swgarden0902Pruning branches during the summer is the way to grow if your goal is to control tree size. Mulberry limbs were scraping the rain gutters at my house (shown above), so I’ll prune them back this month. Photo credits Marisa Thompson.Question: Is it okay to prune landscape trees now? Should we wait until it cools off a bit? Wait till fall?

- Submitted via Lynda Garvin, Sandoval County Extension Agriculture Agent 

Answer: It’s totally fine to prune in summer, but we’re getting to the end of the safe season. Depending on your goals, summer actually might be a better time to prune.

We tend to teach about pruning in late winter and early spring because 1) it’s easier to see the branching pattern on deciduous trees and therefore easier to teach proper pruning techniques, 2) we may be pruning for increased fruit tree crop load, and 3) disease transmission from unclean pruning blades is expected to be lower when temperatures are cooler. Pruning just before the start of the growing season, when buds are still mostly dormant, also encourages a flush of new growth, so you can imagine why many consider late winter and early spring to be a great time for pruning fruit trees.

However, if you want to control branch growth and control the size of your tree, summer pruning is better. Pruning during the active growing season means it’s the perfect time for the tree to seal pruning wounds from the inside. Pruning sealant is no longer recommended for pruning cuts or any open wounds. For more on how tree wounds seal naturally on their own, visit the blog version of this column that’s chock full of resource links (https://nmsudesertblooms.blogspot.com/).

Because I’m trying to control the growth of big trees in my own yard, I’ll prune the mulberry limbs that are scraping against my house’s rain gutters now. I’ll also prune back some of the longer branches that are shading newly planted trees to give them more sunlight for next year. These include Chinese pistache, New Mexico olive, bur oak, and desert willow. But I’d better go ahead and get these cuts done soon!

Fall and early winter are usually considered to be the worst times for pruning because either the trees are dormant or they’re growing very slowly, so the wounds are more likely to be open for months. Pruning sealant is still not recommended, though.

It’s always a good time to prune out dead, diseased, rubbing, or dangerous limbs, no matter what season we’re in. And plenty of municipal and commercial tree pruners work through the fall and winter months, partly because there’s otherwise no way they’d be able to get it all done each year.

No matter what time of year you prune, always clean blades between cuts. I recommend any household disinfectant. Diluted bleach solutions are no longer recommended, especially for beloved and expensive tools, because over time they can cause pitting and corrosion on blade surfaces. I prefer disinfectant wipes for the convenience (homemade or store-bought). And aerosol disinfectant sprays are great for getting in between grooves on pruning saws.

The rules about not taking too much off of the canopy at once might be even more important in summer because of the difference between sun leaves and shade leaves. Here’s how my predecessor Dr. Curtis Smith explained them in a 2006 column about houseplant leaves getting damaged by too much sunlight when moved outdoors:

“If plants are grown indoors through the winter in a location with bright, but indirect light, they often form ‘shade leaves.’ Shade leaves develop under conditions of relatively low light. They efficiently produce food for the plants, but many lose the ability to tolerate exposure to full sunlight. They must be gradually exposed to full sunlight (hardened off). This is done by gradually increasing their time in full sunlight from a few minutes to longer and longer periods.

“Some plants will adapt without drastic changes, but others will drop their shade leaves and form new sun leaves. Sun leaves have protective mechanisms to protect them from full sunlight. If your plants begin dropping leaves as you change their environmental conditions, this is probably the cause. Leaf drop can also happen when plants are moved indoors in the fall. In the fall, sun leaves may be discarded, and new shade leaves formed.”

The same explanation works within a tree canopy where outer leaves with more exposure are adapted differently than the shaded leaves in the canopy center. By pruning off the sun leaves that are accustomed to high light levels, we’re stressing those newly exposed shade leaves underneath. So don’t overdo it if you prune in summer. Remember, stressed trees are likely to become even more stressed by major pruning, so be sure to irrigate deeply, infrequently, and out at the canopy dripline and beyond, not just at the base of the trunk.

Interestingly, it used to be customary to remove many of those shaded leaves and small branches on the inside of tree canopies. That growth was considered to make a “trashy trunk,” and people believed that by removing them we were reducing competition within the canopy and allowing air to get through the canopy better to reduce pathogens that thrive in moist conditions. Recommendations and practices continue to change. For one, in the semi-arid Southwest we just don’t have much to worry about in terms of harboring pathogens in the middle of tree canopies. Even with a super “trashy trunk,” it doesn’t stay moist enough in there to be a real threat. Also, we now know more about the benefits of keeping those inner canopy shade leaves because they tend to be able to photosynthesize more efficiently with less light stress, and they actually contribute their photosynthates (aka sugars, carbohydrates) to the health of the greater tree.

Another big reason to leave smaller branches in place is that we now know more about the natural overall branching patterns of trees. I’ll save this concept for another column, but for now I’ll just tell you that trees have evolved to develop branching patterns that tend to be the same across tree species and plant families, and that play a major role in long-term tree health and avoiding branch breakage throughout the lifespan. If you’re dying to read more, Google “Leonardo da Vinci’s rule of trees.”

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the NM Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent (http://aces.nmsu.edu/county/) and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center.

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