Comparing Alternative Weed Management Practices to Black Plastic in CBD Hemp Production

Results

Environmental Conditions

The 2021 and 2022 growing seasons were one of marked extremes.  Table 3 is a presentation of average temperatures,  and the very low rainfall in May and June and the abnormally warm weather in June allowed an earlier than expected planting date.  In 2022, we delayed planting about week at UMRF, but about 3 days later at VHF.   The 2022 season was more favorable for production as it was dry for planting, and there was enough moisture through the season for growth and development, but with less moisture, there was less disease pressure, particularly at VHF.  

Table 3.   Average monthly temperature and rainfall collected at UMRF – 2021 and 2022

  compared to 30-year averages

Early Growth and Development

Early Growth and Development

Weed management treatments led to differences in early hemp growth over both years (Figures 1a and 1b).  The 2022 season was slightly more moderate and the plants coming out of the greenhouse were healthier and better hardened off to get established quickly in 2022, leading to significant year*treatment and year*location effects.  The soils at the VHF farm with higher organic matter, sandy loam textures were more conducive for early growth than the Rogers Farm (UMRF) location.  We found a significant treatment x location interaction caused by the apparently slower growth in 2022 at the VHF site in the mowed rye (Figure 1c).  It is difficult to determine if the rye cover crop leaches out allelopathic chemicals that slow early hemp growth, if the biomass ties up N at the soil surface as microbes begin to break down the mulch, or if the lack of tillage reduces early root growth compared to the tilled soils slowing hemp production in these treatments.  Given the significant treatment by location interaction, it seems like the amount of rye biomass on the soil does affect production.  Given that the lack of tillage is consistent, it would seem that the winter rye could pull too much N and our fertilization was insufficient, and the reduced rye crop in 2022 could have left more N in the soil for the growing cannabis to utilize and produce larger plants, or the amount of immobilization is reduced because there is less mulch biomass to break down.  The UMRF site had about 40% less cover biomass in 2022 than in 2021 due to winter kill, and the hemp in that mowed rye treatment at UMRF produced more biomass and area in 2022 than it did in 2021.

Perhaps not surprisingly, the black plastic standard created the best early season growing conditions and the plants responded accordingly.  The sandy loam, high organic matter soil at VHF was significantly more conducive for early growth, and plants were almost twice as tall at VHF than at the UMRF at one-month post transplanting (Figure 1a and Figure 1b).

Figure 1a.  Hemp growth at one month after transplanting - 2021

Figure 1b. Hemp growth at one month after transplanting - 2022

Figure 1c. Hemp growth at one month after transplanting averaged over years.

Plant Analysis

We used a Minolta SPAD meter to evaluate leaf color on the last opposite and first alternate fan leaf by experimental treatment one month after transplanting.  Analyzing the last opposite leaf SPAD values collected across both sites with similar weed management treatments, we found that the mowed rye was significantly paler in color over both locations and years.  We also found the 2022 hemp plants had significantly deeper green color compared to 2021 likely due to plants adjusting better to the transplanting in 2022. 

The lowest SPAD value on the last opposite fan leaf and first alternate fan leaf was found with the mowed rye treatments at both sites (Figure 2a).  This could suggest that the paler green color is due to lower N due to possible immobilization, poorer root formation due to the untilled soil, due to allelopathic chemicals leaching out of the rye causing leaf yellowing, or a combination of the three factors.  We did not collect leaf tissue samples at this stage in plant development, but a later sampling would show lower leaf tissue total N in the mowed rye plots showing some immobilization occurring leading to the less deep green leaf color in those treatments over the two years.

Figure 2a.  Last opposite fanleaf SPAD values as affected by weed management over 2021 and 2022 growing seasons.

The ratio of the last opposite to the first alternate SPAD values were also different at the two sites across both years.  The growing conditions at VHF were more conducive to growth and development and had an average value of 1.  The UMRF location's was 0.965 which probably reflected the poorer soil conditions at that location. 

