A moderate response rate was achieved (43%), with the majority of respondents coming from within
the sampling frame, i.e. organically certified producers of herbs and vegetables. The reported farm
sizes were small (median = 2.4 hectares) and the respondents had relatively little experience with
organic farming (median = 6 years). Producers grew a wide range of herb and vegetable crops
(median = 3) and reported numerous types of weeds on their farms. The most common weeds were
predominantly those with persistent underground parts that resist common forms of organic weed
control such as cultivation (by hand or plough) and mulch. Heavily seeding annuals were also
frequently reported. Respondents expressed strong concern about weed management, particularly
regarding the difficult and time consuming nature of OWM and the impact of weed competition in
reducing crop yields.
The respondents reported using a diverse range of weed management techniques, with over 40
specific methods or strategies being mentioned. The most common method was manual weeding,
which was sometimes used as a central management technique or as a final “clean up” after relying
on other methods such as tillage or mulch for the bulk of the weed control. Other commonly
reported methods were organic mulches; tillage (especially rotary hoes); cultural methods such as rotations, bed preparation, timing of operations, cover crops and inter-cropping; slashing and/or
mowing; and grazing. Less frequently reported OWM methods included thermal methods, synthetic
mulches, organic sprays and farm hygiene practices.
The results for perceived success of weeding methods used were loosely correlated with the
regularity of use. Therefore, manual weeding, organic mulches and tillage were reported to be the
most successful weeding methods. No relationship was found between the perceived expense of a
weeding method and the regularity of use of that method. This suggests that growers were not
primarily motivated by the cost of a weeding method; instead, they were more concerned that it was
successful in controlling weeds. Synthetic mulches and thermal weeding were considered to be the
most expensive weed control methods.
Results from the OWM survey and the US-based Organic Farming Research Foundation (OFRF)
survey (Walz 1999) highlight a number of possible research issues of concern to organic growers.
Improved control of particularly problematic weeds, such as couch, sorrel and nutgrass, was a
common area of concern. Studies on the biology and ecology of such weeds may identify lifecycle
vulnerabilities that can be exploited by modified management practices. There also appeared to be
some interest from respondents in novel techniques such as flame weeding, steam weeding, improved
tillage implements and organic sprays, although these methods require further development too.
Cultural strategies such as techniques for weed seed bank reduction and the development of weed
suppressing cover crops that don't become weeds themselves were also of interest to respondents.
For lettuce, a crop with a short growing season, cheaper weed control methods such as tillage with
limited follow-up hand weeding may be sufficient to ensure a reasonable crop yield. The acceptable
economic return of the control treatment suggested that good weed control in the cropping area prior
to planting may even be adequate. Hay mulch provided good yields but was less cost-effective due
to the high labour requirement for mulch laying and follow-up hand weeding. However, this
treatment greatly reduced bolting in the lettuces and may give more flexibility in terms of harvesting
than tillage or hand weeding. Paper mulch stunted the growth of lettuce, presumably due to nitrogen
immobilisation, although it also provided excellent weed control. It could not be recommended
without reformulation with a nitrogenous fertiliser or some other method of improving available soil
nutrient levels.
The results from the echinacea trials suggest that more expensive weeding methods are cost-effective
for longer-season and higher-value crops. Hand weeding and hay mulch both provided cost-effective
weed management. The differing effects of those two treatments on soil structure, carbon and
moisture conservation were not evaluated, although these factors may be considered important by
growers. The paper mulch again reduced crop yields and, combined with high purchase cost,
appeared to be unreliable. The moderate adjusted crop value (ACV) (derived by subtracting the cost
of each treatment at each site from the gross crop values) for paper mulch at Yarrowitch suggested
that the treatment may have some potential as a weed control method in certain circumstances.
The economic analysis of the treatments is specific to the time and location of the trials, and
outcomes may vary depending on the availability of labour, machinery and implements, and suitable
mulch materials. However, similar trends have been reported in other organic and conventional
trials.
For both crops the ranking of the weed control treatments for their weed biomass accumulation at
crop harvest was hand weeding = paper mulch followed by hay mulch, tillage and then the control
treatment. Weed control levels ranged from 98% through to approximately 60% for the tillage
treatments.