1 - It is difficult to manage weeds invading raised beds, but there are some possible management techniques.
Yard and Garden July 27, 2013
Q.
I have got an interesting situation I could use some help with. I have a raised bed I constructed last year. I built the raised bed to avoid the compacted nature of the soil beneath the bed. The compaction comes from a mystery root which infiltrates this and another bed, and will bind the plants therein. So I built a raised bed, lined it with carpet, and it seems the roots have either infiltrated again, or new weed roots form the soil I imported have started to grow and make the soil somewhat compacted. So my question is, "should I worry about this, will the plants push aside these other roots?" Is there anything you would recommend to remedy this weird situation? I have invested so much energy into this bed that it is a bit frustrating.
A.
I am guessing that the mysterious plant whose roots are causing the problem is bindweed, a perennial relative of morning glory vines. This vine is called bindweed because it binds-up plants it grows near and can kill or weaken those desirable plants. Tree roots can also compete with plants in garden beds. Both can invade the soil in raised beds and cause problems for plants growing in the raised beds. Controlling such weed invasion can be very difficult. Proper management will involve accurately identifying the weed causing the problem. In my answer below, I will assume the problem is bindweed.
One management technique is to very persistently manually remove the plants as they appear. Your goal will be to prevent leaves from developing sufficiently to develop a strong root system in the soil inside the raised bed. Herbicides labeled for managing the bindweed can also be used, but you should be sure the herbicide is labeled for use around the plants you are growing in the raised bed. To avoid problems, the herbicides, especially those with soil activity (ability to be absorbed from soil by plant roots) should be applied directly to the weed by directed spray at only the weed and avoiding runoff into the soil, or by painting the herbicide on the leaves of the weed while protecting the desirable plants. This should be done as frequently as manually removing the weeds, so manual removal may prove to be the simplest management method. Your local NMSU Cooperative Extension Service County agent can help you identify the offending weed.
Another potential option to protect the soil in the raised bed from invasion by weed or tree roots is to put a barrier at the base of the soil in the raised bed. You attempted this with carpet, but the carpet failed to exclude the invading roots. A commercial product called Typar Biobarrier (R) may prove more successful. This product consists of a spun-bonded polyester fabric which contains spaced plastic dots which are impregnated with herbicide which specifically halts roots. This product should continue to be effective for several years. In time it may become necessary to remove the soil, replace the barrier at the base and the soil. Without an effective barrier you will need to remove the soil frequently and, in addition, fight the development of weeds throughout the growing season. If the weed finds gaps in the barrier, you will need to fight the weeds by manual or chemical removal earlier, but you should still have several successful gardening seasons.
While using the raised beds to manage this problem, you should also manage the weeds outside the raised bed. If the weeds are kept weak in the surrounding soil, they will have a more difficult time invading the raised beds.
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.
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