The amount of water held by peat moss can be 15 to 20 times its own weight, depending on peat moss type . By week 4, water retention was equal or greater in peat moss compared to pine bark across both experiments. Greater water retention by pine bark early in the experiment was likely due to initial hydrophobicity common in sphagnum peat moss. As the experiment progressed over time with daily irrigation events, this initial hydrophobicity was overcome and peat moss retained a greater volume of water compared to pine bark. The layer of mulch within the round Buchner funnels of this experiment was disc-shaped with a height of 2.5 cm, vertical grow system diameter of 13.1 cm, and volume of 342 cm3 . Thus volumetric water content in each mulch layer could be calculated for the volumes of water retained listed in Table 3. Pine bark and peat moss VWC ranged from 0.40 to 0.50 cm∙cm−1 at hour 0 throughout the two experiments, and decreased to 0.2 to 0.4 cm∙cm−1 by hour 24. In contrast, VWC in rice hulls never exceed 0.20 cm∙cm−1 at hour 0 and declined to less than 0.10 cm∙cm−1 by hour 24 in the experiment.
O’Meara et al. used hydrangea and gardenia in a pine bark substrate to show that the ability of plants to extract water from a soilless substrate is species-dependent, and for these two species transpiration declined at VWC of 0.20 to 0.28 cm∙cm−1 and slowed to near zeroonce VWC dropped below 0.12 to 0.16 cm∙cm−1 . Rice hulls do not exclude enough light at depths of 2.5 cm to prevent bitter cress or creeping wood sorrel germination. The amount of PAR penetrating rice hulls from 0.6 to 2.5 cm averaged 1.3 µmol∙m−2 ∙sec−1 . While flexuous bitter cress germination declined when PAR dropped below 41.3 µmol∙m−2 ∙sec−1 , germination still occurred in complete darkness. Creeping wood sorrel had 92% germination across all light levels. Numbers and shoot fresh weights of both species decreased with increasing rice hull depth when seed were placed beneath the mulch layer. The mechanism of this control is not likely due to light reduction or light exclusion, especially with creeping wood sorrel. It is widely accepted that germination rate of seed decreases with increasing burial depth . Decreasing numbers and shoot fresh weights with increasing rice hull depth in our study was likely a function of burial depth as well.
A meta-analysis of seedling recruitment in natural grasslands found that plant litter up to 500 g∙m−2 improved seedling recruitment, cannabis grow equipment while greater litter amounts inhibited seedling recruitment . Teasdale and Mohler showed a similar response in germination of four weed species beneath various mulch materials over a silt loam soil. The mass of rice hulls applied in our experiment was 46 g at a depth of 2.5 cm in a 15 cm diameter container, resulting in an application equivalent to 2523 g∙m−2 . Cumulative germination and shoot fresh weights of the flexuous bitter cress and creeping wood sorrel in our experiment were reduced at 1.3 and 2.5 cm depths , but not reduced at 0.6 cm depth . Flexuous bitter cress and creeping wood sorrel establishment beneath rice hull mulch seem to follow the general rule of seedling establishment beneath litter proposed by Loydi et al. . Altland and Krause speculated that gaps in the mulch barrier common with applications of 0.6 cm rice hulls were responsible for greater establishment of liverwort and bitter cress; however, it is likely that lack of rice hull mass also contributes to poor control. Flexuous bitter cress cumulative numbers that established from beneath the 2.5 cm rice hull layer was reduced 68% relative to the non-mulched control.
Likewise, cumulative shoot fresh weights of these bitter cress were reduced 78% compared to the nonmulched control. In contrast, creeping wood sorrel numbers and shoot fresh weight from beneath 2.5 cm rice hulls was reduced 88% and 94%, respectively, compared to non-mulched controls. Flexuous bitter cress seed used in this study were 0.9 ± 0.13 mm long and 0.6 ± 0.06 mm wide , while creeping wood sorrel were larger with length of 1.4 ± 0.14 mm and width of 1.0 ± 0.05 mm . Others have shown decreasing sensitivity to mulch depth with larger seed Thus it was somewhat surprising that the larger seed of creeping wood sorrel were more affected by burial depth than the smaller-seeded bitter cress. Rice hulls retain less water than pine bark or peat moss, and this seems to be the primary mechanism by which rice hulls provide weed control when seeds are applied to the mulch surface. Weed seed can germinate in soils with water potential as low as −1.5 MPa, as summarized by Bullied et al. .