Weed Control in Cut Flowers

“Specialty cut flowers” encompasses a wide range of flowering plants. Controlling weeds in production systems requires an equally diverse number of options.

Due to the wide diversity of genus and species among cut flowers, managing weeds often requires different approaches depending on the specific weed, specific flower or woody stem, time of year, and level of weed infestation. We are often trying to manage one weed while producing another for sale. Most growers use a combination of methods in order to increase soil organic matter, and avoid the buildup of pest and pathogen populations. These methods can be loosely grouped under the headings: Preplant Soil Preparation, Cultivation and Hand Removal, Chemical/Herbicide, Barrier, and Flame.

Preplant Soil Preparation can include any of the following: sequential cover cropping, fallow ground with burn or plow down, and solarization. Cover crops are an excellent way to prepare the ground for cut flower production. This is especially true when you are taking ground from pasture or turf and trying to prepare it for production. Among the cover crops commonly used are cereal rye, buckwheat, rape, and various legumes. Each has inherent advantages and limitations a grower must factor in prior to planting. For example, while cereal rye is an excellent smother crop that yields 4,000-8,000 pounds of dry matter per acre, will tolerate very late season planting, and does an excellent job of scavenging nitrogen, it can suppress the germination of some direct-seeded flowers through the secretion of allelopathic chemicals, and can be difficult to work into the soil with small equipment. Once cereal rye is worked into the soil it is ideal for most transplants and does an excellent job suppressing the germination of oxalis, chickweed and purslane. Legumes are an excellent method to build soil nitrogen, increase soil organic matter and smother weed seedlings, but there is the potential to increase nematode, pest and pathogen populations. Buckwheat makes for an excellent fast turnaround, summer cover crop, but it is critical that it be either mowed or killed before any mature seeds are set or it can become a weed.

Fallowing ground, that is leaving it unplanted and regularly killing any germinating or emerging weeds, is an excellent tool for reducing tough perennial populations and reducing weed seed banks. Regularly tilling any weeds that emerge or the application of a contact herbicide such as glyphosate, Gramoxone, Scythe, Axxe, Finale, Reward, or 20% acetic acid (heavy vinegar) will generally reduce tough weeds that are very difficult to control in a cropping situation.

Soil solarization is an effective low-input tool that accomplishes many of the same goals as chemical fumigation. By covering the ground with clear plastic after tilling and a good watering in the summer, you can reach temperatures in excess of 120F at 6-8″ deep under the plastic. This combination of clear plastic which allows weed seeds to germinate, and high heat to kill the seedlings can significantly reduce weed and pest populations. It is very important to maintain moisture levels under the plastic in order to get the deepest penetration of the heat. Unfortunately, purslane often thrives under solarization, so it may become necessary to remove the plastic long enough to use a burndown herbicide, then reinstall the plastic. Be sure to use a biological inoculant such as Actinovate AG or RootShield Plus to help reestablish a beneficial soil microflora after solarization.

Cultivation for cut flowers is no different than the practices used in vegetable production, with the added challenge of a high plant population to work around. Most cut flower farms are small (less than 2 acres) operations, making rototillers of various sizes very practical for weed control. Hand weed removal may be the only option when working in tightly planted perennial cut flowers.

Chemical Weed Control     A number of pre- and post-emergent herbicides are labeled for use in cut flowers.
See the table at the following URL as a starting point in selecting specific herbicides for cut flowers:
http://ipmguidelines.org/GreenhouseOrnamentals/Chapters/CH07/default-7-24.aspx Considering the wide range of cut flowers grown, selecting a single pre-emergent may be impossible, but with careful selection of chemicals, you may be able to get by with just a few.

Steve Bogash

Steve Bogash is Horticulture Educator for Penn State Extension in Carlisle, Pennsylvania. Contact him at [email protected]