Learn how to manage weeds of field-grown cut flowers

Larger direct seeded species such as zinnia, above, and sunflower usually can tolerate registered herbicides. But most smaller seeded species, if direct seeded and immediately exposed to a pre-emergent herbicide, will be susceptible to injury. (Getty Images photo)

HERSHEY, Pa. — Dr. Andrew Senesac, of Cornell University Extension, outlined numerous options for managing weeds of field grown cut flowers at the Mid-Atlantic Fruit and Vegetable Convention.
Senesac pointed out, “The first opportunity for weed management often occurs the season before the crop is planted. It is possible to avoid planting on land that is seriously infected with difficult-to-control weeds if the grower knows what to look for.”
He added, “Creeping perennial weeds such as yellow nutsedge, mugwort and hedge bindweed are very difficult to control once a flower crop is planted.”
If site avoidance is not possible, then manage these during the season before planting, he noted. Repeating applications of a non-residual postemergence herbicide and/or repeating disking or harrowing will usually bring the weeds down to a more manageable level.
Growers may consider chemical soil fumigation in the fall or spring prior to planting.
Consider fumigation if other serious soil and soil-borne pests exist or if no other chemical means of weed control are considered. While effective for seed-propagated weed species, fumigation is expensive and it controls creeping perennial weeds poorly.
Pre-emergent herbicides are sometimes applied pre-plant, but are usually applied post-plant or post-transplant, and always pre-emergent to the weed.
They are generally active on germinating weeds, but usually need either incorporation by cultivation or water to activate and move the chemical into the soil where the weed seeds are germinating. Some can be applied prior to transplanting. However, having the transplant roots below the herbicide layer enhances the cutflower safety.
The larger direct seeded species such as zinnia and sunflower usually can tolerate registered herbicides. But most of the smaller seeded species, if direct seeded and then immediately exposed to a pre-emergent herbicide, will be very susceptible to injury. Senesac cautioned growers to always follow the label.
In general, crop safety increases with larger transplants, the lowest level of application, application after transplanting for better root establishment, granular rather than spray formulations, and sprays directed to the base of the plant.
Senesac noted that some post-emergence herbicides selectively control only emerged grassy weeds. Sethoxydim (Segment, Poast) and clethodim (Envoy and others) are now labeled for over-top application. They control grasses but not sedges or broadleaves but can rescue the crop if grassy weeds are not a problem.
Nonchemical methods can also help prevent weeds from becoming economically damaging. Proper fertility and placement, irrigation and pH management as well as well-adapted cutflower cultivars can be effective. Senesac pointed out, “Weeds are great opportunists and will take advantage of any condition which tends to stress the crop.”
He reminded growers that mulches can be effective and practical for control, particularly with transplants in multi-crop, low acreage operations. Crops compete better when harmful insects and diseases are controlled.
Hand weeding and rogueing escaped weeds before they disperse their seed will help the following year’s problem.
Also, narrower spacing in the rows and between rows allows quicker bare ground coverage which shades weeds more effectively.
Senesac stressed the importance of developing some or all of these tools well before the growing season starts.
Senesac recommended consulting the “Cornell Pest Management Guidelines for Greenhouse Crops and Perennials.” Cornell’s guidelines are available at cropandpestguides.cce.cornell.edu. Or, a specific search with the above title will yield the latest versions.