Rice Hull Mulch Affects Germination of Bittercress and Creeping Woodsorrel in Container Plant Culture

Mulches are commonly used to control weeds in container nursery crops, especiallyin sites where preemergence herbicides are either not labeled or potentially phytotoxicto the crop. Parboiled rice hulls have been shown to provide effective weedcontrol when applied 1.25 to 2.5 cm deep over the container substrate surface. Theobjective of this research was to determine if weed seed placement, above or belowthe 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 comparedto when seed were applied above the mulch. Light penetration through varyingdepths of rice hulls was determined with a spectroradiometer. Photosynthetically activeradiation (PAR) decreased exponentially with increasing rice hull depth, and wasless than 1 μmol·m-2·s-1 beneath depths greater than 1 cm. Germination of both specieswas 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 woodsorrelgermination was not affected by light level and was high (92%) after 3 weeks. Therole of light exclusion by rice hulls as a mechanism for controlling buried weed seedis discussed. Water retention immediately after irrigation, and for 24 hr following irrigation,was determined for a 2.5 cm layer of rice hulls, sphagnum peat moss, andpine bark. Rice hulls retained less water, and dried more quickly than peat moss orpine bark. The volumetric water content of the rice hull layer is less than 0.20cm·cm-1 and what has been shown necessary for plant growth. Lack of water availabilityin the rice hull layer is discussed as the primary mechanism of control of weedseed above the mulch layer.