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 peat moss 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 peat moss, and pine bark. Rice hulls retained less water, and dried more quickly than peat moss 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.
Highlights
Many economically important weeds of field crops spread via perennating organs such as stolons, rhizomes, tubers, bulbs, or corms
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
Weed number decreased linearly and quadratically with increasing rice hull depth in containers seeded above the mulch, and decreased linearly for those seeded below the mulch
Summary
Many economically important weeds of field crops spread via perennating organs such as stolons, rhizomes, tubers, bulbs, or corms. Substrates of nursery container crops are isolated from each container, weeds cannot spread via perennating organs as they do in field soils. 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. Some crops such as hydrangea (Hydrangea macrophylla (Thunb.) Ser.), azalea (Rhododendron obtusum (Lindl.) Planch.), and many herbaceous perennials are sensitive to preemergence herbicides [1], and no preemergence herbicide is currently labeled for use on container crops inside enclosed structures such as greenhouses. The most common alternative to herbicides is the use of mulches
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