April 2, 2011

1 - Raised beds are good for gardening, but keeping tree and shrub roots out can be a challenge.

Yard and Garden - April 2, 2011


I have a question regarding plant roots, including trees, getting into built-up four square gardens from the underside. We have a four square garden which worked out very well to start with, 10 + years ago, but then started declining to the point where we abandoned it a couple of years ago. We used horse manure which I understand now is not good, mixed with other soil, but I have been mainly blaming tree roots--one seed juniper and pinion, in addition to other plant roots getting into the gardens searching for and finding a water source. Also, treated lumber for the sides of the gardens may be a problem.

In view of the fact that we would like to succeed at this, would you have some advice, please?

- Jim H.

Via internet


Raised bed and container gardens are a good idea in New Mexico because the soil can be "tailored" to the crops. However, the drawback is that the soil must be renewed periodically. Open-bottom raised beds, such as yours are also subject to root invasion as you have described.

Renewing the soil may involve removing the soil from the raised bed and replacing all of it, or just the addition of compost or another source of organic amendments and nutrients. If high levels of mineral salts from irrigation have accumulated in the soil, total replacement may be best. If irrigation has leached the nutrients from the soil, and if over time, the organic matter has diminished you may just work nutrients and compost into the soil. A soil test will help decide which is appropriate. Your local NMSU County Extension Service office has free information regarding soil testing that you can request.

Excluding tree and shrub roots can also help prolong the useful life of the tailored soil in the raised bed garden. If the soil surrounding the bed is deep enough, you can build an underground "fence" to slow root entry into the beds. This involves digging a trench around the outside of the raised bed and placing a vertical root barrier into the trench to block root growth into the soil under the raised garden. The root barrier material may be a sheet of solid plastic (of the same material that large nursery pots are constructed) or a fabric material with plastic dots impregnated with root inhibiting herbicide (usually trifluralin). Compact the soil at the base of the trench to discourage roots that try to grow under the barrier. If possible, turn some of the root barrier outward away from the garden (toward the source of invading roots). This will further discourage downward root growth. Be sure that there is no seam or gap between the bottom of the vertical barrier and the horizontal barrier at the bottom of the trench for maximum effectiveness. If you combine a solid plastic vertical barrier with the trifluralin treated fabric (fabric outside the solid plastic), you can make a vertical barrier from both and the bottom barrier with the fabric. This technique will isolate your garden plants from the herbicide. Such a root barrier should slow root entry into the garden, but may require replacement or re-setting if roots eventually do enter the garden.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!