Root Competition and Native Plant Vigor
A Grower's Perspective

Richard T. Haard, Ph. D.
Plant Propagation Manager
Fourth Corner Nurseries

Since native plants grow all around us unaided, it is easy to entertain the misconception that they do not require any care. This is only the case for established plants in their preferred habitat. An established plant has a root system that is fully integrated into the surrounding soil and is capable of supporting the top growth. It may take anywhere from one to four years with proper maintenance for a transplant to achieve this condition. Native species, just as horticultural and agricultural species, do have cultural needs and do experience transplant stress.

How can we minimize transplant stress and achieve establishment and healthy growth? First of all, what kind of growth should you expect from tree and shrub species? The following table shows a reasonable growth rate for some species receiving minimal care. This study indicates that one way to minimize transplant stress and achieve healthy growth is by eliminating root competition.

This is a comparison of native plant growth in bare soil compared to growth at sites with competing vegetation (pasture grass). This data is anecdotal and based on our experiences at Fourth Corner Nurseries, versus our monitored revegetation plantings over the past 10 years, particularly in grassy meadows. All plants began as 12", 24", or 36" bare root seedlings, transplanted into nutrient-rich, moist sites with full sun. The bare soil at the nursery field was maintained with a standard mix of herbicide and hand cultivation. No fertilizer or irrigation was used on either site. The observations are all taken in September of the fourth growing season after planting. Stem caliper is at 6" from soil line.

In comparing transplanting native plants into a nursery field with bare soil to transplanting into a grassy meadow we see that some of the species in the meadow accomplish modest growth while others appear stunted. Plants free from root competition grew more vigorously, even without fertilizer and irrigation (on moist rich soil). This is a repeatable pattern. The cause of this meadow-stunting effect is subject to interpretation. There are some who say that since red cedar and vine maple are shade-tolerant understory species, the sun inhibits their growth. The answer to this meadow-stunting question is not in the sky but in the soil: the theory does not hold up when considering the same species are commonly grown in full sun by nursery growers.

Healthy roots lead to a healthy plant. Farmers and gardeners have found over the centuries that plants grow better when their roots are not competing with other roots for space, water, nutrients, and oxygen. Weeds are pulled and over-seeded crops are thinned so each plant has room to develop optimally. When we plant shrub seed at our nursery field, it is the seedling density that determines size and survival at the end of a single growing season. For example, a planting of Holodiscus discolor, Ocean Spray:

It appears that a unit of soil volume will support a certain amount of biomass no matter how many individuals are present. This 'biomass constant,' dependent on soil type, structure, and microorganisms, is a useful concept for growers and restorationists. When this 'biomass constant' concept is applied to a mixed grass/shrub population, the meadow-stunting effect is explained by root competition. The soil cannot support root growth of a transplanted shrub if it is already 'full' of grass roots. Native plant ecology bears witness to this. Red alder and big leaf maple are primary succession species that normally grow onto bare mineral soils. Natural colonization by these plants into meadow sod is a rare event. Published studies have implicated drought stress as the primary cause of poor plant performance. Competition in the root zone exacerbates drought stress. Regular irrigation may help keep a plant alive, and indeed is crucial in many cases, but plant establishment and independence from irrigation requires a reduction in root competition for most species. We find that reducing competition for space, nutrients, water, and oxygen are the critical factors in transplant establishment.