It’s a reflex for many of us around this time of year to spread fertilizer over our lawns. We may do this out of habit, but like any habit, this one has consequences, many of them not so good.
If you use a synthetic fertilizer, for example, you should know that it comes with a considerable carbon footprint. The synthetic nitrates that form the bulk of such fertilizers are manufactured with huge quantities of natural gas and represent a very considerable release into the atmosphere of carbon dioxide. In fact, as Dr. David Wolfe, a professor at Cornell University, told me, that process typically releases four to six tons of carbon dioxide or its equivalents in terms of greenhouse gases for every ton of nitrates produced. Then, when you spread the synthetics on your turf, they release a sudden burst of fertility, much of which won’t be absorbed by the grass and which will instead wash away to pollute local streams, lakes and rivers.
If you pay the higher price and apply an organic fertilizer to your lawn, that reduces the carbon footprint of the product’s manufacture and may reduce water pollution, as organics typically release their nutrients more slowly, at a rate that is better utilized by the lawn. However, organic turf fertilizers do share a defect with their synthetic competitors. Both are rich in nitrates, which tend to escape into the atmosphere as nitrous oxide, a gas that is approximately 300 times as potent a greenhouse gas as carbon dioxide.
Fortunately for the gardener who wants to green their lawn, there is a natural, non-polluting alternative that is also cheaper and easier. That is white clover. White clover is a legume, a type of broadleaf plant that absorbs nitrogen gas from the atmosphere and converts it in its roots to nitrates that serve as a natural fertilizer. The initial benefit is to the clover, but as its roots spread through the soil and eventually die and decay, the clover releases the nitrates, and they are absorbed by the grass roots.
Our turf grasses co-evolved with white clover, so the two types of plants associate very comfortably. Indeed, the clover fills an ecological niche that might otherwise be occupied by a broadleaf weed, so its benefit to the lawn is two-fold.
Why, then, is clover so often labeled as a weed itself? That was not always the case. Older lawn manuals, in fact, typically hail white clover as an essential ingredient of any good quality lawn. It was only after World War II, and the introduction of chemical broadleaf weed killers, that clover’s status changed. As a broadleaf plant, it was vulnerable to the new herbicides, and so, in the view of the weed killer salesmen, it automatically qualified as a weed.
Nevertheless, I have found white clover to be very useful in making lawns more sustainable and more hospitable to bees and other pollinators. It works especially well with turf grasses such as turf-type tall fescues and fine fescues that flourish on less fertile soils; a greedier Kentucky bluegrass lawn may need some supplemental fertilization to really flourish. I also like white clover as a lawn additive because it stands up to heat and drought better than the classic lawn grasses; white clover retains its rich green color even as the grasses’ hue dulls in mid-summer.
If you want to include white clover in your lawn, mow the lawn short in mid spring and rake it to remove any thatch and scratch up the soil; renting a power rake will make this job easy. Then mix the white clover seed with sand and spread it over the prepared lawn with a broadcast spreader. The proportion of white clover seed to sand will be small, as the clover seed is very fine; two ounces of clover seed is enough to overseed 1,000 square feet of lawn, so you’ll probably want to mix it with at least 20 lbs. of sand to achieve a relatively even spread.
One characteristic of white clover that some will view as a defect is that its nectar-rich flowers attract bees in mid-summer. Those who are allergic to bee venom may wish to avoid the use of clover in their lawns or mow the lawn more frequently during the blooming season to remove buds before they open. Gardeners who favor supporting pollinators will probably regard the insects that clover blossoms attract as an added benefit.
Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Thomas Christopher is a volunteer at Berkshire Botanical Garden.