Amur maple, Acer ginnala, has been a top pick for Minnesota landscapes. With hardiness, adaptability, fast growth, and radiant fall color, tens of thousands have been planted. However, recently some people have raised concerns about Amur maple becoming a "weed" tree. They're seeing seedlings popping up along roadsides and even in sidewalk cracks. The fear is that like buckthorn, a notoriously invasive shrub from Europe, Amur maple is displacing native species.
This is unfortunate, as Amur maple has been a valuable asset for hedges, windbreaks and as a small yard tree. On the brighter side, a number of small maples remain which do not appear to be invasive. They will be discussed in this Brief.
Visitors to the Minnesota Landscape Arboretum can see some outstanding small maple specimens in their maple collection. The trees are labeled so you'll be able to envision what they'll look like as they grow and mature in your landscape.
Three Flowered Maple
Three Flowered Maple, Acer triflorum, is an outstanding small tree. This tree can grow twenty feet tall and fifteen feet wide. When blooming, you will see three flowers in a cluster. Its leaves are also in clusters of three, a leaf formation which sets it apart from most other maples commonly seen in this area. However, it does have the same great fall color that many maples share. This color can vary from year to year, but a rich fusion of orange red and yellow is standard. The bark is also outstanding, with tan and brown flaking strips.
A few garden centers have begun selling these trees in recent years, and they are now finding their way into landscapes. Three flowered maples are best suited to planting singly as specimen trees or in small groupings. Site them in full sun where their attractive bark and fall color will provide seasonal interest in the landscape.
Purple Blow Maple
Purple blow maple, Acer truncatum, is a well-kept secret. This tree has an air of quality and distinction that is difficult to put into words. Its habit is spreading, twenty feet tall and about as wide. Both summer and autumn foliage are extremely attractive. The leaves are almost star-shaped; red-purple when emerging and a glossy green throughout the summer. Fall color can be a wonderful fusion of yellow, orange and red. Like three flowered maple, this choice tree is best planted as a single specimen or in a small grouping.
Photo credit: Nancy Rose
Mountain maple, Acer spicatum, is a nice, wide spreading small tree suitable for wildflower gardens and specimen use. Plants typically range from ten to twenty feet in height. In the Twin Cities, you can see mountain maples at the Eloise Butler Wildflower garden in Theodore Wirth Park. Perhaps the best place to see them, however, is along Lake Superior. There they grow among the balsam fir and white pine, providing spots of red, orange and gold during the fall.
Mountain maples can be hard to find commercially, but a few retailers now sell them locally. Plants seem adaptable to a variety of soil conditions, thriving in the naturally acidic soil along the North shore but also doing just fine elsewhere in more typically neutral soil conditions. Mountain maples will grow in shadier sites, however they exhibit the best fall color when planted in full sun.
Moosewood or striped maple, Acer pensylvanicum, is another well-kept secret. Moose love to eat it, hence the common name – moosewood. Its other name refers to the white stripes that run vertically along the stems. Some people think the bark looks almost snake-like. This lends it yet another name – snake bark maple. In fact, there is an entire group of related small maples with this feature that are known collectively as snake barks.
Of this group, we're most familiar with Acer pensylvanicum because it is native to eastern North America. This plant is an understory species, found in moist woodlands. You can expect it to grow fifteen or twenty feet tall. In the Twin Cities, you can see some nice specimens at the Eloise Butler wildflower garden. In the landscape you'll need to mimic these conditions for your moosewood to succeed.
Plant moosewood in a spot that stays moist, but not wet throughout the growing season. In other words, avoid both droughty locations and spots with standing water. An eastern exposure with morning sun and afternoon shade would provide enough light to foster good growth, but offer some relief from afternoon heat. If the trunk is exposed on the south side in winter, wrap it each fall to prevent sunscald, until the bark has grown thicker. Be sure to unwrap the trunk each spring.
Unfortunately, horticulturists and gardeners have yet to find a Japanese maple, Acer palmatum, durable enough for southern Minnesota's zone four winters. Garden centers do sell these seductively beautiful plants and occasionally one will succeed in a very protected site. But among the hundreds of cultivars, none is known to be fully hardy.
Our best alternative may be Korean maple, Acer pseudosieboldianum. This plant looks very similar to a green-leafed Japanese maple. It can grow fifteen feet tall and ten feet or more wide, and some reports claim it is hardy down to forty degrees below zero. This is probably a stretch, though, and the plant needs more trial before it can be recommended whole-heartedly. However, it's a great choice for the adventurous gardener who wants to try something unusual. Avoid sites where it will be exposed to afternoon sun.
Caring for Small Maples
All these trees will benefit from annual fertilization. A slow release, granular fertilizer with equal parts nitrogen, phosphorous and potassium should be applied according to label instructions around the root zone in spring. Maintain adequate moisture throughout the season by watering whenever natural rainfall is inadequate – especially when the tree is young and not yet well-established.
Apply and maintain a large circle of wood chip or shredded bark mulch around each tree at planting time to help hold moisture and keep roots insulated from heat in summer and extreme cold in winter. Mulch also protects tree trunks from accidental weed whip and lawnmower encounters. Keep the mulch an inch or two away from the actual trunk, though, to maintain good air circulation around the bark.