Tree and shrub collections make up a significant portion of the Minnesota Landscape Arboretum. The planting of collections for preservation purposes began as soon as the Arboretum was founded in 1958, and a number of plants went in the ground even before the Arboretum was dedicated. The most extensive plantings occurred from 1957 to 1965. In 2012, the pine and ornamental grass collections were officially included in the North American Plant Collections Consortium, a program administered by the American Public Gardens Association.
The arborvitae (Thuja sp.) collection consists of 1 species and 51 cultivars with a total of 64 specimens. The collection is located across the pond from the iris collection just southwest of Green Heron Pond on Three-Mile Drive. When viewing the collection please note that the unusual pruning (all branches removed to the height of a deer's reach) is due to our resident deer population. They do best in sun and partial shade but will take full shade as well. They prune easily and can be kept at most any height and shape.
The ash (Fraxinus sp.) collection is located across Three-Mile Drive from the Crabapple Collection and adjacent to the hawthorns. (The ‘old' ash collection is located to the left of the gatehouse). There are 7 species and 26 cultivars and a total of 86 specimens in the collection. Ash have a large number of cultivars (hybrid trees) and some have very good fall color and form. Ash is one of our good native shade trees and withstands city conditions. Green ash (Fraxinus pennsylvanica) varieties are more hardy in all situations but white ash (Fraxinus americana) are cleaner trees with maroonish red fall color.
The Birch collection (Betula sp.) is one of our most often changed collections. All birch species have a relatively short life span in the managed landscape of somewhere around 50 years. However, they are one of the most beloved genera of trees. The popular white and gray birch with their white bark are spectacular in the landscape, whether wild, native, or managed and they look great in all seasons. All birch have a beautiful yellow fall color and are easily spotted in the landscape.
The birch genus (Betula) however seems to attract diseases and pests, including the bronze birch borer which kills the tree. The most resistant species to plant is the River Birch (Betula nigra). There are 16 species and cultivars and 65 specimens in the old collection (by the Rhododendron garden) and 44 species in the new collection (by the Frerichs Garden for Wildlife).
No buckeyes are native to Minnesota, however, many grow well in our climate. This genera of trees located just west of the Ornamental Grass Collection has 8 species and 2 cultivars and a total of 23 specimens. They range in size from large shade trees to medium sized shade trees and can make a handsome addition to the home landscape.
One of the showiest of flowering trees in June are the Catalpas. In Victorian times they were all the rage with the large, pillowy white, orchid-like flowers set on candelabras above the bold, tropical-esque foliage. In post World War II years, they fell out of favor, considered too messy for the immaculate lawns that were common (the flowers, large leaves, and bean-like seed pods drop through the year).
There are two American species of Catalpa that are commonly planted in our region: Northern Catalpa (Catalpa speciosa) and Southern Catalpa (Catalpa bignonioides). Northern Catalpa is more common. Southern Catalpa was originally found along rivers from Virginia to Texas – and is surprisingly fully hardy there. Both have since naturalized well to the north.
Northern Catalpa blooms first with larger flowers with fewer in a cluster; the Southern Catalpa begins to bloom as the Northern fades and has smaller flowers with more of them in a cluster. Both are great for bumblebees! Northern Catalpa grow into a very large, upright shade tree, readily reaching more than 50 feet in height, while the Southern Catalpa is usually around 35 feet tall, rarely getting to 50 feet at maturity. The bean-like seed pods are also longer on the Northern Catalpa. Hybrids can occur – some can be seen at the Arboretum.
Look for the Arboretum’s catalpa collection between the Dahlia parking lot (opposite the Chinese Garden) and the Grass Collection.
The Arboretum's largest collection consists of 16 species and 120 cultivars and a total of 268 specimens of crabapples (Malus sp.). All apples are in the rose family along with peaches, pears, cherries, apricots, and around 100 other genera. There are some 3000 species and cultivars including roses! The rose family is susceptible to many diseases and insects. This generally means heavy spraying, shabby looking trees or hunting for cultivars which are least susceptible to major diseases.
