Coprosma acerosa is a prostrate shrub that grows mainly from a central stem and only roots occasionally from stem nodes. It is therefore not surprising that the two treatments using weed mat produced the largest spreading plants, since weed mat very effectively keeps out weed competition for soil resources. The paper mulch also produced good plant growth while it lasted, since it produced a similar barrier between light and soil resources. This prevented weed seeds from both wind-carried sources and the dormant seeds in the soil seed bank from germinating and establishing. However, plant growth suffered once the paper cover degraded. The wood chip treatment produced larger plants near the end of the trial, when it had thinned out considerably. The wood-based mulches did not produce larger plants, perhaps due to soil nitrogen immobilisation during the decay process. The bare soil plots produced the smallest plants, presumably due to increased weed competition and increased water loss from evaporation.
For the herbicide treated plants, there was also the possibility of a growth check on the plants as a side effect, although no visible damage symptoms were observed in this trial on C. acerosa. The weeds in the herbicide treated plots also remained in the plots after herbicide application, and so may have continued to exert competitive pressure on the ground cover plants if they recovered during the interim period between herbicide applications. Such effects would have been absent in handweeded plots and accounted for the better growth provided weed volume was low. Furthermore, the full results from Chapter 5 were not available at each herbicide application. With the knowledge from trials in Chapter 5 which showed that each of these three species were tolerant of a wide range of herbicides suited for control of different weed conditions, better results could have been obtained. Except for the simazine application in Sedum mexicanum plots, no other residual herbicides were used. The performance of the ground covers in this trial may improve in
herbicide treated plots if well-informed decisions can be made about herbicide choices and suitable combinations.
Persicaria capitata, with its scrambling nature and freely rooting stem nodes, established best in treatments which allowed full access to bare soil, these being the handweeded and herbicide treated plots. The paper mulch plots also produced good plant growth after the paper had degraded. The weed mat treatment also produced larger plants despite not providing bare soil access. However, this may be a temperature related effect, as the black weed mat produced the highest soil temperatures during the day, and this treatment appeared to produce better results only in warmer months. It is likely that it did not perform well in wood-based mulches due to difficulty in accessing soil nutrients from the shallow roots from stem nodes. Amongst the three wood-based mulches, the wood chip treatment produced the largest plants, but presumably because this treatment had thinned out the most and so had more bare soil. The weediness of the plots also did not seem to deter Persicaria capitata growth; perhaps the freely scrambling nature of the stems enabled it to quickly root at the next most opportune spot.
Despite the speed with which P. capitata plants covered the whole plot area and the lush foliage in summer, this species is unfortunately very frost tender. An early frost in April caused a rapid decline in ground coverage as frost damage caused the leaf canopy to dwindle and the area covered became exposed in winter. This led to problems with ground cover establishment as weed seeds which germinated in spring shaded the new leaves of the ground cover. The prolific seeding of
P. capitata was also limited in its ability to compete with weed seedlings, as the earlier germinating
P. capitata seedlings were killed by late frosts. The weed deterrence ability of this species was unfortunately hampered by its deciduous habit.
Sedum mexicanum was noted to be susceptible to damage from rabbit burrowing and
pūkeko prodding due to the brittle stems. The most heavily burrowed treatment was the sawdust
plots; ironically, the resilience of Sedum stems led to the biggest plants to be found in sawdust plots. The scatter and subsequent regeneration from stem fragments led to best ground coverage. The next best treatments were the weed mat and bark nugget and paper mulches. These treatments also showed no evidence of rabbit burrowing, though some burrowing occurred on paper mulch plots after the paper degraded. The weed mat under wood chip treatment also dissuaded further damage from rabbit burrowing once the wood chips were scattered, hence this treatment managed to aid plant establishment without relying on regenerative chance. The bare soil in handweeded and herbicide plots had the most burrowing action. The wood chip treatment also revealed bare soil after the wood chip layer was scattered, and just as much damage to the plants was done as well. The
better stem regeneration in sawdust may have been due to the improved friability of the sawdust particles after burrowing which aided root establishment. The better warmth retention of sawdust in summer months, plus the moisture absorbance, may have also contributed to successful recovery.
6.9 Conclusion
In general, ground cover plant species which do not, or poorly, root along their stems like
C. acerosa probably perform best when planted with mulches which blanket the soil from light for as long as possible. This contrasts with ground cover plants with rooting stem nodes which perform better when contact with soil or friable media is permitted, which allows for more nutrients or warmth. If the plant species could also regenerate from plant fragments, chance damage may inadvertently promote its spread.
The performance of herbicide treatments had been handicapped in this trial because full information from the herbicide trials in Chapter 5 were not yet available, so the best herbicide solutions were not implemented to assist with ground cover establishment. The decision to allow the weeds to be left in the plots to die when the most effective herbicides were not used, also led to poorer mortality and may have allowed dying weeds to successfully set seed. With results from Chapter 5, the right herbicides may be used for more effective results.