After using this test method in two experiments over two years, we now do not think the plants have adjusted from the transplanting or developed enough of a root system to reflect the real treatment differences at the early sampling period.  The sampling done at flower initiation has shown more sensitivity to treatment differences.  Unfortunately, we do not feel sampling leaf chlorophyll at a month post-transplant is an accurate test. This would be the preferred time to try to add N because the plant has another month to grow and develop before flowering, but the floral hemp plants do not exhibit deficiency well at this stage of growth.  We knew that different cultivars would likely have a different starting leaf chlorophyll value due to the different shades of green found on various cannabis varieties, and as such we were hoping that the ratio of the first alternate to last opposite leaf would reflect real N status, but as one can see it did not reflect that (Figure 2b).  Ratios of the first alternate branch pair fan leaf and last opposite branch pair were not sensitive to N status as hoped; there were no statistically significant differences in ratios across the weed management strategies (Figure 2b).

Figure 2b – Ratio of the first alternate to last opposite fan leaf SPAD values as affected by weed management over 2021 and 2022.

Flower Initiation – growth and development

By flower initiation, the resinous hemp plant growth had increased in size by three times at VHF and four times at UMRF over first month post-transplant plant sizes.  In all cases, the VHF site had larger and more advanced plants except in the mowed rye which had a significantly greater amount of winter rye mulch material to immobilize N than the amount of winter rye which survived the ice damage at the Rogers Farm site.  This led to a significant treatment by location interaction in the size of plants (Figure 3a).  Where the plants were allowed to thrive with little competition, the VHF black plastic and cultivated plants were almost twice the size of those at UMRF.  This speaks to the need to carefully select production sites (high organic matter, well drained soils are ideal) which characterized the VHF site.  Also, due to the lack of rainfall in June and early July in Stillwater, weed growth was considerably less productive in the controls than that found at VHF.  Thus, the hemp in the control plots at VHF were larger than those at UMRF, because those weeds received more natural precipitation and better competed for the resources available.

Figure 3a.  Plant size as affected by weed management at flower initiation (2-months post-transplant)

Flower Initiation – Number of Branch Pairs

Along with plant area at flower initiation, weed control influenced the number of branch pairs the plants produced.  One can further imagine that the number of branch pairs would affect the number of flower production sites. 

In this case, the response of the plants in the number of fully developed branch pairs was similar to what we found in plant size (area) (Figure 3b).  There was also a significant treatment x location interaction again caused by the mowed rye slowing growth.  If weeds were controlled by plastic mulch or cultivation, the plants at both locations were larger, and they had more branch pairs. 

Figure 3b.  Plant node number as affected by weed management at flower initiation (2-months post transplanting). 

Image #3.  Height differences caused by weed management measures.

The more intensive broadleaf weed population at VHF likely limited N and affected growth more than at the UMRF location, particularly during the dry June and July periods in 2022 (Table 3).  As well, plants in the mowed rye treatment were three times smaller than the black plastic at VHF while only two times smaller at UMRF.  Black plastic appeared to be more useful at UMRF than at VHF.  Again, the soil type was more conducive for cannabis production at VHF than UMRF, and the difference in development in the mowed rye plot was greater at VHF than UMRF. 

Flower initiation – leaf SPAD

Although the fertility management was different at the two locations, leaf color was surprisingly similar across locations.  To check the fertility status of cannabis plants, the CSDRE (2018) recommends sampling the fan leaf from the 4^th fully developed node from the top of the plant at flower initiation.  Following this model, we measured fan leaf SPAD values at the 4^th most fully developed branch pair from the top of the plant.  We also counted four branch pairs down from that and collected SPAD measurements at the 8^th fully developed branch pair from the top.  We then developed ratios of the 8^th to the 4^th to see if N might have been limited by weed management at either location (Table 4).  Fan leaf total N was also collected at VHF in both 2021 and 2022, but due to a misunderstanding was not done at UMRF, in 2021, but was collected in 2022.

Table 4.  Fan leaf SPAD values at the 4^th and 8th plant nodes and the ratio to assess apparent plant health and N status.

In all these cases, leaf SPAD values' response to the weed management were generally similar across locations.  The 4^th node leaf SPAD value reflected well the plants' response in size and node number (Figures 3a and 3b).  Reducing weeds by black plastic or cultivation generally led to the largest plants and deepest green plant color.  Interestingly, the fan leaf SPAD values at the 4^th node correlated the most highly to the most fully mature fan leaf N content and the relationship was statistically significant (Figure 4).

