Farm Tile Drains and Tree Roots
Trees can grow on agricultural land in many useful and profitable ways. These include: windbreaks, treed fence rows, fruit and nut orchards, stream buffers, shelter for pastures, shelter for farm buildings, plantations, woodlots and reforested marginal land. Cropped farm fields and pastures are often under-drained with tile to remove excess water to enhance crop production. Some species of trees can aggressively plug farm drain tiles with roots while other tree species rarely plug drains. If conditions are favorable, it is believed that any tree species has the ability to plug farm drains with roots. Where crop production is to continue, tree planting projects on drained land must be designed to sustain the use of tile drains and not interfere with removal of excess field water.
Figure 1. The section of plugged tile must be located and cut out from the system. These dense clods of fibrous roots from a nearby tree entered through perforations in the tile and blocked water flow. The car keys help show the pipe diameter and size of the root balls. These clods can break free and plug the line further on down stream.
Sustaining functional farm drains
A properly designed drainage system should quickly remove water from farm soil whenever an excess amount of water is present. Excess soil water can occur any time of year, however, is typically most abundant from late winter to early spring, late summer to early winter and sporadically during the growing season due to heavy or prolonged rains. The need for field drainage can vary from season to season. Field tiles that have remained fairly dry for several years may run water frequently or constantly during wet seasons if water tables become high. Tiles that drain natural springs will always run water. During most seasons, farmers realize production advantages by having field drains in place.
Drain lines blocked by tree roots will disrupt proper water drainage. How can this problem affect crop production? Winter melt water and spring rains may not drain adequately to allow early tilling and seeding especially on heavy soils. Wet soils are cooler and can delay crop germination and growth, reducing crop yields. Sporadic summer flooding can remain pooled too long in low spots, resulting in crop damage. In late summer and fall, soil may be too soft to permit heavy harvest equipment onto the land. It costs time and money to locate the plug and replace it with new pipe. Farmers depend on a functional drainage system to remove excess field water.
Farm drainage is a production investment that can cost $700 /acre to install. Tree plantings added to farmland can also be designed as production investments. In order to establish trees and shrubs on farmland in beneficial ways without risk of plugging drain tiles with roots, recommendations on tile drain installation are outlined in OMAFRA Publication 29, Drainage Guide for Ontario (see below). These recommendations have been established through field experience and are recognized by drainage contractors. If questions on tile and tree root plugging arise, farmers should consult with their drainage contractor or OMAFRA Agricultural Engineer.
Drain Management Problems:
Trees: Drains that are
within 15 m of trees and which carry water for prolonged periods during the
growing season may become plugged with tree roots. If possible, all water-loving
trees, such as willow, soft maple, elm, and poplar, should be removed for a
distance of 30 m from the drain; other trees should have a clearance of 15 m. If
a tree cannot be removed or the drain rerouted, use continuous non-perforated
pipe for a distance of 15 m on either side of the tree.
Figure 2. A lengthy section of 10 inch mainline is plugged solid with roots. The roots originated from a large willow tree located more than 50 feet away from the buried drain. The plugged section, about 80 feet in length, was cut out and replaced with non-perforated pipe to prevent roots from entering again.
Precautions for stream (riparian) buffers
Where trees, shrubs and weeds associated with wet soils are established or are permitted to grow naturally along a watercourse, perforated tile that passes under the buffer to an outlet can quickly become plugged by roots. For field tiles that drain into buffered streams, intermittent watercourses or ditches, a section of non-perforated tile should be installed. The non-perforated section of tile should extend from the outlet, pass under the vegetated buffer, and continue for at least 15 meters into the cultivated field where it can then connect to standard perforated pipe. Roots will not penetrate non-perforated pipe. Worry-free drainage of field water will continue with added benefits realized by having buffered vegetated watercourses.
How tree roots grow in the soil
Roots of trees grow to new areas of soil to increase the root surface area. Nutrient uptake, water absorption and anchorage are key functions of roots.
Roots grow proportionately in size with the above ground tree and maintain a specific root-to-shoot ratio.
Roots of many species of trees, some weeds, several shrub and crop species can grow close to and within tile drains as they expand their ability to acquire water and nutrients.
Back-filled soil over drainage tiles provides easy access for roots to the tile due to the breakup of compacted soil layers and loosening of the back-filled soil by the drainage installation equipment.
Roots do not actively search the soil for moisture and nutrients but grow more vigorously as they randomly encounter more favorable growing conditions such as, increased moisture and nutrient levels.
Root growth conditions can continue to improve until moisture becomes excessive or nutrients reach toxic levels at which point root growth declines.
The ideal amount of soil moisture or the ideal amount of each nutrient is entirely dependent upon the tree species.
Physical soil properties can become more favorable for root growth with increasing moisture. Roots may develop more vigorously towards an increasing humidity gradient and moisture gradient. Roots can push their way through soil easier towards an increasing moisture gradient as soil becomes more pliable due to increasing amounts of water.
Other soil factors such as oxygen concentrations and soil particle size also contribute to ease of root growth.
How tree roots plug drain tiles
Drainage tiles that are perforated with holes (modern plastic pipe), have gaps (sectional clay tile) or are damaged by cracks can be plugged by roots. Non-perforated pipe cannot be plugged by roots since there are no entry points.
