Ask the Garden: Your Tree Questions

The Missouri Botanical Garden is celebrating its colorful fall foliage with Tree Week from October 29 to November 4.  The week will highlight the Garden’s impressive tree collection, and the staff who care for it, with tours and other opportunities for guests to learn more about it.

As part of that focus, we’d also like to answer some of your questions about trees in your yard. We asked visitors to submit their tree questions via social media, and our Kemper Center for Home Gardening provided the following answers.

 

What are the best evergreen trees for St. Louis climate/soil?

-Ann E.

Evergreens can be tricky for the St. Louis area.  The combination of poorly drained, slightly alkaline soil and hot, humid summers can make it difficult for most evergreens to reach their full potential.  However, there are a few which the staff here at the Missouri Botanical Garden have found to be successful in our area. Some examples include eastern red cedar, Norway spruce, arborvitae, yew, Japanese plum yew, and southern magnolia.

Of course, keep in mind that the more you can do to create the preferred environment for a tree, the higher your chances of success. This could mean amending the pH or texture of your soil, which is a small price to pay compared to losing an entire tree.

There are other evergreens that may do well in our area but are less likely to do so. These include Colorado spruce and eastern white pine. Although it’s possible to grow these trees successfully, we recommend speaking to nursery professionals in your area to find out which do best.

See these resources for more information about evergreens in our area:

Best Conifers (Evergreens) for St. Louis Gardens

Selecting Landscape Plants: Needled Evergreens

Broadleaf Evergreens for St. Louis Gardens

 

Eastern redbud, Cercis canadensis 'Little Woody'
Eastern redbud, Cercis canadensis ‘Little Woody’ | Photo by Tom Incrocci

What flowering trees do well in our area and which prefer sun and which prefer shade?  

-Susan T.

Different trees have different light requirements and tolerance levels.  Most flowering trees need some sun to flower. Flowering trees which do well in our area and tolerate some shade include serviceberry, eastern redbud, fringe tree, kousa dogwood, flowering dogwood, silverbell tree, and Japanese snowbell. There are also cultivars available for most of these species.

Some flowering trees do best only in full sun. Examples include yellowwood, most hawthorns, tulip tree, ornamental crabapples, most ornamental pears, and most ornamental cherries.

 

How can I best trim Elderberry bushes and control their shoots so the birds and I can still enjoy their benefits?  What are their best alternative smaller trees and bushes to plant?

-June M.

A newly planted elderberry bush can be pruned after the first three years of growth. Before this time, the plant should be left to grow vigorously and establish fully in the garden. After this, you can begin selectively pruning older canes. Elderberries only produce flowers at the tips of new growth. One and two-year-old canes produce the most flowers and fruit. Canes older than this should be cut back. New branches will continue to develop from the base of the shrub, as well as laterally from older canes. With careful pruning, you can keep your elderberry fruitful and productive for many years.

Alternatives to elderberry that have similar wildlife and foraging benefits include black chokeberry, purple-flowering raspberry, and highbush blueberry.

 

What can/should be done about oak tree galls? Is this a sign the tree is dying?

-Ann W.

Galls are difficult to stop, but trees rarely need treatment. Galls on oaks rarely cause more than cosmetic injury to the plant. They can look very unsightly, but do not affect the long-term health of the tree. The one exception is the “gouty oak twig gall.” This gall causes round, swollen growths to form on the twigs, and a large infestation can result in branch dieback and even tree death.   

Using chemical insecticides to treat gouty oak twig gall is not recommended. There is only a small window of time in the spring when the wasps are active and achieving adequate coverage for a large tree while minimizing spray drift is difficult. The best course of action is mechanical control. When the galls are small and are just developing on twigs and branches, where possible, prune and destroy the infested plant material. Rake and destroy all infested leaves. Maintain the health of the tree by watering during dry periods and fertilizing if needed.

For more information on gouty oak twig galls and galls in general, see these resources:

Gouty, Horned, and other Twig Galls

FAQ: What are these growths or gall on my oak tree?

 

Knolls_Incrocci09.JPG
Eastern white pine, Pinus strobus | Photo by Tom Incrocci

 

My 20+ year old white pines have been dying over the past 10 years. Another suddenly died this summer and I see other large dead ones in neighboring yards. Do you know why?

-Gloria R.

Eastern white pines are a wonderful evergreen tree native to the Northeast and northern Midwest, as well as the Appalachian Mountains. Unfortunately, these trees can struggle in our area. The combination of clay-rich, alkaline soils, hot, humid summers, and urban pollutants mean that these trees rarely reach their maximum potential. The stressful environment can cause them to enter a state of slow decline. Lightening and drooping of the needles, as well as disfigured bark and oozing sap are symptoms of white pine decline. Although there is no contagious aspect to white pine decline, it will result in the eventual death of the tree.

See these resources on eastern white pine problems in the St. Louis area:

White Pine Decline

Eastern White Pine Problems

 

How can I prevent my sweet gum trees from producing gumballs?

-Dory L.

The “gumballs” produced by sweet gum trees are one of the few downsides to this large, Missouri native shade tree. These trees are adaptable to many soil conditions, and produce reliable fall color in shades of yellow, orange, red, and purple.  The gum balls are, however, quite a serious problem. The hard, bristly fruit clusters are slow to decompose and can create slip hazards on sidewalks and driveways. It is best to never plant one of these trees near a pedestrian area.  

If you want to reduce the number of gum balls produced by your tree there are two chemical methods: foliar spray and injectable treatments. The chemicals used in these treatments are quite strong and can cause surrounding plants to not set fruit. Keep in mind, this may be the desired outcome for your sweet gum, but perhaps not for your neighbor’s crabapple. There is also a very short window of time when the treatments can be applied.  For these reasons, both methods are best left to professional arborists.

Another way to greatly reduce the number of gum balls in your yard is to replace your sweet gum tree with the cultivar ‘Rotudiloba.’ This cultivar has similar characteristics to the species plant but has, as the name suggests, rounded leaf lobes instead of pointed. It also does not produce the infamous gum balls. There is some question as to the hardiness of this cultivar in the St. Louis area, and some branches of the tree may revert and produce fruit.

 

Still have questions about trees?

Email the plant doctor at plantinformation@mobot.org. You can also call our horticulture answering service at (314) 577-5143 from 9 a.m. to noon Monday through Friday.

 

Catherine Martin
Public Information Officer

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