Growing plants beneath evergreen trees such as spruce and pine can be one of the most challenging tasks faced by gardeners worldwide. First of all, the overhanging branches can create an extremely shady environment for understory plants, depending on the density of the branches and their distance from the ground. Without adequate sunshine, photosynthesis is inhibited and the plants cannot produce enough food to meet growth and maintenance requirements. Most flowering plants require a great deal of sunshine in order to acquire enough energy to drive the flowering process. Some flowering plants may survive under the shade of large trees but the number and quality of flowers will be greatly reduced.
Secondly, understory plants must compete with shallow and wide spreading tree roots for water and nutrients. Contrary to popular belief, tree roots generally don't penetrate great soil depths, but rather reside near the soil surface where moisture and oxygen are most abundant. In fact, most of a tree's absorbing roots can be found within the top 30 cm of the soil. It is plain to see why this would create a problem for understory plants occupying the same soil area. Without proper irrigation and fertilization, these plants cannot compete with aggressive tree roots and often succumb to extremely dry and infertile soil conditions.
Finally, falling evergreen needles may increase the acidity of the soil to levels that many plants cannot tolerate. In some cases, a tree may shed so many needles that the understory plants simply become smothered. Although soil acidity is a contributing factor, inability to grow plants beneath evergreen trees is mainly due to inadequate water and sunshine. Implementing a regimented watering plan and a balanced fertilizer program will improve both the survival rate of understory plants and the overall health of the tree. Water deeply and frequently and use foliar fertilizer instead of soil applied to ensure that the understory plants receive the nutrients. Soil applications tend to supply the aggressive tree roots only.
Understory plant selection begins with choosing shade-tolerant varieties, remembering that the amount of shade beneath a tree is related to both the density of the overhanging branches and their distance from the ground. Most shade plants, unfortunately, require evenly moist, well-drained soil and will not tolerate long periods of dry conditions. Irrigation is essential. On the Canadian prairies, consideration must also be given to winter hardiness. Ample snow cover will increase the survival rate of perennials but loss can be quite severe following winters with extreme lows and little snowfall. The following perennials are recommended for growing beneath evergreen trees in the prairie garden:
In partial shade conditions, bigroot geranium (Geranium macrorrhizum) may be a suitable choice. Lily-of-the-valley, solomon's seal, bergenia, gentian, and bigroot geranium will also tolerate slightly acidic soils. Forget-me-nots (Myosotis sylvatica) are true biennials that perpetuate in the shade garden by self-seeding. They will tolerate almost any light exposure and soil pH, but tend to have high moisture requirements. Johnny-jump-ups (Viola tricolor) are also prolific self-seeders, appearing to be perennials in the prairie garden. They are known for their resiliency and ability to tolerate inhospitable growing conditions. Introduction is often avoided, however, due to the fact that expelled seeds can travel great distances, often landing where they are not welcome. Shade-tolerant annual plants that may make suitable understory plants include impatiens, begonia, browallia, torenia, coleus, lobelia and dusty miller. Growing turf underneath the shade of evergreens is often impossible and may not be the best choice anyway. Turfgrass is an extremely aggressive competitor, often drawing moisture and nutrients away from tree roots. Furthermore, it invites trunk damage from lawncare equipment, which could be detrimental to the health of the tree.
If growing plants beneath evergreen trees has proven entirely impossible, there are alternative ways to make the space more attractive. Consider growing shade tolerant plants in containers underneath the trees, thereby eliminating the elements of competition and soil acidity. However, avoid placing too many, heavy containers on top of the soil as this can increase soil compaction and suffocate tree roots. Placing a few, small container gardens at the base of a tree can enhance the landscape while providing a solution to a common problem area of the garden.
Using decorative mulches to remedy problem areas of the garden has become a common practice in the prairie landscape. Mulches can not only add color and texture to the garden, but also provide many benefits to trees and other landscape plants. They help to stabilize soil temperature, conserve soil moisture, reduce weed competition, reduce soil compaction, and eliminate damage from mowers and weeders. There are many types of mulches and they fall into two basic categories: organic and inorganic.
Commonly used inorganic mulches include gravel, lava rock and decorative stone. Inorganic mulches tend to be low maintenance as they do not blow away and do not decompose over time. These materials, however, tend to reflect a tremendous amount of heat. Such temperature increases can be undesirable in hot, dry areas of the garden and adjacent to homes and buildings. Certain types of gravel and stone can also raise the pH of the soil, thereby contributing to the development of iron chlorosis. When the pH of soil exceeds approximately 6.5, soil iron becomes insoluble and unavailable to the plant. In recent years, spruce and Scots pine have exhibited susceptibility to iron chlorosis in alkaline prairie soils.
Organic mulches such as bark nuggets and wood chips are generally preferred by horticulturists. These materials decompose over time, adding organic matter to the soil and improving both soil fertility and soil structure. Unfortunately, decomposing wood chips can often lead to nitrogen deficiency in the soil. If plants become symptomatic, a simple fertilizer application will remedy the situation. Because organic mulches decompose, they must be replenished every so often. From a landscaping perspective, organic mulches also have the aesthetic benefit of creating a natural, often fragrant, woodland setting. Natural needle drop of evergreens is essentially nature's organic mulch - it's free and the work is done for you!
Using container plants and mulches to enhance the space beneath an evergreen limits the amount of root disturbance inflicted upon the tree. For many trees, approximately 99% of the root system resides within the top meter of the soil and most of the absorbing roots are within the top 30 cm. Digging and cultivating around the bases of trees increases the probability of root damage and subsequent tree decline. Injury and stress also heighten susceptibility to disease and insect invasion. Maintaining as natural a setting as possible will improve the overall health of the tree and increase its value in the landscape.
Relevant web sites:
Armitage A.M. 1989. Herbaceous Perennial Plants. Varsity Press, Inc, Georgia. 646 pages.
Toop, E.W. and Williams, S. 1991. Perennials for the Prairies. University of Alberta Faculty of Extension, Alberta. 188 pages.