Douglas asks: I am growing lots of native plants from seed in my small home nursery but I am spending many hours weeding so that the seedlings can thrive without competition. A friend suggested that I try using a pre-emergent herbicide in the pots to control the weeds once the seedlings have sprouted. I want to be organic. Is there an organic weed suppressor I could use?
Tropical Gardener answer: The short answer is yes, but the longer response is that only a few (maybe only one) are actually OMRI (Organic Materials Review Institute) certified. In order to be used in a certified organic operation, all inputs must be OMRI certified. If you don’t have organic certification but want to use an organic product, you have more choices.
A pre-emergent herbicide is a product that covers the soil and kills seeds as they begin to germinate. Pre-emergents do not affect plants that have already germinated and are putting out their true leaves. Many commercially available pre-emergents include seed-killing chemicals. Some, however, contain only corn gluten meal. This is a naturally available organic substance but it can’t qualify for use in certified organic operations unless it is made from organic corn.
Eartheasy carries a certified organic fertilizer made from corn gluten that serves as a weed suppressor as well as a fertilizer containing 8 percent nitrogen. Look for it in stores or online if you need an OMRI approved certified organic product. Many other pre-emergents are also made from corn gluten and are chemical free. Though corn qualifies as organic matter, not all the corn in these products is certified organic. Since the products are made from organic matter (corn) and contain no chemicals, they can be called organic even though they are not certifiable. Are you confused yet?
For those who do not need to use OMRI approved products, I recommend the “organic” corn gluten products since they are nontoxic as well as pet, human and environment friendly. The products come in granular, pelletized, powder and liquid forms. They are comparably effective and all inhibit root formation in weeds at the time of germination. When the seed cracks open to sprout, corn gluten meal dries out the seed and roots do not form.
Corn gluten meal is a powdery byproduct of the process of milling corn and it has been used for years as a supplement in animal feed. In the 1990s, researchers at Iowa State University discovered the seed killing effect of corn gluten meal almost by accident. Since corn gluten also contains nearly 10 percent protein, they could market the product to both weed and feed.
Some tips will make whatever corn gluten product you use more effective.
• Read the instructions that come with the product as different formulas may have different coverage requirements.
• Corn gluten needs to be wetted to activate it. Some formulas recommend waiting a day or so to wet the product while others require immediate wetting. A drying period is required after initial wetting for the product to be fully effective. Soil that is wet during weed germination may reduce efficacy.
• An application usually lasts from four-six weeks. Repeated applications during times when weed seeds are germinating will increase efficacy.
Some local hardware or big-box stores may carry these corn gluten pre-emergents. Several online suppliers also carry them. Check to see who has them and do some price comparison as prices seem to vary wildly online.
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.
Gardening Events
Saturday: “Work Day at Amy Greenwell Garden,” 9 a.m.-12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers will be able to help with garden maintenance and are invited to bring a brown bag lunch. Water and snacks provided. Info: Call Peter at 323-3318.
Farmer Direct Markets
Wednesday: “Sunset Farmers Market,” 2-6 p.m. in the HPM parking lot, 74-5511 Luhia St., in Kailua-Kona
Wednesday: “Hooulu Farmers Market,” 9 a.m.-2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay
Friday: “Pure Kona Market,” 9 a.m.-5 p.m. at Amy Greenwell Garden in Captain Cook
Saturday: “Keauhou Farmers Market,” 8 a.m.-noon at Keauhou Shopping Center
“Kamuela Farmer’s Market,” 7 a.m.-noon at Pukalani Stables
Sunday: “Pure Kona Green Market,” 9 a.m.-2 p.m. at Amy Greenwell Garden in Captain Cook
Tuesday-Saturday: “U-Pick greens and produce,” 10 a.m.-4 p.m. at Tropical Edibles Nursery in Captain Cook
Plant Advice Lines
Anytime: konamg@ctahr.hawaii.edu
Tuesdays, Thursdays: 9 a.m.-noon at UH-CES in Kainaliu at 322-4892
Mondays, Tuesdays, Fridays: 9 a.m.-noon at UH-CES in Hilo at 981-5199 or himga@hawaii.edu