Early autumn has traditionally been the best time of year to plant white ash, usually from a container-grown or ball-and-burlap nursery plant. Young white ashes start out with a somewhat pyramidal shape, and they grow at a moderate rate to eventually be more rounded and quite tall. But although it is still possible to buy white ash trees, planting them is now discouraged by scientists and civic officials seeking to slow the spread of emerald ash borer, which thrives when ash trees are plentiful and in close proximity. If you are still inclined to purchase and plant an ash tree, prepare yourself for the expense of the eventual removal of the tree. In regions not yet affected by emerald ash borer, white ash is a fairly low-maintenance tree. Plan to fertilize annually and prune every few years, as well as prune problem areas such as broken branches as needed. Also, water as needed to maintain at least moderate soil moisture. And be on the lookout for signs of disease, which should be treated promptly. You may also face the expense of chemical treatment to prevent emerald ash borer infestation.
Light
White ash trees grow best in full sun. This means that young trees should be in a spot that gets at least six hours of direct sunlight on most days.
Soil
These trees prefer organically rich loamy soil with good drainage. They are not fussy about soil pH, doing well in acidic, neutral, or alkaline soil.
Water
White ash trees like even moisture in the soil, but don’t want to be soggy due to poor drainage. They can tolerate short dry spells, though prolonged drought can damage a tree. So make sure to water them during stretches without rainfall to maintain soil moisture. Precisely how much and how often to water will vary depending the age of the tree, the composition of the soil, and the climate in your area, but in general, these trees should do well if they receive the standard recommendation for landscape plants—1 inch per week through a combination of rainfall, and irrigation. For young trees (one to two years old) it will be important to follow this prescription—and in sandy soils young trees might need even more water. In soils that are very dense and poorly draining, however, this much water could be too much.
Temperature and Humidity
White ash trees thrive in the range of climates of USDA growing zones 3 to 9. Mature trees will survive temperatures as low as minus 30 degrees Fahrenheit, though young specimens may be vulnerable for the first 10 years or so. In extreme climates, they are best planted in a spot that is protected from strong winds. Humidity is not a concern for white ash trees as long as there is adequate soil drainage.
Fertilizer
White ash trees can benefit from an annual application of an all-purpose tree fertilizer each fall. Apply it according to label instructions under the tree’s canopy, then water it into the soil.
Types of White Ash
Several varieties of white ash trees have been commonly planted in recent years, including:
‘Autumn Applause’: This tree features maroon fall foliage and notably dense branches. ‘Chicago Regal’: This is a fast-growing variety that has purple leaves in autumn.‘Greenspire’: This tree has a narrow, upright growth habit and turns dark orange in the fall.‘Skycole’: This cultivar has a very symmetrical growth habit with a strong central leader, topping out at about 50 feet. It turns orange or red in the fall.
Pruning
Pruning a white ash tree will both help to train the tree’s growth and help established trees remain healthy. Prune in the fall just after leaves start to drop. Young trees should be pruned annually while established trees will need pruning every few years. With larger trees, pruning work is best done by a reputable professional. On young trees, you will likely see several trunks, also known as leaders. Select the strongest one and prune off the rest to ensure healthy upright growth. Plus, on both young and old trees, prune off any thin branches in the fall to improve airflow throughout the tree. And take off damaged, diseased, or dead branches as you spot them. Before disposing of any diseased portions, check with your local cooperative extension office to see whether there are any special procedures for the disposal, which is often the case in areas plagued by emerald ash borer, where storing or transporting ash wood may be restricted.
Propagating White Ash
Although propagation of any kind is now discouraged due to emerald ash borer, white ash trees are relatively easy to propagate by taking green stem cuttings during the growing season and rooting them in a growing medium. This is actually one of the methods being used to experiment with new cultivars that might show resistance to EAB. When a particular mature tree seems to show natural genetic resistance to the insect, controlled propagation of that specimen from stem cuttings by plant scientists may ultimately provide a reproducible cultivar that allows ash trees to once more be a viable landscape option. For the moment, though, there is no reason for homeowners to attempt the propagation of this tree.
How to Grow White Ash From Seed
The seeds found in the winged samsara require a period of cold stratification in order to germinate. In practice, this often occurs simply by seeds that overwinter in the soil, then sprout up as volunteers in the spring. If not for the problems of emerald ash borer, these volunteers can easily be transplanted into new locations—or the seeds can be planted wherever you want a new tree to grow up.
Overwintering
Planted within its recognized hardiness range, white ash requires no special winter protection or preparation, other than making sure the tree is well-watered going into winter. However, late fall after the leaves have fallen is a good time to do any necessary pruning on the tree. And fallen leaves should be cleaned up to prevent fungi from overwintering.
Common Pests & Plant Diseases
The other pests and diseases to which white ash is susceptible are nearly irrelevant, due to the seriousness of a single pest: the emerald ash borer Gardening experts do not currently recommend planting white ash trees due to their vulnerability to EAB. It is an invasive beetle native to Asia that was first spotted in the Midwestern United States in 2002. It has since spread quickly through more than 35 states and parts of Canada, and it’s expected that this single pest may eventually virtually eradicate ash trees from North America. Once the emerald ash borer infests a tree by boring under the bark, it can quickly kill the tree within a few years. Eradication and prevention methods are thus far not very successful. Dead leaves and premature fall leaf coloring are two signs of emerald ash borer infestation. You also might see dead branches around the tree’s crown, as well as new leaf shoots coming out of the trunk or branches. These shoots are a sign the tree is making an effort to produce extra leaves to help in photosynthesis. In addition, you might notice D-shaped holes in the tree, which are a telltale sign of a fully developed beetle boring out of the tree. Other lesions on the tree could be a sign of beetle larvae feeding. If you see any of these signs, consult with a professional arborist to see whether anything can be done to save the tree, but in nearly all instances, the recommendation will be the removal of the tree. In many communities, civic authorities are authorized to identify affected trees and order their removal by homeowners. Although the danger from other pests pales by comparison to ash borer, these trees can also be susceptible to aphids, scale, leaf miners, and webworms—none of which really require much in the way of treatment. Ash trees are also susceptible to various fungal leaf spot diseases, powdery mildew, rust, anthracnose, and ash yellows caused by a bacteria-like organism. Ash yellows causes slow growth and may prompt “witch brooms,” in which the branch tips form small clusters of branches. There is no cure for ash yellows, though affected trees may persist for many years without succumbing.
How to Get White Ash to Bloom
Male and female purple flowers appear on these trees in April and May, before the leaves appear. They are not particularly showy, so there is no real reason you should want the trees to flower. Female flowers are followed by clusters of winged samaras that cling to the tree before drying and falling in the fall. For this reason, female cultivars are quite messy, and landscapers sometimes prefer male cultivars that don’t produce seeds.
Common Problems With White Ash
In addition to the many insect problems and diseases to which white ash is vulnerable (see above), these are relatively brittle trees that are easily damaged by wind. Homeowners should be wary of planting them too near structures and be prepared for regular clean-up after storms and heavy winter snows.
That said, a healthy tree that is treated every other year by a professional can hold off EAB for a considerable length of time—enough time for a replacement tree of a different species to get a good start, for example. But be wary of any business that guarantees they can prevent EAB on any particular tree.