The spectacular bloom, generally sometime around Mother's Day brings thousands of visitors to see the pinks, whites and reds of the many trees. It is best to jot down the names of those you like, then return in mid-summer to see if their leaves are still intact and not disease or pest prone. Because most of the crabapples have decorative fruit (there are some fruitless varieties) you should return in the fall to check fruit color, size and density. Finally, a visit in winter will give you a seasonal view which you will see five months of the year.
Crabapples come in all shapes and sizes, including spreading, weeping, dwarf, vase-shaped and columnar.
Our wonderful midwestern and eastern street tree, the American elm, (Ulmus americana) has been nearly exterminated because of Dutch Elm disease. Though there are several elms that are less susceptible to the disease, Siberian elm, (Ulmus pumila), and the closely related hackberry (Celtis occidentalis) there has yet to be a disease resistant elm with the beautiful arching gracefulness of the American elm. We continue to add cultivars to the collection which are supposedly resistant. To date we have not lost any of the "resistant" cultivars. ‘Accolade' is one of the better hybrids. I sometimes call the elm collection the ‘stump collection'!
Located across from the crabapples, the Hawthorns, or Thornes, (Crataegus sp.) have 17 species and 9 cultivars and a total of 32 specimens. Hawthorns are in the rose family and therefore subject to many of the same diseases and pests as crabapple. However, they have wonderful twiggy, spreading or upright forms, small clusters of tiny white or pink flowers and fruit that attracts birds.
The Larch Collection (Larix spp.) contains 5 species and 1 cultivar and a total of 27 specimens. This northern deciduous conifer (cone bearing but loses its leaves) includes the native Tamarack (Larix laricina) common in Northern Minnesota bogs and swamps. It is easily spotted in the spring when its light green needles arrive and in the fall with its striking yellow fall color. In winter it may look like a dead spruce from a distance but en-masse it provides a cypress–like grove and in summer it is easy to distinguish from spruce. Definitely an underused tree as it is adaptable to many conditions and is perfectly hardy.
Lindens (Tilia sp.) are planted world-wide as street and shade trees. Often called ‘Lime Trees' in Europe, (both ‘Linden' and ‘Lime' are derived from the Germanic root ‘lind'), they are generally slightly or very conical in form and many hybrids exist which have been bred or selected for specific characteristics. The linden's form makes for a very uniform, well ordered boulevard. The American linden (Tilia americana) or basswood has many upright cultivars and is an excellent yard tree without quite the formality of many of the European and other hybrids. The lindens are a major Landscape Arboretum collection with 10 species and 22 cultivars. There are 46 specimens in the collection.
There are several kinds of locust and they can be wonderful trees for light shade but some can be problematic. The two main genera are honey locust and its many cultivars (Gleditsia sp.) and black locust and its cultivars (Robinia sp.). Both types send up suckers from roots near the surface of the ground but both are very tolerant of city conditions.
The magnolia (Magnolia sp.) is a wonderful genera with many species and cultivars. Of 125 species only a few are hardy in Minnesota. Several of these are cultivars of Magnolia x loebneri, a hybrid of the star magnolia (Magnolia stellata) and Kobus magnolia (Magnolia kobus). The magnolia flowers are fragrant and beautiful and it is no wonder that gardeners wish to have them in their gardens. This collection is near the end of Three-Mile Drive just as one emerges from the woods but there are some in the Shade Tree Exhibit, the Home Demonstration Gardens and elsewhere. We have 17 species and cultivars and a total of 19 specimens in our collection.
Maple is another world-wide genus with many species. Among those that grow native to Minnesota are our best trees for fall color, sugar maple (Acer sacharum), red maple (Acer rubrum) and their cultivars. Three Arboretum introductions, Northwoods Red Maple (A. rubrum ‘Northwoods'), Autumn Spire red maple (A. rubrum ‘Autumn Spire') and FirefallTM Freeman maple are excellent shade trees and street trees. They all have good form and good fall color.
Oak and Nut Collection
The Minnesota Landscape Arboretum has 3 species of hickory and 8 cultivars (Carya spp.) and 8 species and 10 cultivars of walnuts and butternuts (Juglans sp.). There are 35 specimens in the collection. Many of the nut trees are native but because of their tap root system they are impossible to transplant except as seedlings. Black walnuts, which are handsome trees, give off the compound Juglone from their roots which allow very few plants to grow beneath them. Therefore, it is not the best tree for the home landscape.
