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.
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, thus weeds cannot spread via perennating organs as they do in field soils. 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, some crops such as hydrangea (Hydrangea macrophylla (Thunb.) Ser.), azalea (Rhododendron obtusum (Lindl.) Planch.), and many herbaceous perennials are sensitive to preemergence herbicides [
Numerous mulch products have been evaluated in container crops [
Mulch products would likely be applied at or near the time of potting, and presumably would be applied to containers free of weed seed. Any weed seed introduced into the container thereafter would have to germinate and establish on the surface of the mulch product. Alternatively, mulches could be applied sometime throughout the production cycle of a crop. Containers might be hand-weeded to remove existing weeds, and then mulched. In this scenario, a large weed seed bank could be present on the substrate surface. Seed present at the time of mulch application could germinate from beneath the mulch product. A third scenario would be the carryover of weed seeds from liners into larger containers at the time of transplant. In this situation too, a seed bank present on or near the surface of the newly potted liner would have to germinate and establish through the mulch. Establishment of weed seed above and below mulch products has been studied previously. Cochran et al. [
Parboiled rice hulls (Riceland Foods, Inc., Stuttgart, AK) are dry rice husks removed from rice grains with steam or hot water. Hereafter they will be referred to as rice hulls. Rice hulls are commercially available for horticultural use, and are currently used as a component in greenhouse and nursery substrates. Rice hulls can also be used as a container mulch. One manufacturer (Riceland Foods) recommends a rice hull mulch depth of 3.8 to 5.0 cm for effective weed control in container crops. Previous research has shown that rice hull mulch at a depth of 2.5 cm provides excellent flexuous bittercress (Cardamine flexuosa With.) and liverwort (Marchantia polymorpha L.) control when seed or propagules are disseminated onto the mulch surface [
On 17 Jan. 2014, 15-cm diameter and tall containers (trade-gallon) were filled with an 80 pine bark : 20 sphagnum peat moss substrate to within 2.5 cm of the container top. It was established that a 2.5 cm deep layer of rice hulls (Riceland Foods, Inc., Stuttgart, AK) on the substrate surface would weigh 46 g. Containers were randomly assigned to receive rice hulls at a depth of 0, 0.6, 1.3, or 2.5 cm by weighing 0, 11.5, 23, or 46 g of rice hulls and spreading them evenly over the surface. Half the containers were seeded with flexuous bittercress and the other half with creeping woodsorrel. These two weeds were selected as the test species due to their prevalence in greenhouse and nursery container crops. Weed seed were either placed on the substrate surface prior to mulch application, or on top of the mulch surface following mulch application. For containers receiving weed seed below the mulch layer, 40 seeds were applied to the substrate surface prior to mulch application. For containers receiving weed seed above the mulch surface, 10 seeds were applied to the container surface each week for 15 weeks throughout the course of the experiment. There were six single pot replications for each weed species, seed placement, and mulch depth combination. Containers were arranged in a completely randomized design with the two weed species randomized separately.
Containers were placed in a glass-covered greenhouse in Wooster, OH, and received natural photoperiod with heat and cool set-points at 20˚C and 23.9˚C, respectively. Containers received overhead irrigation consisting of city tap water injected with a commercial complete fertilizer with micronutrients (Jack’s 20N-4.4P-16.6K-0.15Mg- 0.02B-0.01Cu-0.1Fe-0.05Mn-0.01Mo-0.05Zn, JR Peters, Inc., Allentown, PA) at a concentration of 100 mg∙L−1 N. Irrigation was run twice daily for 7 min each cycle, so that each plot received approximately 1 cm∙d−1 water. Irrigation was provided daily through- out the experiment.
Weed number and shoot fresh weights were determined 4, 8, and 12 weeks after potting (WAP). Because the number of seed applied, and the timing of seed application differed between containers in which seed were applied above or below the mulch, the treatment factor of seed placement is discussed but was not compared statistically (with the exception of data collected 4 WAP and % germination). Weed number were arcsine-square root transformed for analysis, although actual data are presented. Data were subjected to repeated measures analysis of variance (ANOVA) using the general linear model (GLM) procedure in SAS. Regression analyses were conducted with orthogonal contrast statements within the GLM procedure. The least significant difference (LSD) for treatment means was calculated using Fisher’s protected LSD test where α = 0.05.
