Rice hull mulch affects germination of bittercress and creeping woodsorrel in container plant culture

Authors: Altland, James; Boldt, Jennifer; KRAUSE, CHARLES

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2016
Publication Date: 11/28/2016
Publication URL: https://handle.nal.usda.gov/10113/5801824
Citation: Altland, J.E., Boldt, J.K., Krause, C.R. 2016. Rice hull mulch affects germination of bittercress and creeping woodsorrel in container plant culture. American Journal of Plant Sciences. 7:2359-2375.

Interpretive Summary: Weeds of nursery container crops spread primarily by seed dispersal. The most common method of weed control in nursery container crops is the use of preemergence herbicides applied to the substrate surface to inhibit weed establishment from seed. However, these herbicides cannot be applied safely to all crops, nor can they be applied to crops inside enclosed structures such as greenhouses and hoophouses. Rice hulls can also be used as a container mulch for weed control in crops or sites where preemergence herbicides are not feasible. Previous research has shown that rice hull mulch at a depth of 2.5 cm provides excellent flexuous bittercress (Cardamine flexuosa) and liverwort (Marchantia polymorpha) control when seed or propagules are disseminated onto the mulch surface. The objective of this research was to determine if application of seed above or below the mulch surface affects bittercress or creeping woodsorrel establishment. This research shows that seed present on the substrate surface at the time of mulch application are not controlled as well as those introduced after rice hull application. Rice hulls do not exclude light from reaching the substrate surface. A rice hull layer at least 0.5 in thick is necessary to form a physical barrier to prevent establishment of bittercress or woodsorrel from beneath the mulch layer. The ability of rice hulls to dry quickly after irrigation events is the primary mechanism by which rice hulls provide excellent weed control of weed seeds on the mulch surface.

Technical Abstract: Mulches are commonly used to control weeds in container nursery crops, especially in sites where preemergence herbicides are either not labeled or potentially phytotoxic to the crop. Parboiled rice hulls have been shown to provide effective weed control when applied 1.25 to 2.5 cm deep over the container substrate surface. The objective of this research was to determine if weed seed placement, above or below the mulch layer, affects flexuous bittercress or creeping woodsorrel establishment. Seeds of both species were placed either above or below rice hull mulch layers 0, 0.6, 1.3, or 2.5 cm deep in nursery containers with a 80 pine bark : 20 sphagnum peatmoss substrate. Establishment of both weeds decreased with increasing mulch depth. Establishment of both species was generally greater from beneath the mulch compared to when seed were applied above the mulch. Light penetration through varying depths of rice hulls was determined with a spectroradiometer. Photosynthetically active radiation (PAR) decreased exponentially with increasing rice hull depth, and was less than 1 µmol m-2 s-1 beneath depths greater than 1 cm. Germination of both species was determined in Petri dishes placed beneath varying densities of shade cloth. Flexuous bittercress germination responded quadratically to decreasing light level, but still germinated (13%) in complete darkness after 3 weeks. Creeping woodsorrel germination was not affected by light level and was high (92%) after 3 weeks. The role of light exclusion by rice hulls as a mechanism for controlling buried weed seed is discussed. Water retention immediately after irrigation, and for 24 hr following irrigation, was determined for a 2.5 cm layer of rice hulls, sphagnum peatmoss, and pine bark. Rice hulls retained less water, and dried more quickly than peatmoss or pine bark. The volumetric water content of the rice hull layer is less than 0.20 cm cm-1 and what has been shown necessary for plant growth. Lack of water availability in the rice hull layer is discussed as the primary mechanism of control of weed seed above the mulch layer.