USDA tests PBH rice hulls for weed control in container crops

The USDA-ARS in Wooster, Ohio, has begun a new five-year research program to develop weed management options for herbicide-sensitive crops. This is a multi-pronged approach to develop a wide variety of tools for weed control in production settings where herbicides are either not labeled or cannot be used safely.

A very promising part of this project thus far has been the use of parboiled rice hulls as a mulch in containers. Riceland Foods, Inc. has been marketing their parboiled rice hulls for weed management in horticulture crops, and some nursery producers in Ohio have already successfully used rice hulls for weed control. The goal of our research was to determine, in controlled research, what quantity of rice hulls provided effective weed control of liverwort (Marchantia polymporpha) and bittercress (Cardamine ­exuosa).

A Few Basic Points

Before I get into the details of our experiments and results, it’s important to review some basic concepts on how mulches provide weed control. Seeds of container weeds are small, and must germinate on or near the container substrate surface. When you cover the substrate surface with mulch, small weed seeds don’t have enough stored energy to grow through the mulch and establish themselves on the surface. This is primarily how mulches provide weed control, at least temporarily. The problem is that most mulches don’t provide long-term weed control because the mulch itself becomes an excellent substrate for weed germination. After the mulch is applied, new weed seeds that land on the surface of the mulch will soon germinate in the mulch itself. A truly e­ffective mulch, especially for container production, is one that persists for a long period of time and off­ers an inhospitable site for weed seed germination.

Eff­ective mulches for container crops should have a combination of the following properties: they provide little or no available nutrients, they dry quickly after irrigation, and they resist decomposition. Weed seeds require available nutrients to establish successfully. They may germinate in the absence of nutrients, but they will fail to develop much past the cotyledon stage without sufficient nutrition.

It almost goes without saying that mulches must resist decomposition. If the mulch decomposes, the barrier is lost and weeds are free to germinate. Unfortunately, abundant fertilization and irrigation are conducive to organic matter decomposition. The nature of nursery and greenhouse crop production renders many mulch products unacceptable due to decomposition after just a few weeks in production. To review, the ideal mulch will be low in nutrient composition, retain little water even after irrigation, and resist decomposition. With that in mind, let’s look at some recent experiments to see how eff­ective rice hulls are in preventing liverwort and bittercress establishment, two of the most problematic weeds in nursery production.

Will fertilizers a­ffect rice hull mulch?

The first experiment we conducted examined bittercress and liverwort growth in containers with 0, ¼, ½, and 1 inch of rice hulls applied to the surface. We filled 48 containers with standard greenhouse growing substrate and applied each rice hull treatment (mulch depth) to 12 containers. Of the 12 containers, six were placed on a bench that received overhead irrigation with regular tap water twice daily, and six were placed on a bench that received overhead irrigation with a standard commercial water-soluble fertilizer injected (100 ppm N) twice daily. We applied liverwort gemmae (spores) to the surface of the container weekly to encourage liverwort establishment. We applied bittercress seed to the surface twice, immediately after applying the mulch and about 4 weeks later. We observed weed control in the pots to determine if rice hulls could prevent these weeds from establishing.

Rice hulls at ½ or 1-inch depth provided 100 percent weed control. No weeds grew in these pots. In containers with ¼ inch of rice hulls, both bittercress and liverwort grew, albeit a lot less than the un-mulched pots. This may have something to do with the way we applied the rice hulls. For the 1-inch mulch depth, we carefully weighed the containers before and after applying rice hulls to a depth of 1 inch. The weight of rice hulls in those pots was 44 g. So, for the pots receiving ½ inch of rice hulls, we simply weighed 22 g of rice hulls for each pot, and 11 g for the ¼ in pots. This seemed a fair, consistent, and accurate way to meter out the rice hulls. In doing this, we observed that ½ and 1-inch treatments were completely and thoroughly covered with rice hulls. However, the ¼ inch depth left some gaps in the mulch layer so that you could see the substrate surface through the mulch. Invariably, it was in these gaps that liverwort and bittercress found a footing and successfully established.

Photo: shows oxalis seed (in red circles) on the rice hull topdressed container surface, still unable to germinate after six weeks.

Based on our earlier discussion on the role of nutrition in weed seedling establishment, you might expect weed growth to be more vigorous on the bench with fertilizer injected into the irrigation stream. And you’d be correct, that’s exactly what happened. Weeds in the non-mulched pots (no rice hulls) that received fertilizer were a lot larger than those in the control pots with no fertilizer. However, to my surprise, rice hulls applied at ½ or 1-inch depth provided perfect liverwort and bittercress control even with fertilizer injected into the irrigation stream. Quite frankly, this surprised me a great deal. I was expecting weeds to germinate into the rice hulls as long as fertilizer was applied via the irrigation system. But they never did in the 8-week trial.

After observing this trial for several weeks, I was impressed with how quickly the rice hulls dried following irrigation. Since nitrogen was not limiting in these pots, I concluded that it must have been the water that was limiting. Perhaps the rice hulls dry too quickly for weed seed to successfully germinate and establish? We didn’t have any sensors or fancy gadgets to measure moisture levels in the rice hulls. However, we irrigated all these pots twice daily, and we could see with our own eyes how quickly the rice hulls dried after irrigation. We could also see the weed seed and liverwort gemmae (reproductive spores) sitting on the surface of the rice hulls without ever germinating or establishing.

Research and photos provided by Dr. James Altland, Research Horticulturist with the USDA-ARS in Wooster, OH. He can be reached by email at james.altland@ars.usda.gov. Mention of any commercial product in this article is for educational purposes only, and should not be considered an endorsement by USDA-ARS.

Visit Riceland PBH's website to learn more, including how shade canopy affects weed control with rice hulls.