A spectroradiometer (PS-200, Apogee Instruments, Logan, UT) was placed within an opaque, black, 12 L nursery container (#3 Nursery Supplies, Chambersburg, PA). An initial spectral scan of photosynthetically active radiation (PAR, 400 to 700 nm) was recorded outdoors (Toledo, OH) on 28 Aug. 2014 at mid-day on a sunny day. A 24-cm diameter glass pie plate (Pyrex, World Kitchen LLC, Rosemont, IL) was placed over the top opening of the container where the outer edge of the plate rested on the rim of the container while the bottom of the plate nested within. The sides of the pie plate were covered with black electrical tape to prevent light from reflecting or refracting through the edges of the plate. Another spectral scan was taken to provide the percent light transmission through the plate. Rice hulls were placed in the plate at depths ranging from 0 to 2.5 cm. It was established that a 2.5 cm layer of rice hulls in the glass plate weighed 154.6 g. The mass of rice hulls needed for each depth was subsequently calculated based on this rice hull depth to weight ratio (2.5 cm: 154.6 g). Spectral scans were taken from approximately 5 cm beneath the glass plate filled with varying levels of rice hulls using the spectroradiometer. Five scans were recorded for each rice hull depth, using a different pre-weighed batch of rice hulls for each scan. The order in which they were taken was blocked in time and completely randomized within each block. Spectral scans of full sunlight and with the empty glass plate were taken at the end of the measurement period and averaged with the initial scans. The intensity of full sunlight at the beginning and end was 1880 and 1790 µmol∙m−2∙s−1 PAR, respectively. Light intensity for PAR and each waveband of light passing through the rice hull depths was subjected to ANOVA and means separation. Data were also fit to exponential functions in the form of I = a + be−cx where x = mulch depth, I = intensity, the sum a + b equals the maximum light intensity when mulch depth is 0, and c is a scaling factor (
Flexuous bittercress and creeping woodsorrel germination in response to irradiance intensity was determined in a growth chamber (BDR16, Conviron, Winnipeg, CN) using
Petri dishes (95 mm × 15 mm). The growth chamber was programmed to provide a constant 20˚C air temperature, 10 hr photoperiod, and 30% relative humidity (RH). Irradiance was provided by cool-white fluorescent lamps (FT2T8/TL841/HO, Philips Lighting, Somerset, NJ). An agar base was made using 15 g∙L−1 granulated agar in a modified Hoagland solution (in mM: 7.5 N, 0.5 P, 3 K, 2.5 Ca, 1 Mg, 1 S, 0.071 Fe, 0.009 Mn, 0.0015 Cu, 0.0015 Zn, 0.045 B, 0.0001 Mo, 0.024 Cl, 0.0002 Na). The agar was autoclaved for 20 min, then 25 mL was poured into each Petri dish. Ten seeds of each weed were placed onto the agar media so that the seed from each species was confined to one half of the dish. Petri dishes were exposed to six different light treatments by placing them individually within a 2.5 cm tall section of 10 cm o.d. black ABS pipe (Charlotte Pipe Products, Monroe, NC). One group of Petri dishes was placed inside of uncovered ABS pipe sections. Other groups of ABS pipe sections were covered with a single layer of 30% shade cloth, a single layer of 63% shade cloth, two layers of 63% shade cloth, or four layers of 63% shade cloth (DeWitt Woven Shade Cloth, Sikeston, MO). Shade cloth was attached to the top of the ABS pipe and kept taut and in place with a zip tie (Panduit Co., Tinley Park, IL). A final group was covered with opaque foil tape (3M, St. Paul, MN) to exclude 100% of the light. The actual light reduction beneath each shade treatment was measured with a spectroradiometer and determined to be 0%, 22%, 61%, 88%, 99.9% and 100% shading, respectively, which corresponded to 336.3, 261.6, 131.5, 41.3, 0.3, and 0 µmol∙m−2∙s−1 PAR, respectively. There were ten replications per shade treatment arranged in a randomized complete block design within the growth chamber. Petri dishes from five replications were destructively harvested to count the number of germinated seed 1 and 3 weeks after seeding (WAS). Weeds were considered germinated if both roots and cotyledons were visible with a hand lens. Data were arcsine-square root transformed for analysis, although actual data are presented. Transformed data were subjected to ANOVA and means separation using SAS (Version 8, SAS Institute, Cary, NC). The experiment was repeated.
