RENEGADE GARDENERâ„¢

The lone voice of horticultural reason

Unsuitable trees for Zone 4

From those frigid, friendly foresters at the University of Minnesota’s School of Horticultural Sciences, here’s a new list of trees available from nurseries throughout Zone 4 that are proving to be less than adequate for the task at hand. Avoid the following:

Catalpa
Northern catalpa [C. speciosa] Hardiness limitations. Consider it Zone 5.

Corktree
Amur corktree [Phellodendron amurense] Hardiness limitations. Consider it Zone 5.

Crabapple
Siberian crabapple [Malus baccata Jackii] A magnet for rabbits. Select only if providing complete winter protection from our furry, hasenpfeffer-inspiring friends.

Elm
Lincoln elm [Ulmus pumila x U. rubra ‘Lincoln’] Hybrid selection from a cross between Siberian elm and red elm that has apparent resistance to Dutch Elm Disease (DED) but has not been widely tested. Subject to severe defoliation by the elm leaf beetle.

Red (Slippery) elm [U. rubra] Species native to Zone 4, but susceptible to DED.

Regal elm [U. carpinifolia ‘Regal’] A DED resistant elm that is subject to severe defoliation by the elm leaf beetle. Use should be restricted to the southernmost edges of Zone 4

Rock elm [U. thomasii] Native to Zone 4, but susceptible to DED.

Sapporo Autumn Gold elm [U. carpinifolia ‘Sapporo Autumn Gold’] A DED resistant elm that is subject to severe defoliation by the elm leaf beetle.

Siberian elm [U. pumila] Widely planted as shelterbelt or windbreak tree that is resistant to DED but is subject to winter injury and defoliation by the elm leaf beetle. This tree is not suitable for urban plantings.

Hawthorn
Downy hawthorn [Crataegus mollis] Very susceptible to rust and fire blight.

Washington hawthorn [C. phaenopyrum] Very susceptible to fire blight. Markedly less susceptible to rust than many other hawthorns. Cold hardiness limits the use of this species to the southernmost portions of Zone 4.

Note: Hawthorns may need winter protection from rabbits and are sensitive to deicing salt spray and run-off.

Honeylocust
Thornless honeylocust [Gleditsia triacanthos var. inermis] Zone 4 hardiness and Nectria cankers are concerns. Cultivars include ‘Halka’, Imperial, ‘Moraine’, ‘Shademaster’, Skyline, Summer Lace, and Sunburst.

Magnolia
Cucumbertree magnolia [Magnolia acuminata] Plant in protected locations, winter hardiness may be an issue.

Acer x fremanii (Freeman Maple)

Maple [Acer x fremanii] Existing cultivars have performed poorly due to an apparent lack of hardiness. Freeman maples should be used with caution in Zone 4.

Norway maple [A. platanoides] Winter hardiness and susceptibility to Eutypella canker limit the usefulness of this species and its cultivars in Zone 4.

Note: Maples other than Norway maple are sensitive to deicing salt. Maples perform better in lawn or park settings and in groups, rather than as single specimens in fully exposed boulevards. Planting with the root collar at ground line is important to see and treat girdling root syndrome.

Oak
Eastern pin oak [Quercus palustris] Intolerance to alkaline soils and marginal winter hardiness limit the usefulness of eastern pin oak within much of Zone 4.

Plum
Canadian plum [Prunus nigra] A small tree with white flowers in the early spring. ‘Princess Kay’ is a double-flowered cultivar that was collected from Itasca County and introduced by the University of Minnesota Landscape Arboretum. This species is very susceptible to black knot. Needs winter protection from rabbits.

Poplar
Lombardy poplar [Populus nigra ‘Italica’] Lombardy poplar is an upright, fast-growing cultivar that is often used in windbreak and shelterbelt plantings. It is extremely susceptible to several canker diseases and as a consequence is short lived.
All poplars have shallow and aggressive roots. Do not plant near pavement where shallow roots can cause damage.

Class dismissed!

Don Engebretson
The Renegade Gardener