Numerous mulch products have been evaluated for use in container crops. Pine bark nuggets have been shown to provide effective control of mulberry weed [Fatoua villosa (Thunb.) Nakai] (Penny and Neal, 2003), prostrate spurge (Chamaesyce maculata L.), eclipta (Eclipta alba L. Hassk) (Cochran et al., 2009), bittercress (Cardamine spp.), and oxalis (Oxalis stricta L.) (Richardson et al., 2008). In each of the aforementioned studies, weed control was shown to improve with increasing depth of pine bark mulch. Wilen et al. (1999) showed that composted greenwaste, pecan [Carya illinoinensis (Wangenh.) K. Koch] shells, and pine (Pinus taeda L.) bark at a depth of 2.5 cm provided excellent control of creeping woodsorrel (Oxalis corniculata L.), northern willow herb (Epilobium ciliatum Raf.), and common groundsel (Senecio vulgaris L.), but only moderate to poor control of annual bluegrass (Poa annua L.). Ferguson et al. (2008) showed that a 3.7 cm layer of wood chip mulches from southern redcedar [Juniperus silicicola (Small) E. Murray] and southern magnolia (Magnolia grandiflora L.) provided control of redroot pigweed (Amaranthus retroflexus L.) and large crabgrass [Digitaria sanguinalis (L.) Scop.].
Parboiled rice hulls have been shown to provide excellent control of flexuous bittercress (Cardamine flexuosa With.), liverwort (Marchantia polymorpha L.) (Altland and Krause, 2014), and creeping woodsorrel (Oxalis corniculata L.) (Altland et al., 2016a). Parboiled rice hulls prevent establishment of new weed seed by drying quickly following irrigation, resulting in insufficient moisture to support germination and establishment of new weeds. Parboiled rice hulls are less effective in preventing germination of weed seed preexisting on the substrate surface before mulch application. To prevent germination and establishment of preexisting weed seed, a sufficient mass must cover the substrate surface to physically impede germination (>500 g·m−2 or a depth of 1.25 cm) (Altland et al., 2016).
A common concern about organic mulches is that they impose nitrogen (N) deficiency on crops they surround. After initial reports on the utility of PBH for weed control in container crops (Altland and Krause, 2014), a similar concern of N immobilization was voiced by nursery growers (J. Altland, personal observations, unpublished). In a review of mulch use in urban landscapes, Chalker-Scott (2007) explained that high carbon to nitrogen ratio (C:N) is incorrectly assumed by many practitioners to immobilize fertilizer N and thus deprive plants of sufficient nutrients. Chalker-Scott (2007) goes on to assert that mulches do not affect nutrition of landscape plants. There are numerous experiments in field soils that support this assertion and show mulches do not adversely affect nutrient uptake of field-grown crops (Broschat, 2007; Ram et al., 2003; Trinka and Pritts, 1992). However, this generalization about interactions between mulch and fertilizer in field crops might not be applicable to container crops. Altland and Lanthier (2007) reported that container-grown hydrangea [Hydrangea macrophylla (Thunb.) Ser. ‘Fasan’] with controlled release fertilizer (CRF) placed below the mulch were larger and had higher foliar N concentration compared with those with CRF placed above the mulch. The implication was that N from CRF placed above the mulch was partially immobilized by the mulch layer. Likewise, Glenn et al. (2000) showed that recycled paper mulch reduced petunia (Petunia floribunda Hort. ‘Midnight Madness’ and P. grandiflora Hort. ‘Ultra Blue’) growth and leachate N when CRF was placed above the mulch layer. The objective of this research was to determine specifically how PBH mulch affects the nutrient concentration of irrigation water passing through the mulch layer, and subsequently how this affects substrate and plant nutrition.
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