Skip Weed-and-Feed Fertilizers
A lot of people out there are trying to sell you weed-and-feed fertilizer. Don’t buy them — or you’re buying trouble. Here’s why they don’t always work — and may cause problems.
Two different types of weed killers, aka herbicides, are blended with lawn fertilizers in formulating the so-called weed-and-feed blend. One kind is advertised to kill broadleaf weeds; another to kill crabgrass.
The formulation of weed-and-feed fertilizer advertised as killing broadleaf weeds such as dandelions, ground ivy and plantain is a phenoxy compound (2,4-D, Dicamba, MCPP, etc.) in granular form. These compounds can be absorbed through the foliage or by the roots. But only under certain conditions.
They don’t function when the soil is dry or when weeds are dormant. If applied in early spring, the killing agent may well deteriorate or leach deep into the soil beyond the reach of the roots of weeds. But the roots of trees and shrubs can absorb them, causing injury such as twisting and curling of leaves and damage to new growth. Another problem is leaching into groundwater, which contributes to the pollution of the Bay.
Delay application until after grasses and weeds have resumed growth, and the fertilizer is likely to cause a disease problem such as fusarium. Fusarium, aka frogeye, kills grass. The symptoms appear as a tuft of green surrounded by a dead brown zone. Another problem: Late application of high-nitrogen fertilizer on cool-season grasses such as bluegrass or fescue forces the grass to produce lush growth at a time when lawn grasses are stressed by the heat of summer. Cool-season grasses grow best in cool weather.
Weed-and-feed fertilizers promoted to control crabgrass often do not perform as advertised when applied early. These weed killers are called pre-emergent, meaning that they must be applied before the weed seeds germinate. Their effectiveness at killing germinating weed seeds lasts only four to six weeks after they have been applied. But crabgrass is a summer annual weed whose seeds do not begin to germinate until a week or two after forsythia has dropped its flowers. So if crabgrass-killing weed-and-feed fertilizer is applied too early, its effectiveness at killing germinating crabgrass weed seeds is severely reduced. Delay applying the product until after forsythia flowers have dropped, and you risk fusarium problems.
Weed-and-feed fertilizers can also contribute to the pollution of the Bay. Granules that fall on sidewalks and driveways either float or dissolve and become part of the storm water that contributes to the non-point source of pollution. If your property abuts the Bay or its tributaries and a heavy rain occurs soon after application, the granules can find their way into the Bay.
A healthy lawn does not allow weeds to grow.
Have your soil tested to make certain that the pH is between 6.2 and 6.8. If the results indicate that your soil contains adequate levels of phosphorus, purchase only fertilizers with 0 phosphorus. If the test indicates your soil has adequate levels of phosphorus and potassium, consider using calcium nitrate.
Also set your mower to cut the grass no shorter than three inches and taller if possible. Cut it tall and let it fall for a healthy turf that gives no ground to weeds.
To Test Your Soil
Find instructions for submitting soil samples for testing at www.al-labs-eastern.com. For sandy soil, choose the S3 test. Save money by specifying no crop. I offer free follow-up directions to Bay Weekly readers. For my recommendations, add my email (as well as your own): [email protected].
Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.