Roots are more likely to be found within tile after a prolonged dry period as root systems expand downward to increase their ability to absorb water.
A root will likely stop growing once it enters a dry tile but can remain alive. The root will not proliferate to plug the tile if the root does not encounter a water source.
Once running water or standing water becomes available inside a tile, tree roots that are present may proliferate and plug the tile.
The rate of root growth and an ability to plug is dependent on the species of the root occupying the tile. Roots will plug tile slower if other sources of water are available outside the tile during the same period of time.
Figure 3. At first glance a plug tile may appear blocked by soil. Try washing the soil out with a garden hose to see if anything else is causing the blockage.
Figure 4. The same tile as in figure 3 after rinsing. Washing out the soil may reveal roots of trees or other vegetation that has entered through perforations. The roots are often fine and fibrous, packed very tight within the drain. Although soil was caught in the root mass, the real cause of the blockage was by roots.
How do you know when field drains are plugged?
The root growth which created the plug could have progressed the previous season in early autumn and continued as late as December. The plug may have developed over several seasons. An old tile system may be losing the ability to effectively drain the land due to accumulations of sediment or pipe collapse. Plugged sections of tile will need to be located, cut out and a new section of tile spliced into the line.
Root masses that form within tile can occasionally break free from the parent plant and travel downstream inside the tile, eventually blocking water flow at a different location. These shifting plugs have been found blocking drains of interconnected neighbouring farms causing crop damage. Determining where the root mass originated from can sometimes be difficult.
Conditions that could cause plugging
Water can run through tile constantly or it can flow for an extended duration into the growing season due to drainage of natural springs or drainage of areas in a field where the water table meets a tile. Indication of this water flow can usually be observed at the tile outlet.
Depth to the water table can
vary from one season to the next and is dependent upon seasonal rainfall
patterns. Tiles that are dry during average growing seasons may have late flow
of water during wet seasons. Field locations having wet tile will be a risky
area to establish any tree species since plugging by roots may eventually occur.
At a low field elevation, plugging in tile by roots may encourage proliferation of roots of other trees upstream in the line since water remains present until the plug is noticed and subsequently released.
Since trees take up soil water, could trees be used to drain farmland naturally?
By transpiration, trees, like crops, can remove significant amounts of water from soil. If planted dense enough, could trees act as an effective natural substitute for tile drainage on farmland? Trees remove little soil water during times of critical farming activities. In order to remove soil water through transpiration, water must evaporate through photosynthesizing deciduous leaves or needles of conifers. Trees remove water from soil when crops absorb water from soil, during the growing season, from mid-spring to early autumn. At other times of the year, trees transpire very little water out of the soil. Although some moisture evaporates from trees during winter dormancy, the amount is small. Trees remove very little water from soil before early spring and after mid-autumn since leaves are absent or are no longer functional, and for most conifers, needles are either ending or beginning their winter dormant period. Early spring and fall are critical times when land needs to be drained to allow access by heavy farm equipment. Field tile will adequately drain field water at these important times.
During heavy summer rains and occasional flooding, soil oxygen levels can be depleted enough to cause many tree species to stop water uptake altogether, especially dry-site trees. Tree species that tolerate wet soil and are able to continue absorbing water during floods would not remove field water fast enough to enable nearby crops to survive the prolonged saturated conditions. Trees will not function as a natural substitute for tile drainage during sporadic saturation of farm soil during the growing season.
Trees that can plug farm drains
Tree species that naturally tolerate or thrive in wet or flooded conditions and are shallow to intermediately rooted can proliferate and plug wet drainage tiles. Plugging may occur quickly or it may require several seasons of repeated wet conditions.
The following list of tree species can tolerate and grow in saturated soil or free water and should not be planted near perforated field drains:
Shallow rooted trees - Have roots that grow laterally for long distances (30 meters or more have been observed) and develop primarily within 1 meter of the soil surface, have many fibrous roots that can form very dense root systems causing thick blockage of drainage lines.
Speckled alder, gray alder (Alnus
incana ssp. rugosa)
Intermediate rooted - Most roots have uniform thickness and grow outwards and downwards from the tree in a circular pattern, have some deeper lateral roots but are fairly wide spreading in growth, can completely block field tile with many small diameter roots.
American elm (Ulmus
Deep rooted - usually consist of one or two deep taproots that extend straight down deep into the soil for many meters. The roots do not tend to spread out laterally and are least likely to plug wet drain lines unless planted within 1 or 2 meters of the underlying drain.
Bur oak (Quercus
Other plants - Field horsetail (Equisetum arvense) is a common weed that can plug perforated drainage tile. Rhizome roots of horsetail can penetrate to more than 1 meter below ground forming thick mats of root. Other species reported to have plugged farm field drains include canola, sugar beet, kale, rape, brambles, watercress, hawthorn, nettles, dandelion, meadow grass, dock, buttercup, fleabane and rushes.
Trees that rarely plug farm drains
Roots of tree species that
prefer dry or well-drained soil are least likely to plug farm drains especially
when grown on fast draining soils. Roots may encounter conditions within tiles
that are too wet for the roots to survive. It is important to know that drain
plugging by dry-site trees is rare but can still occur depending on the
situation. For example, roots of peach and black locust have been observed
plugging farm tile and both are dry-site species.
Intermediately rooted trees: