Although these organic herbicides are generally recommended for control of emerging weeds, especially during the early spring, I applied these materials during September-October of 2003, primarily because of availability of materials but also to determine their activity in the fall against established weeds. Another note about OMRI, this institute reviews materials primarily in terms of their ingredients and general purpose but not their efficacy. Our objective here was to determine the efficacy of three OMRI-approved organic herbicides in field trials. Trials are also being conducted by other IFAS faculty.

Three of these broad spectrum, contact herbicides herbicides, AllDown, Matran 2, and Xpress were compared with Roundup Pro, a systemic broad spectrum herbicide in a conventional commercial grove in Vero beach, at the Plant Science Research and Education Unit (PSREU) near Citra and in the Horticultural Sciences Department citrus grove in Gainesville, Florida (Table 1). Roundup Pro is not an approved organic herbicide but I wanted to include a conventional comparison standard. At all sites, herbicides were applied once with a backpack sprayer, at approximately 35 psi between 11:00 a.m. and 2:00 p.m. during sunny days with low wind and with daily temperatures ranging from approximately 60 to 80 F. No rain fell within two days after application.

Treatments, including a control, were replicated four times in a randomized complete block design with plots at each site ranging from 25 to 300 ft ₂. Treatments at Vero Beach were applied on September 18 in the grove middles of a conventional young tree planting at recommended rates. Since the three organic herbicides applied at the Vero Beach site did not effectively control grass and broadleaf weeds, compared with Roundup Pro within four days after treatment (Table 2), herbicides were again applied on October 13 at PSREU at recommended rates with and without an OMRI-approved adjuvant, Nu-Film 17. Better but inconsistent control of grass and broadleaf weeds was again observed at the PSREU site until October 31 as compared with Roundup Pro. Consequently, all four herbicides were then applied at twice the recommended concentration and application rate with and without a adjuvant at the Gainesville site, again with the organic herbicides having an inconsistent and largely minimal weed control compared with Roundup Pro. Weed populations were somewhat different and this could account to some degree for the differences in weed control. Herbicide concentrations and application methods must be more carefully controlled when such small plot sizes are involved but materials were applied by the same person and herbicide concentrations and application rates were carefully prepared.

All herbicides were applied during September and October when weed growth was slowing down and some species, especially pigweed and nutsedge, were producing seeds. However, newly emerged nutsedge in recently cultivated areas was not controlled by the organic herbicides either. Neither AllDown or Xpress require an adjuvant but one is recommended for Matran 2. However, the addition of an adjuvant to these organic herbicides did not improve weed control.

The Xpress and Matran 2 labels specify application to actively growing weeds from 1 to 4 and to young weeds less than 2 inches tall, respectively, emphasizing their use as early season herbicides. Our fall applications of organic herbicides to larger weeds, some within the specified size range and others taller and producing seed, could partially explain poor weed control. Repeat applications are also recommended for Matran 2 and for other conventional acidic herbicides and could have improved weed control. However, an earlier July applications of AllDown applied as a spot treatment to grasses and broadleaf weeds was effective on actively growing grasses and broadleaf weeds, perhaps emphasizing the importance of weed growth stage and temperature.

Grasses and broadleaf weed species were different at each site but included Alexander grass, bahia grass, Bermudagrass, carpetweed, crabgrass, dayflower, goosegrass, hairy indigo, lambs quarters, Mexican tea, Florida pusley, goatweed, Mexican clover, nutsedge, pale meadowbeauty, pigweed, shrubby primrose willow, broadleaf signalgrass, Southern sandbur, spurge, torpedograss, Wild radish, and purslane. All weeds were identified at the University of Florida herbarium. Given the mix of grass and broadleaf weeds at each site, weed control for grasses and broadleaf weeds but not specific species was recorded.

At the Gainesville two mandarin hybrid rootstock seedlings, about 2 years old and without a recent growth flush, were planted in each replicate of each of the four treatments and sprayed to runoff with each of the herbicide treatments. Seedlings treated with Roundup Pro had heavy leaf damage and were dying after 2 weeks. Seedlings treated with the organic herbicides had about 10% leaf spotting and dieback but were not seriously damaged.

Early spring applications of these materials are also planned for 2004 to complete this field study. According to manufacturers’ recommendations, these materials are probably best applied as an early post-emergent herbicide when weeds are 1 to 4 inches tall and are actively growing rather than at the end of the growing season in the fall.