Two-piece, polyvinyl chloride Buchner funnels (13.1 cm i.d., 6.6 cm tall, Fisher Scientific, Waltham, MA) were filled with a 2.5 cm layer of rice hulls, sphagnum peat moss (Fafard, Agawam, MA), or pine bark (Buckeye Resources, Dayton, OH). It was established that a 2.5 cm layer of rice hulls, peat moss, and pine bark weighed 46, 70, and 130 g, respectively. These weights were used to uniformly apply the same mass of each mulch material to replicate funnels. A subsample of each mulch material was weighed, oven dried at 72˚C for 4 d, and weighed again to determine percent moisture content of the rice hulls, peat moss, and pine bark, which was 7.6%, 71.2% and 69.1%, respectively. Each mulch-filled funnel was weighed (Wi) and placed on a greenhouse bench equipped with an overhead irrigation system with fixed pattern nozzles (Rain Bird 5H, Rain Bird Corp., Azusa, CA). Each funnel was placed over a 400 mL glass jar (Fisher Scientific) so that all irrigation water passing through the mulch layer would collect in the jar beneath, and furthermore, only water passing through the funnel (and no other extraneous irrigation water) would drain into the jar. The irrigation system was run for 10 min, resulting in an application of approximately 1.3 cm of water. The funnels were weighed after the irrigation ceased and funnels stopped dripping (W0). The funnels were weighed again 1, 4, and 24 hr after the irrigation event (W1, W4, and W24, respectively). The volume of water passing through the mulch and into the jar beneath was measured (V). The percent of water passing through the mulch layer in each funnel was calculated as V/(W0 − Wi + V). Volume of water passing through the mulch layer was expressed as a percent to correct for variation in the volume of applied water due to non-uniformity of the irrigation system. The mass of water retained in the mulch layer at 1, 4, and 24 hours was calculated as W1 − Wi, W4 − Wi, and W24 − Wi, respectively. The mulch filled funnels remained in place on the greenhouse bench where they were irrigated daily. There were six replications per mulch material placed in a completely randomized design. The process described above to measure the volume of water passing through the mulch, and the mass of water retained in the mulch layer over the course of 24 hr was repeated at 2 and 4 weeks after the initial measurement. Data were subjected to ANOVA and Fisher’s protected means separation using SAS. The experiment was repeated.
At 4 WAP, seed placement and rice hull depth interacted to affect flexuous bittercress numbers (P < 0.0001,
Seed | Mulch | Number and percent germination | Shoot fresh weight (g) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
placement | depth (cm) | 4 WAP | 8 WAP | 12 WAP | 16 WAP | Cumulative | Establishment (%) | 4 WAP | 8 WAP | 12 WAP | 16 WAP | Cumulative |
Above | 0 | 11.3 | 29.8 | 22.5 | 23.8 | 87.5 | 58 | 1.3 | 7.5 | 4.5 | 3.6 | 16.9 |
0.6 | 4.2 | 3.3 | 7.8 | 13.7 | 29.0 | 19 | 0.6 | 3.4 | 3.7 | 5.3 | 13.1 | |
1.3 | 0.0 | 0.2 | 0.2 | 1.0 | 1.3 | 1 | 0.0 | 0.0 | 0.0 | 0.1 | 0.1 | |
2.5 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | |
L***Q*** | L***Q*** | L***Q*** | L***Q*** | L***Q*** | L***Q*** | L***Q* | L***Q* | L** | L*** | L***Q*** | ||
LSD0.05y | 3.6 | 3.8 | 4.3 | 7.5 | 3.3 | 2.9 | 2.3 | 5.3 | ||||
Below | 0 | 17.7 | 6.3 | 0.3 | 0.3 | 24.7 | 62 | 7.3 | 5.4 | 0.0 | 0.4 | 13.2 |
0.6 | 23.3 | 5.0 | 0.7 | 0.0 | 29.0 | 73 | 7.3 | 3.0 | 1.2 | 0.0 | 11.6 | |
1.3 | 17.8 | 2.3 | 0.2 | 0.0 | 20.3 | 51 | 2.2 | 1.5 | 0.5 | 0.0 | 4.2 | |
2.5 | 6.3 | 1.7 | 0.0 | 0.0 | 8.0 | 20 | 0.1 | 2.9 | 0.0 | 0.0 | 2.9 | |
L* | NS | NS | NS | L*** | L*** | L** | NS | NS | NS | L*** | ||
LSD0.05 | 7.8 | NS | NS | NS | 7.9 | 13.9 | 3.6 | NS | NS | NS | 6.2 |
zL and Q represent linear and quadratic rate response to rice hull depth, respectively. *, **, or *** represent significant rate responses at the 0.05, 0.01, or 0.001 level, respectively. .yLeast significant difference where α = 0.05.
