AGBB 2031 Module 2 Types of Roots

Transport roots serve as a continuation of the pipeline carrying water and nutrients from the absorbing (feeder) roots to the trunk. These are the major spreading roots of the tree and follow soil oxygen gradients across the rooting area. In compacted areas (where soil oxygen is lower), they will come to the surface. In soils with good structure (higher oxygen content, they will be deeper. They also provide support to the tree in winds above 40 mph. Transport roots are typically thumb-size diameter, long, meandering, and with limited branching. Transport roots do not spread uniformly around the tree. Some areas may be void of roots, in others, heavily concentrated.

Transport Roots

The root plate is the area containing the primary structural roots that extend outward from the trunk. The root plate is a continuation of the pipeline, carrying water and nutrients from the absorbing, and transport roots into the tree trunk. Roots branch readily and taper in diameter. The root plate is the tree's primary support in winds up to 40 mph. Avoid disturbing the soil and roots in the rot plate area. Construction and hardscape features should not encroach into the root plate! When the tree fails by tipping over, often exposing the root plate, it is failure at the edge of the root plate. As a rule of thumb, the radius of the root plate is three to six times the trunk diameter at breast height (DBH, 4.5 feet).

Root Plate

The tap root is the primary root emerging from the germinating seed. However, beyond the seedling stage, the tap root is nonexistent on most trees. As the root system develops, the roots grow into the root plate system due to low soil oxygen. In nursery production, the tap root is cut while tiny, forcing a more branching root system that is tolerant of transplanting.

Tap Root

Absorbing (feeder) roots take up water and nutrients from the soil. These tiny roots are found near the soil surface throughout the entire transport rooting area. Generally, they are found in the top 12 inches on the soils with good tilth, and in the top four inches or less in compacted, clayey soils.Absorbing roots have a short life, being replaced in four to five flushes of growth throughout the growing season. A short-term drought stress (defined as 10 days) can shut down growth for 1-5 weeks. Long-term drought stress (defined as 22 days or more), can shut down growth for 1-2 years!

Absorbing Roots

Sinker roots follow natural openings into deeper soil as soil oxygen levels allow. It is unknown to what extent trees actually have sinker roots in the soils of a landscape. Sinker roots can extract water from deeper soil depths when the surface soil is dry. This helps explain how tress can have good short-term drought resistance. It also helps explain the severe drought stress observed on trees when there are dry seasons with dry subsoil. Sinker roots also provide additional support in strong winds.

Sinker Roots