners (or secateurs) like these are recommended for pruning smaller twigs.

Pruning can help keep your shrubs and trees looking good and increase the production of flowers and fruit if done properly. However, poor pruning techniques can irreversibly damage your plants.

Here are some tips and further resources on how to do it right.

Why prune?

First, you need to have a good reason to prune. The general rule for determining this is to apply the four Ds: dead, diseased, damaged, or dangerous. Don’t prune just to shape a plant into a meatball or a square. In fact, most shrubs do very poorly with topiary-style shearing, a very specific type of pruning used to make plants take on a uniform shape. If a limb is dead, diseased, damaged, or crossing another branch (which will cause damage due to rubbing), it needs pruning. Frost-damaged plants should only be pruned after the chance of frost has passed (around March 15 here in Tucson). Branches that pose a hazard — falling or snagging — also should be pruned off.

If you like the formal look, make sure you choose the correct type of plant. Many shrubs that do well here (like Texas rangers and oleanders) do not respond well to shearing, which ignores the natural structure of a plant and forces growth at the outside edges. Plants that can be sheared regularly and formally include Japanese boxwood (Buxus microphylla japonica), waxleaf privet (Ligustrum japonicum), xylosma (Xylosma congestum), and myrtle (Myrtus communis). One native shrub that can be sheared or left to grow naturally is jojoba (Simmondsia chinensis). For more privacy shrub ideas, check out "Pruning Shrubs in the Low and Mid-Elevation Deserts in Arizona" from the University of Arizona’s Extension office.

When to prune

Deciduous trees and shrubs should be pruned when they are dormant. For other plants, it depends on when they flower. For instance, you do not want to prune spring-flowering shrubs in the winter, because that’s when they form their flower buds, and you will be left with very few flowers for that season. Prune these shrubs after they bloom.

On the other hand, shrubs that bloom in the summer, like oleanders and Texas rangers, can be pruned in early spring; in fact, you may end up with more flowers if you do. The above-mentioned handout on "Pruning Shrubs in the Low and Mid-Elevation Deserts in Arizona" from the University of Arizona’s Extension office has a great table that lists common ornamental shrubs and when to prune them.

How to prune

If you would like to thin out your shrub or tree, you can do this effectively by pruning out branches all the way to their point of origin from the ground or the main trunk. This helps open up the shrub or tree and get more light and circulation inside the plant, which can help reduce disease, branch rubbing, and other problems. The general rule for this method is to remove one-third or less of the shrub per growing season so as not to stress the plant too much. You may need to plan out your thinning over several seasons if your shrub or tree is particularly dense and ungainly.

Pruning tree limbs requires some knowledge and finesse. I recommend against pruning a branch that is thicker than about 2 inches, or one that is hard to reach with your tools yourself — get an arborist to help you. You will need to make three cuts to trim a tree limb, with the first being on the underside of the limb to prevent bark from peeling off and causing damage to your tree. You will also need to space the cut correctly so that the tree can heal itself after the cut. For detailed information, read “Pruning Deciduous Shade Trees” from the University of Arizona Extension office.

If you have a shrub that has been poorly pruned in the past, you can try to salvage it with rejuvenation pruning. This is where you prune all of its branches down to 12-18 inches high. With luck, the shrub will grow back in a more natural form. In the meantime, you will have bare sticks poking out of the ground, so be prepared for that look until your shrub grows back. Keep the shrub well-watered and give it compost to help it recover. Gardeners in other climates sometimes recommend using this technique every season or two for keeping shrubs that are too large for their space a more manageable size. In our harsh climate, this is going to kill your shrub. If it’s too big for the space, move it or replace it with something more appropriate.

Other pruning tips

Avoid pruning plants that don’t need it — if the plant is too large for the space, consider moving it or getting rid of it. Remember the “right plant, right place”  concept, and plan ahead for the mature size of the plant when you’re buying it.

Avoid pruning citrus. You may have seen some formally pruned citrus trees — they’re trimmed into lollipop-shaped small trees with white-painted trunks. This isn’t healthy for the tree and is more work for you. The lower branches shield the trunk from the sun, necessitating painting of the trunk, and you need to keep pruning to keep that formal shape. Citrus do best with very minimal pruning, following the four Ds rule. For more information, check out “Pruning Citrus” from the University of Arizona’s Extension office.

Use bypass pruners rather than anvil pruners. Bypass pruners make a clean cut, while anvil pruners tend to crush the limb, which slows healing and causes unnecessary damage.

Make sure you keep your pruners sharp and well-oiled and clean them after each use with hot soapy water or dilute bleach to avoid spreading disease in your garden. At least once a year, give them a proper cleaning and sharpening.

You can use your cast-off pruned plant material for mulch. Just chop it up into small twigs with your shears, or consider investing in a small yard mulcher, like this one for $125. Then drop your free mulch under your plants. Do not mulch and reuse any vegetation that you think might be diseased or that has lots of pests on it.

I highly recommend the Pima County Master Gardener online lectures on pruning. They have them every other month or so. You can check their online schedule and sign up.

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