The Landscape Arboretum has a sizable oak collection with 15 species and 2 cultivars and 52 specimens. Oaks, as a rule, are one of our longest lived trees, some reaching over 500 years old. Of the many species world-wide, relatively few will grow in Minnesota, but those that do are exceptionally beautiful. The fruit, (acorns), are great attractions for squirrels and other animals and so are good for natural areas but could be considered messy in the home landscape. They can be difficult trees for the boulevard as they like plenty of room for their roots.
Pines (Pinus sp.) create the Arboretum's second largest woody collection with 200 specimens. We have a small selection of the species that grow world-wide. Our natives, red pine (Pinus resinosa) which is our state tree, and white pine (Pinus strobus) which is one of the most beautiful specimen trees that grow in Minnesota. Pines, even with a variety of disease and insect problems are some of our most versatile evergreens and can be kept at hedge height or in other forms. The many weeping forms of P. strobus are extremely exotic and can be seen in the weeping tree collection. The Scot's pine allee (P. sylvestris) creates the backdrop for the Maze Garden and the shelter for much of our azalea breeding program.
The poplars (Populus sp.) include the giant cottonwood, our tallest native tree in Minnesota, and the lovely aspens of northern Minnesota. They are all fast growing, but all are relatively short lived, weak wooded and tend to lose a lot of branches during storms. However, there is nothing quite like a clump of aspen with their brilliant yellow fall color to portray the essence of fall. We have 24 species in this collection.
The Prunus Collection includes cherries, apricots and plums. The better known Prunus are generally the Japanese plums and cherries, which are not hardy in Minnesota. The apricots, Moongold and Sungold, are two introductions from the Horticultural Research Center are both necessary for fruit, but they bear sporadically at best. The flowers are a beautiful shade of pink. Another introduction, Prunus ‘Princess Kay,’ is a spectacular double flowering selection of Canada plum and certainly worth being in the garden, but it is short lived.
Small Tree Collection
Not a generic collection but one based on the fact that the trees are all of relatively small stature. Most home landscapes have the need for small trees as well as shade trees. Some of the best would include serviceberry (Amelanchier sp.) and Japanese tree lilac (Syringa reticulata). A common small tree is the Amur maple (Acer ginnala). It is very popular because of its rounded form, ease to prune, disease resistance and beautiful fall color. There is one drawback and that is its massive production of seed and its ensuing invasiveness in many areas. Use with caution! All the crabapples and hawthorns are generally small stature trees as well.
The Serviceberry Collection is hidden at the bottom of the hill below the Maple Collection and the Shrub Walk. Serviceberry (Amelanchier sp.) are found extensively throughout the Arboretum but probably less so in the average landscape. The most common species grown are small trees which are understory trees in their native habitat. This collections has 8 species and 15 cultivars.
The Spruce Collection adjacent to the small trees and across Three-Mile Drive from the crabapples contain 124 specimens and 21 species of spruce. Spruce (Picea sp.) are distinguished from fir (Abies sp.) by their short, stiff needles. Spruce are evergreen, have conical form and for the most part are large trees. Along with arborvitae they are probably the most common evergreen planted in Minnesota. Some can be easily pruned into hedges. They are not easy to handle due to their spiky needles but make excellent barriers. They are also quite stunning in the winter landscape.
Weeping Tree Collection
Some Arboretum collections are not generic but are planted by characteristic. The weeping tree collection consists of varieties or cultivars of 16 genera and 15 species and a total of 25 specimens. Some of the more dramatic weeping trees are the white pines (mentioned under pine collection), Red Jade weeping crabapple (Malus ‘Red Jade') and Uncle Fogy weeping pine (Pinus banksiana ‘Uncle Fogy") an Arboretum introduction. Most common of all weeping trees are the weeping willows.
The willow collection (Salix sp.) has 7 species, 10 cultivars and a total of 47 specimens. Willows are shallow rooted trees and shrubs and the best known landscape plants are weeping willow trees (generally Salix alba ‘Tristis') and the pussy willow shrub (generally Salix discolor). Willows are most useful in the landscape near water bodies and streams where they have plenty of freedom to grow and their graceful forms displayed. Their roots can clog drain pipes in a matter of months if planted around the house!