Walk into any garden center in the spring and you’ll find shelves of tree spikes, granules and “tree food.” It’s easy to assume your mesquite or palo verde could use some too.
Usually, it can’t. University of Arizona Cooperative Extension puts it plainly: fertilizer is only justified when a tree is showing signs of a nutrient deficiency. Here’s why, what those signs look like, and the times when feeding a tree actually helps.
Why most desert trees do fine without it
Native and drought-tolerant trees grew up in soils with very little organic matter. UA Extension notes they generally don’t need fertilizer, likely with help from beneficial soil fungi (mycorrhizae) that partner with their roots. A healthy, established mesquite, palo verde or ironwood is already getting what it needs.
Too much fertilizer can hurt
Extra nitrogen doesn’t just go to waste. According to UA Extension, over-fertilizing can lead to:
- Weak wood and poor branch structure
- Higher water use
- More damage from insects and other pests
- Nitrogen washing down into groundwater
Fast, soft growth is a real liability once monsoon winds show up. A tree that’s been pushed to grow quickly is carrying more weight on weaker wood.
Fertilizer also won’t fix the problems behind most struggling trees. Poor drainage, compacted soil, a bad planting job or watering that’s off are not nutrient problems. UA Extension warns that adding fertilizer in those situations can actually speed up a tree’s decline.
Signs a tree might need nutrients
Nitrogen deficiency
Look for slow growth, sparse foliage and older leaves turning pale or yellow. If that’s what you’re seeing, UA Extension recommends light applications of nitrogen during the growing season, spread out near the drip line where the feeder roots are. Things should start to improve within a few weeks.
Iron chlorosis
This one is common in East Valley yards. New leaves come in yellow while the veins stay dark green. It happens all over Arizona because our alkaline soils lock up iron, even though there’s plenty of it in the ground. Symptoms tend to be worst on new spring growth and often fade as the weather warms up.
A foliar spray of chelated iron usually greens things up within a couple of weeks, though it typically has to be repeated every year. For a longer-lasting fix, soil sulfur can temporarily lower the soil pH so the roots can get at the iron. UA Extension advises against trunk injections, since the wounds can lead to serious rot.
Before you treat anything, test your soil pH. And because the same yellowing can come from overwatering or root damage, it helps to have someone look at the whole picture first.
When fertilizing does make sense
- Confirmed deficiencies, like the ones above.
- Trees with cramped roots, boxed in by pavement, curbs, walls or containers. UA Extension notes these can benefit from feeding.
- Citrus and other fruit trees, which are heavy feeders with their own schedule. See our guide to fertilizing citrus in the East Valley.
If you do use a granular fertilizer, water it in right away. Otherwise some of the nitrogen can escape into the air. Tree spikes are easy, but they concentrate nutrients in just a few spots around the tree.
And skip fertilizer for newly planted trees. A new tree needs to grow roots, not a flush of new leaves. Our post on when to plant trees in Gilbert covers first-year care.
Not sure what your tree needs?
If a tree looks pale, thin or stressed, start with a diagnosis instead of a bag of fertilizer. Heartwood’s certified arborists provide tree consultations and targeted tree fertilizing when a tree truly needs it, in Chandler, Gilbert, Scottsdale, Paradise Valley and the rest of our service area. See all of our tree services, request a free estimate, or call (480) 269-5636.
Sources
- University of Arizona Cooperative Extension, Fertilizing Woody Ornamentals (Backyard Gardener #212)
- University of Arizona Cooperative Extension, Iron Deficiency (Backyard Gardener #232)