tainers seeded below the mulch layer compared to those above (16.3 vs. 3.9 seedlings, P < 0.0001). A similar response with respect to both rice hull depth and seed placement was observed with flexuous bittercress shoot fresh weight.
Repeated measures analyses showed that flexuous bittercress numbers and shoot fresh weight when seeded above the mulch layer were affected by the interaction of time and rice hull depth (P < 0.0001) (
Weed numbers among containers with flexuous bittercress placed beneath the mulch changed over time (P < 0.0001). Flexuous bittercress numbers declined sharply from 4 to 8 WAP, as many of the applied 40 seed had germinated by the first harvest date. Flexuous bittercress numbers continued to decline over time in each treatment. There were no differences in bittercress number or shoot fresh weight with respect to rice hull depth from 8 to 16 WAP. Lack of differences among mulch treatments was due to there being relatively few bittercress germinating after the first harvest.
There was an interaction between seed placement and rice hull depth on percent establishment (P < 0.0001). Non-mulched controls in both groups had similar percent establishment (
At 4 WAP, creeping woodsorrel number and shoot fresh weight were affected by an interaction between seed placement and rice hull depth (P < 0.0001;
Repeated measures analyses showed that rice hull depth and time interacted to affect
Seed | Mulch | Number and percent germination | Shoot fresh weight (g) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
placement | depth (cm) | 4 WAP | 8 WAP | 12 WAP | 16 WAP | Cumulative | Establishment (%) | 4 WAP | 8 WAP | 12 WAP | 16 WAP | Cumulative |
Above | 0 | 10.0 | 20.5 | 28.8 | 30.2 | 89.5 | 60 | 2.4 | 12.4 | 5.8 | 4.5 | 25.1 |
0.6 | 1.0 | 2.2 | 4.5 | 10.5 | 18.2 | 12 | 0.6 | 4.7 | 1.2 | 2.4 | 9.0 | |
1.3 | 0.2 | 1.0 | 2.8 | 12.7 | 16.2 | 11 | 0.1 | 1.6 | 1.8 | 2.8 | 6.3 | |
2.5 | 0.0 | 0.0 | 0.2 | 0.7 | 0.8 | 1 | 0.0 | 0.0 | 0.4 | 0.5 | 0.9 | |
L***Q** | L***Q*** | L***Q*** | L*** | L***Q*** | L***Q*** | L***Q** | L** | L* | NS | L***Q* | ||
LSD0.05y | 3.6 | 4.3 | 13.5 | 18.2 | 6.5 | 3.5 | NS | 10.2 | ||||
Below | 0 | 24.0 | 6.5 | 1.2 | 1.3 | 33.0 | 83 | 7.3 | 4.9 | 2.0 | 2.4 | 16.6 |
0.6 | 20.8 | 6.2 | 1.5 | 0.7 | 29.2 | 73 | 6.5 | 3.9 | 1.5 | 0.2 | 12.1 | |
1.3 | 6.7 | 6.8 | 1.0 | 1.2 | 15.7 | 39 | 0.6 | 3.4 | 0.1 | 1.1 | 5.2 | |
2.5 | 0.8 | 2.2 | 0.8 | 0.0 | 3.8 | 10 | 0.0 | 0.0 | 0.9 | 0.0 | 0.9 | |
L*** | NS | NS | NS | L*** | L*** | L*** | L*** | NS | NS | L*** | ||
LSD0.05 | 6.9 | NS | NS | NS | 6.5 | 14.0 | 2.4 | 2.7 | NS | NS | 5.3 |
zL and Q represent linear and quadratic rate response to rice hull depth, respectively. *, **, or *** represent significant rate responses at the 0.05, 0.01, or 0.001 level, respectively.. yLeast significant difference where α = 0.05.
creeping woodsorrel number seeded above (P = 0.0454) and below the mulch (P < 0.0001). Among containers seeded above the mulch, numbers increased from 4 to 16 WAP in non-mulched controls. Among these same treatments, all rice hull depths reduced creeping woodsorrel numbers compared to non-mulched controls, and there were no significant differences between those mulched with 0.6 to 2.5 cm.
