You have probably seen a tree that was trimmed wrong: thick, bare limbs with a puff of leaves at the very tips, like a lion’s tail. Arborists call it lion’s tailing, and Arizona arborists warn that it leads to sunburn, sprouting and limbs that break more easily in storms.
Mesquite and palo verde are two of the most popular desert landscape trees, and they do not want the same treatment. Here is what University of Arizona Extension and Arizona arborists say about trimming them.
Two trees, two personalities
Mesquites grow fast in hot weather when they have water, and they can get big. Extension lists hybrid mesquites at up to 40 feet tall with a spread of up to 30 feet, which is why it calls them often unsuited to the small yards of newer subdivisions. A mesquite planted in a small yard can outgrow it.
Palo verdes vary. Blue palo verde generally needs more pruning than other cultivated palo verdes because it spreads more and carries more secondary branches. Foothill palo verde is the slowest grower of the group, and in the landscape its maintenance is mostly removing dead branches, mistletoe and witches’ broom, or lifting the canopy slightly to show the lower stems. Foothill palo verde can also shed branches during a long drought, so a dead limb is not always a storm story.
When to trim
John Eisenhower, an ISA Certified Arborist who has appeared on Rosie on the House, gives this timing for Arizona desert trees:
- Mesquite and other vigorous desert trees: spring or early summer. Pruning at that time can prevent damage during the summer monsoon storms.
- Palo verde: they are more sun-sensitive. Avoid summer pruning, and as a rule take less off than you would from a mesquite.
- Smaller, less vigorous desert trees that do not need preventive monsoon pruning: fall. Pruning later in the year also tends to hold shape longer, because growth slows down through fall and winter.
Not every arborist draws the line this tightly. LoriAnne Barnett Warren of the Arizona Department of Forestry and Fire Management told the Arizona Republic that low desert trees in Phoenix can be pruned successfully year round, with one firm rule: never remove more than 25 percent of the branches and canopy in a given year, because it takes out too much living tissue. The point of the seasonal advice is moderation and timing, not a single magic month.
The City of Chandler’s storm guidance adds a general rule that applies to every species: avoid removing foliage during seasons when sunburn can result. In Mesa, that mostly means summer.
How much is too much
The Chandler guidance recommends pruning no more than 20 percent to open a canopy, keeping top growth and foliage in proportion to the root mass. Taking more than that removes too much of the living tissue the tree depends on.
Eisenhower is firm on two practices that should never be done to your trees:
- Topping. Cutting large stems at a set height to shrink a tree encourages sprouting at the ends of the cut branches, and the big cuts are entry points for decay fungi. The better method is to cut a branch back to a side branch that is at least one-third the diameter of the one being removed. That reduces sprouting and leaves the tree looking natural.
- Lion’s tailing. Over-thinning the interior leaves long, slender limbs with foliage at the end. It can lead to sprouting and to sunburn on the trunk and lower limbs, and the heavy ends catch more wind, so they break more easily in a storm.
One more tip from the same article that surprises people: on young trees, do not rush to raise the canopy by cutting off lower branches. Those branches are producing the energy the tree needs. Leave them until the top reaches its mature height, then remove one or two each year.
Problems specific to mesquite and palo verde
- Mistletoe. Extension says manual removal is the only practical control, and that cutting it off does not kill it, since it keeps growing inside the wood. Heavy mistletoe can add enough weight to break a branch. On a lightly infested tree it is partly an aesthetic call, because the berries are a primary food source for desert birds.
- Slime flux. This bacterial problem shows up on mature mesquites as dark, foul-smelling liquid seeping from cracks. Extension says the bacteria probably get in through wounds, including pruning cuts. That is one good reason to insist on clean, correctly placed cuts. Extension does not recommend the old remedy of drainage tubes, because they may create more entry points.
- Thorns. On young mesquites, thorns can be pruned off and will not grow back from that spot.
- Irrigation. Not a trimming problem, but it decides how well your trimming holds up. Extension notes that mesquites can become unstable with too much water, since rapid crown growth outpaces the root system. Moving drip emitters out to the drip line helps build roots that anchor the tree.
Put objectives in the work order
Eisenhower’s most useful advice is about how you ask. Instead of telling a crew to “trim,” “crown” or “round” a tree, ask for pruning objectives: the minimum and maximum branch diameters to be cut, and the percentage of foliage to remove. Extension’s contract checklist makes the same point with an example, such as pruning all dead, dying, diseased and weak branches over a stated diameter, and it recommends specifying that the work follows ANSI A300 pruning standards.
A work order built that way might read: remove dead and crossing branches, no cuts into limbs larger than a stated diameter, no more than 20 percent of foliage removed, no topping, and cleanup included. That gives you something concrete to compare between estimates.
Tree trimming in Mesa with Heartwood
Heartwood Tree Care provides tree trimming and pruning across Mesa and the East Valley, with an ISA Certified Arborist (#WE-11505A) on the team. If you are not sure whether a tree needs trimming at all, or how much, our arborist consultation is a good first step. Estimates are free and done on site: request yours here or call (480) 269-5636.
Related reading: what to do with a storm-damaged tree and how to hire a tree company.
Sources
- University of Arizona Cooperative Extension, Mesquite and Palo Verde Trees for the Urban Landscape (AZ1429)
- KTAR, Rosie on the House, Here’s when and how to prune your trees in Arizona
- City of Chandler, Reduce the Risk of Storm Damage (PDF)
- University of Arizona Cooperative Extension, How to Hire a Tree Expert (AZ1003)
- Arizona Republic via AOL, Should you trim your trees for the monsoon? Here’s what an arborist wants you to know