Figure 4.  Relationship of 4^th node fan leaf to plant N status.

Harvest - Weed Biomass

Weed biomass samples were collected, dried, and weighed to assess weed management effects just prior to harvest.  The black plastic weed biomass was collected from between the rows.  The remainder of the samples were collected within the plot at random.  The weed dry matter production is presented below in Table 5. 

Generally, weed biomass was lower in 2022 than in 2021, in part due to the very dry June and July. 

The biggest difference was the weed production in the mowed winter rye plots; there was much more production in 2021 than in 2022.  Due to the dry weather, we were able to water the cannabis but not the weeds at large, particularly at UMRF.  Only weeds growing in the drip line moisture produced well during that period.  The weed management treatments led mostly to similar weed levels by treatment and location.  Tilling the soil appeared to bring up more annual broadleaf weeds which when not controlled led to significantly higher biomass compared to the mowed rye which had no tillage.   Despite plowing under the previous crop residue and clover remaining in the plot from the previous year, the rye residue was not sufficient by itself to keep the clover from growing through the residue and competing particularly in the first season

Table 5.  Weed dry matter yield at harvest 2021 and 2022 across location.

Numbers followed by letters that are different are statistically significantly different.  

Tmt signifies Treatment; Yr signifies Year; and Loc signifies Location.  

Plant Size at Harvest

Cannabis plants were larger at VHF at harvest over both years due to the much more favorable soil type and high organic matter matter content (Table 2).  In 2022, the mowed rye treatment at VHF had smaller plants the other weed management treatments, and the difference between the mowed rye at VHF and UMRF was less than the other treatments.  This again was due to the higher amount of winter rye slowing down development at VHF relative to UMRF. 

Black plastic and cultivation treatments managed weeds well, and the plants were not limited by weed pressure, allowing the plants to approach and occasionally exceed 3 m2 in size (Figure 5a). 

Figure 5a.  Plant area at harvest.

Harvest:  Yield and Cannabinoid Content

Plant yield, whole plant N, and cannabinoid concentrations and yield are presented in Table 6.  The 2022 production year was generally more favorable across all weed management measures.  Dry matter yield was higher in 2022 than 2021, but there was a treatment by year interaction caused by the improved yield of the mowed rye hemp at UMRF in 2022 relative to 2021 (Figure 5b). 

Figure 5b.  Significant treatment x year interaction due in part to the improved mowed rye yield at UMRF in 2022. 

Both whole plant and flower samples were tested for cannabinoid content and cannabinoids yield (Table 6).  Weeds limited yield in plots with either no cultivation or winter rye mulch likely due to weed competition for water and nutrients. The VHF site yielded significantly higher amounts of biomass due to the improved soil conditions, more heat, and more intermittent showers in June and July than what was found at UMRF.  Flower dry matter yields also mirrored whole plant yields. 

Whole plant CBD (measured in mg/g) oppositely mirrored yield as the larger the plant, generally the lower the whole plant CBD concentration.  What was less expected was the lack of a response of flower cannabinoid concentration by weed management.  Little research has been done to evaluate plant stress on cannabinoid concentration, but it was generally assumed that the plant produced cannabinoids and terpenes as a protection against stressors like insects, drought, etc.  We initially expected that intense weed pressure in the control plots would spike CBD and THC concentrations; however, these data and work recently published by Toth et al., 2020 indicated that genetics and time to harvest will influence cannabinoid content more than apparent plant stress.  Analyzed separately or across years, weed pressure did not appear to influence cannabinoid concentration.  The location differences in 2022 were not as strong as 2021.  Plants were transplanted a week later in 2022 than was done in 2021, and the one week less vegetative production likely reduced the differences between locations.  Cannabinoid yield did reflect the weed management based on its effect on whole plant or flower dry matter yield.

Table 6.  Whole plant and flower dry matter yield and cannabinoid concentrations and yield as affected by weed management measures.

As one might expect, whole plant and flower dry matter yield generally reflected plant size differences found throughout the study.