Among containers in which seed was placed beneath the rice hulls, creeping woodsorrel responded similar to flexuous bittercress over time. At 4 WAP, creeping woodsorrel numbers were higher in containers with 0 or 0.6 cm rice hulls compared to those with 1.3 or 2.6 cm rice hulls. From 8 to 16 WAP, there were no differences in creeping woodsorrel number from seed placed below the mulch layer. Shoot fresh weight had a similar response to rice hull depth and time as weed number.
Rice hull depth and seed placement affected percent creeping woodsorrel establishment (
Light penetration through rice hulls decreased exponentially with increasing rice hull depth (
Rice hull depth (cm) | PAR | Blue | Green | Red | Far red | |
---|---|---|---|---|---|---|
(400 to 700 nm) | (450 to 499 nm) | (500 to 569 nm) | (620 to 699 nm) | (700 to 799 nm) | Red: Far red | |
(µmol∙m−2∙s−1) | ||||||
0?unobstructed | 1846.44 | 284.74 | 456.94 | 556.75 | 591.72 | 0.941 |
0?beneath glass plate | 1500.61 | 234.51 | 378.85 | 441.36 | 440.78 | 1.001 |
0.3 | 48.85 | 2.49 | 7.41 | 27.72 | 55.40 | 0.500 |
0.6 | 3.30 | 0.09 | 0.51 | 2.06 | 6.97 | 0.296 |
1.0 | 1.06 | 0.07 | 0.36 | 0.36 | 0.63 | - |
1.3 | 0.92 | 0.08 | 0.35 | 0.24 | 0.07 | - |
1.9 | 0.56 | 0.05 | 0.26 | 0.09 | 0.01 | - |
2.5 | 0.71 | 0.06 | 0.28 | 0.18 | 0.03 | - |
LSD0.05z | 69.31 | 11.208 | 17.446 | 19.615 | 19.584 | 0.234 |
zLeast significant difference where α = 0.05.
ponential reduction in light transmission from litter of the annual grass Setaria faberi Herm., the perennial herb Solidago spp. (mostly S. Canadensis L.), and leaves of the hardwood tree Quercus alba L. All rice hull mulch depths reduced transmission of blue and green spectra below that which passed through the glass plate alone, and there were no differences among mulch depth (>0 cm) with respect to blue and green light intensity. Rice hulls selectively allowed greater penetration of light in the red to far red wavebands (>600 nm) (
Flexuous bittercress germinated in Petri dishes at 1 WAS beneath all shade treatments (
length and etiolated. At 100% shade, seedlings were etiolated, had elongated hypocotyls up to 1 cm in length, and yellow cotyledons.
At 1 WAS, creeping woodsorrel germination increased linearly from 0% shade up to 99% shade (
Shade | PAR | Flexuous bittercress | Creeping woodsorrel | ||
---|---|---|---|---|---|
1 WAS | 3 WAS | 1 WAS | 3 WAS | ||
(%) | µmol∙m−2∙sec−1 | (%) | (%) | (%) | (%) |
0 | 336.3 | 36 | 63 | 14 | 80 |
22 | 261.6 | 26 | 76 | 22 | 96 |
61 | 131.5 | 29 | 78 | 26 | 98 |
88 | 41.3 | 34 | 90 | 40 | 96 |
99 | 0.3 | 41 | 71 | 49 | 93 |
100 | 0.0 | 8 | 13 | 16 | 90 |
NSz | Q*** | L* | NS | ||
LSD0.05y | 16 | 18 | 22 | NS |
zNS, Q, and L represent non-significant, quadratic, and linear response to shade level, respectively. * or *** represent significant rate responses at the 0.05 or 0.001 level, respectively. yLeast significant difference where α = 0.05.
