U of M Student
Arbor Day is celebrated Saturday and May is Arbor Month in Minnesota. It is a great time to consider planting a tree. In 2009, Emerald Ash Borer was discovered in St. Paul, emphasizing the need to find alternatives to ash and diversify the trees planted in our cities. People tend to select trees they are familiar with, but there are more choices than you might think. Here are five trees to consider.
• Hackberry, celtis occidentalis — Hackberry has become a common replacement for ash and elm in many cities in Minnesota. However they are rare at best in International Falls. It is very adaptable to adverse soil conditions found in urban environments, making it a good choice as a street tree. It grows into a medium size tree reaching approximately 40-50’ in height and spreading 30-40’ wide. Trees are pyramidal to oval when young, becoming vase shaped with age, but can vary with growing conditions. The bark is gray and develops interesting warty ridges.
Leaves are a medium green and yellow to greenish yellow in the fall, and the tree produces a small, round dark purple fruit. The bark and form of older trees are quite attractive in the landscape. The trees natural range extends into portions of northern Minnesota. While hackberry is very hardy, planting trees grown from a northern seed source is recommended. Delta and Prairie Harvest hackberry are two seed strains developed for hardiness.
• Bur Oak, quercus macrocarpa — Bur oak is the most adaptable and perhaps the hardiest oak that we can grow, yet it remains an underused tree. This is partly due to it being a tap-rooted species, and slow to establish after transplanting. Some sources actually recommend directly seeding acorns in the fall where you want trees to grow. Nursery techniques such as root pruning and air pruning in special containers can also help overcome the tap-root issue.
While bur oak has a reputation as being a slow grower, once trees are established growth rates are respectable. Bur oak achieves its best growth on better soil such as along river valleys. However, it can grow in tough sites where it maintains a scrubby growth habit. Bur oak can develop into a large tree 50-60’ tall (or more) with a similar spread, but may never reach that size on poor soils. Bur oak has recognizable fiddle shaped leaves with rounded lobes and yellow to brown fall color. The fruit is an acorn with a fringed cup. The bark is gray-brown, developing ridges and deep furrows with age. Bur oak has a bold, broad branching structure. The impressive silhouette is very picturesque in the winter. Bur oak is also long lived with some trees living 400 years or more.
• Ohio Buckeye, esculus glabra — Ohio buckeye is a medium sized tree 20-40’ tall by 20-40’ wide. It is not native to Minnesota, but the native range extends as close as southern Iowa. This tree has proved itself remarkably hardy in Minnesota. If you want to see one growing in International Falls, there is a nice specimen on the north side of Smokey Bear Park. The leaves are palmately compound with 5-7 leaflets. It has somewhat of a tropical or exotic look to it. Fall color can range from yellow to bright red. Trees are susceptible to leaf scorch and early defoliation, especially when planted in less than ideal conditions.
This is not a tree for droughty soils and is best planted in moist soil conditions. In nature Ohio buckeye is frequently found along river bottoms and stream banks. While this tree has it’s limitations, if you can provide the right site conditions it is a handsome tree that can add diversity to our palette of plants to choose from. There are a few cultivars that have been selected based on foliage quality and fall color. ‘Autumn Splendor’, ‘Sunset’, and ‘Prairie Torch’ are three cultivars to try.
• Discovery Elm, ulmus davidiana var. japonica ‘Discovery’ — An American elm was at one time a favorite street tree in American cities and a common tree in our forests. In the 1960s the arrival of Dutch elm disease (DED) in Minnesota, destroyed large populations of elms. Recently, with the introduction and evaluation of DED resistant elms, there has been renewed interest in planting elms in the landscape.
One such tree is ‘Discovery’ elm, a selection of Japanese elm that has excellent resistance to Dutch elm disease. In addition to being DED resistant, it is very hardy, has excellent foliage quality, good branching, and a vase-shaped form. It grows into a medium sized tree 40’ tall by 30’ wide. DED resistant elms deserve a place in our landscapes, but we should learn from the lessons of the past, and not line our streets with all the same species or a single cultivar. Diversity is key to having a healthy urban forest.
• Japanese tree lilac, syringa reticulata — This is another tree that has proven itself remarkably winter hardy. Japanese tree lilac maintains a tree like form and matures into an attractive small tree 20-25’ tall by 15-20’ wide. It can be grown in a multi stemmed clump or a single stem form. It has dark reddish-brown bark and creamy white flowers in June, well after other lilacs have bloomed. Cinnamon-brown upright seed pods are attractive into the winter.
Japanese tree lilac is very adaptable to different soil types, and is very tolerant of de-icing salt, making it a good street tree, especially where a small tree is required. Because it remains a small tree it is ideal for planting under or near power lines, or in small residential yards where space is limited. In International Falls, there are some planted along main street, but it deserves much wider use. The plain species can be planted as well as several cultivars including ivory silk, snow cap, golden eclipse and snow dance.
Hogan graduated from Falls High School Class of 1998, has a bachelor degree in forest resources from the University of Minnesota and is currently finishing the master of agriculture in the Horticulture Program at the University of Minnesota.
