Weed Control in Overwintered Shallots (1984)

Research report from OSU's North Willamette Agricultural Research and Extension Center

Delbert Hemphill
OSU Dept of Horticulture, NWREC

Shallots can be planted in October for harvest the following summer. A major problem in overwinter shallot culture is weed control. Winter rains make tractor cultivation or hand-hoeing nearly impossible and effective herbicides have not been available. These trials were designed to test the weed control effectiveness of four herbicides applied immediately after planting and their effect on crop fields.


In both 1982 and 1983, "French red" shallots were planted on 0.5 foot x 2.0 foot spacing. Plots were 4 rows wide x 10 feet long with 4-foot borders. Six treatments were applied in completely random design with three replications of each treatment. The plot area received a broadcast application of 500 pounds/acre of 10-20-10 which was incorporated by rotary tilling. Shallots bulbs were set on October 14, 1982, and October 10, 1983. Herbicides were applied immediately after planting followed by an irrigation of 0.6 inches. All herbicides were applied with a hand-held sprayer in 50 gallons/acre of water. Treatments were: 1. Non-weeded check, 2. Hand-hoed (1983 only), 3. propachlor at 4.0 pounds/acre, 4. linuron, 1.0 pound/acre, 5. napropamide at 4.0 pounds/acre in 1982 --and 3.0 pounds/acre in 1983, 6. clorpropham at 4.0 pounds/acre. Herbicide-treated plots were not cultivated and no further herbicide applications were made. Plots were rated for weed control in February and May 1983, and in May 1984. Shallots were harvested July 21, 1983, and July 17, 1984.

Results and Discussion


Propachlor controlled all weeds for approximately five weeks after planting. By February, control was ineffective, with chickweed, several grasses, many mustards, dog fennel, and groundsel present on propachlor-treated plots. No injury to the shallots was observed. Linuron and chlorpropham provided much more effective and lasting weed control, although linuron did not control grasses. No crop injury was observed with either material. Napropamide provided nearly complete weed control. The only weed present at harvest was miner's lettuce, and at less than 0.1 plants/ft2. However, crop injury was severe. Severe shallot root dieback was evident by the May rating period.

Gross yields were highest with linuron and chlorpropham, lowest with napromide and the non-weeded check (Table 1). The number of shallots harvested/plot was significantly lower on napropamide treated and non-weeded plots than for the other treatments. Moreover, mean bulb weight was also lowest on non-weeded and napropamide-treated plots.

  Table 1. Effect of herbicides on shallot yield, 1983                Treatment       Yield (T/A)   # shallots/plot    Mean bulb wt. (g)    check               1.4             266                8.8              hand-hoed           5.6             450               19.6           clorpropham         7.3             496               24.8            linuron             8.5             495               28.8             napropamide         3.1             281               18.2         propachlor          5.0             399               21.1     LSD(0.05)        3.5             101                8.1          



All herbicides provided effective weed control for about five weeks after planting, and all herbicides except propachlor continued to provide effective control until early December. A severe freeze in late December (5°F) killed most emerged weeds. All herbicides continued to provide some control until early May (Table 2), when napropamide-treated plots had the smallest weed mass present. By harvest in July, all plots except the hand-hoed had extensive weed infestation.

No treatment completely controlled false dandelion and groundsel, but napropamide effectively controlled all grasses, chickweed, miner's lettuce, and shepherdspurse (Table 1). Weed control by linuron and chlorpropham was not as effective as in 1983, when these two herbicides were clearly superior to propachlor. The apparently improved performance of propachlor relative to linuron and chlorpropham in 1984 may have been from the winter-kill of emerged weeds on propachlor-treated plots.

All herbicides increased yield and mean bulb weight when compared to non-weeded check plots, but yields for all herbicide treatments were lower than those on hand-hoed plots (Table 3). Unlike 1983, there was no severe crop injury with napropamide, which was applied at a lower rate in 1984 than in 1983.

The importance of effective weed control in shallots is seen in the six-fold yield increase on hand-hoed compared to non-weeded plots. Effective early weed control can be obtained with several herbicides. However, complete reliance on chemical weed control will only be feasible if an effective burn-down herbicide can be applied in late winter without undue crop injury, followed by reapplication of a germination inhibitor such as propachlor.

The fact that these herbicides were included in this trial should not be taken to mean that the materials have been registered for shallots or other members of the onion family.

  Table 2. Effect of herbicides on fresh weight of weeds present on May 9, 1984                   Weed fresh         Weed species present in                Treatment        wt.(lb/ft2)        descending order by number                 Check               0.74            groundsel, fescue, orchard grass, annual                                       bluegrass, false dandelion, chickweed,                                       miner's lettuce, shepherdspurse.   chlorpropham        0.09            groundsel, false dandelion, annual                                       bluegrass, chickweed, shepherdspurse.  linuron             0.05            groundsel, chickweed, false dandelion,                                       annual bluegrass, shepherdspurse.  napropamide         0.01            false dandelion, groundsel.  propachlor          0.10            groundsel, false dandelion, chickweed,                                       fescue, annual bluegrass.        LSD(0.05)     0.06                                                           Table 3. Effect of herbicides on shallot yield, 1984                                   Total yield	   # bulbs	           Mean bulb  Treatment	   (T/A)       harvested/plot               wt. (g) 	  check               0.6              139                      7.6           hand-hoed           3.8              244                     25.5   chlorpropham        1.1              136                     14.0         linuron             1.6              149                     18.6    napropamide         1.1              132                     14.2      propachlor          1.6              187                     14.4         LSD(0.05)    0.8               67                      5.1