averaged 92%. This is in contrast to work by Holt [
In Expt. 1, a greater percentage of water passed through rice hulls than peat moss at the initiation (week 0) of the experiment (
In both experiments, rice hulls retained less water than pine bark and peat moss at each time point following irrigation in which water content was determined, and at every week data was collected (
Experiment 1 | Experiment 2 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Volume of water retained | Volume of water retained | ||||||||||
Week | Mulch type | Water passed through mulch | 0 hr | 1 hr | 4 hr | 24 hr | Water passed through mulch | 0 hr | 1 hr | 4 hr | 24 hr |
(%) | (mL) | (%) | (mL) | ||||||||
0 | Pine bark | 69 | 138 | 130 | 96 | 64 | 147 | 139 | 116 | 96 | |
Peat moss | 53 | 120 | 104 | 79 | 43 | 128 | 121 | 106 | 79 | ||
Rice hulls | 89 | 26 | 20 | 14 | 88 | 29 | 25 | 18 | 15 | ||
LSD0.05z | 23 | 9 | 19 | 8 | 18 | 8 | 6 | 14 | 5 | ||
2 | Pine bark | 75 | 145 | 136 | 124 | 73 | 92 | 172 | 161 | 147 | 119 |
Peat moss | 68 | 158 | 147 | 135 | 81 | 87 | 159 | 151 | 139 | 112 | |
Rice hulls | 90 | 54 | 48 | 44 | 34 | 96 | 65 | 58 | 53 | 48 | |
LSD0.05 | 13 | 8 | 8 | 8 | 9 | 6 | 10 | 9 | 9 | 9 | |
4 | Pine bark | 77 | 142 | 134 | 122 | 72 | 89 | 169 | 160 | 149 | 126 |
Peat moss | 87 | 173 | 166 | 155 | 101 | 86 | 164 | 173 | 144 | 122 | |
Rice hulls | 91 | 64 | 58 | 49 | 31 | 92 | 68 | 60 | 53 | 43 | |
LSD0.05 | NS | 10 | 9 | 9 | 8 | NS | 9 | 29 | 8 | 7 |
zLeast significant difference where α = 0.05. NS means non-significant.
expected considering one of the most important characteristics of peat moss is its capacity to absorb and internally retain large quantities of water. The amount of water held by peat moss can be 15 to 20 times its own weight, depending on peat moss type [
The layer of mulch within the round Buchner funnels of this experiment was disc-shaped with a height of 2.5 cm, diameter of 13.1 cm, and volume of 342 cm3. Thus volumetric water content (VWC) in each mulch layer could be calculated for the volumes of water retained listed in
Rice hulls do not exclude enough light at depths of 2.5 cm to prevent bittercress or creeping woodsorrel 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 bittercress germination declined when PAR dropped below 41.3 µmol∙m−2∙sec−1, germination still occurred in complete darkness. Creeping woodsorrel 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 woodsorrel. It is widely accepted that germination rate of seed decreases with increasing burial depth [
A meta-analysis of seedling recruitment in natural grasslands found that plant litter up to 500 g∙m−2 improved seedling recruitment, while greater litter amounts (>500 g∙m−2) inhibited seedling recruitment [
Flexuous bittercress 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 bittercress were reduced 78% compared to the non- mulched control. In contrast, creeping woodsorrel numbers and shoot fresh weight from beneath 2.5 cm rice hulls was reduced 88% and 94%, respectively, compared to non-mulched controls. Flexuous bittercress seed used in this study were 0.9 ± 0.13 mm long and 0.6 ± 0.06 mm wide (n = 10 seeds), while creeping wood sorrel were larger with length of 1.4 ± 0.14 mm and width of 1.0 ± 0.05 mm (n = 10). Others have shown decreasing sensitivity to mulch depth with larger seed [
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. [
Flexuous bittercress and creeping woodsorrel disseminate seed with a ballistic dispersal mechanism. A closely related bittercress species, hairy bittercress (C. hirsuta), can project seeds from 1 to 5 m [
Seed present on the substrate surface at the time of mulch application are not controlled as well as those introduced after rice hull application. This may be a limitation to the use of rice hulls for weed control. Based on these results and that of previous research [
This research was in part funded by the Floriculture Nursery Research Initiative and the Horticulture Research Institute.
Altland, J.E., Boldt, J.K. and Krause, C.C. (2016) Rice Hull Mulch Affects Germination of Bittercress and Creeping Woodsorrel in Container Plant Culture. American Journal of Plant Sciences, 7, 2359-2375. http://dx.doi.org/10.4236/ajps.2016.716207