Leaf Cover

Now that autumn is upon us, we are experiencing the return of everyone's favorite landscape activity - raking leaves!

We field many inquiries from customers during this time of year about the appropriateness of applying fertilizer to leaf covered lawns and how it can be that the application is effective.

Fertilizer is comprised of small, dense prills of the individual components that make up the whole of the product. Although they are soluble in water (they must be in order to be metabolized and eventually taken up by the root zone of the plant) it does take a reasonable amount of water and time in order for the prill to dissolve. Because they are dense, they fall through the turf canopy to the soil surface rather quickly even absent a cover of tree leaves. They do not travel but a few inches from where they were initially applied.

Tree leaves are just the opposite. They have a large surface area and are light in weight, giving them the ability to move great distances in the wind. In fact, due to some strange aberration of theoretical physics, all of my neighbor's leaves will eventually be deposited on my lawn, for which my neighbors are eternally grateful. A day that has swirling winds will have leaf piles forming on one side of a lawn and then moving to the opposite side.

Most of the time, tree leaves have dry surfaces - that's how and partially why they evolved the way that they have, to quickly evaporate water in transpiration when they were on the tree. Fertilizer prills seldom stick to leaves for very long. A fertilizer prill resting on a tree leaf will roll off of it like a billiard ball on a billiards table - often with the first gust of wind.

Given that, our rule of thumb for our applicators is that it is acceptable and proper to apply fertilizer to a lawn as long as there are grass leaves visible through the tree leaf cover and those tree leaves are not wet and matted. There are times when the entire lawn is clear of leaves except for one corner where they have all collected. In such circumstances, we can make the application to the lawn and as long as the pile is small and easily moved, we can take a rake, move the pile, and fertilize the area. This is not to be confused with leaf removal, which we are not equipped to perform.

Leaves can inflict a great deal of damage to a lawn if not properly handled. They are not toxic in any way to grass, but when left on the lawn surface, allowed to get wet and mat, they will block out the light to the grass plants below killing them. This will result in a void in your turfgrass canopy. Voids are opportunities for weeds to encroach - dandelions, crabgrass, etc.

You do not need to become a fanatic about daily removal of leaves to avoid the damage that they can cause. Normally, the time that you would normally spend mowing can be devoted to leaf removal in the fall on a weekly basis. But be careful to keep an eye out for leaves that are sitting in an area unmolested by the wind that are beginning to mat together - that is where the problems start. Sometimes, just taking a rake and moving the leaves a few feet such that they are no longer collecting against an obstacle is enough.

On my own lawn, I use a blower to remove the leaves from the lawn before mowing. It reduces greatly the volume of debris that I collect in clippings and further reduces the chance that I will have leaf debris remaining that will disturb the uniformity of the turf canopy.

What about mulching mowers? Before I broke down and got a riding mower, I used a 21" mulching mower on my lawn. It would do a nice job of breaking down leaves into smaller pieces that would be deposited on the lawn surface. The problem with this is that although you have made the leaves smaller, you have not removed the mass. Where there was thick leaf coverage, you will now have thick leaf pieces that can cause the same matting and loss of uniformity in the turf canopy. I would recommend blowing the leaves, at least the heaviest concentrations, off the lawn before mowing.

Did you know that the major source of nitrate and phosphorus in drinking water is from grass clippings and tree leaves that are improperly left on impervious surfaces, such as streets and parking lots? It is not the proper use of fertilizers on lawns, as you might assume, as long as that the product is not applied to the same kind of impervious surface. It may seem counterintuitive, but applications of fertilizers containing nitrogen and phosphorus will actually decrease the amount of nitrogen and phosphorus that reaches the aquifer from beneath a lawn surface. This makes sense as the dense, fibrous root system of the grass plant is a superb filter. The lawn's thatch layer is chemically attractive to all manner of substances. It grabs hold of many, many things that we would not want in our drinking water and won't let them pass through. Case in point, when we try to control grubs in a lawn curatively, the product we use is highly soluble in water, yet we have a heck of a time getting it down to where the grubs are feeding due to the influence of the thatch layer. That is why we insist that customers water that application in thoroughly or wait for a rainy day to make the application.

Leaves, by definition, contain phosphorus that must eventually be released by decomposition. Composting leaves or blowing them back into the woods or sending them to a municipal composting site are the proper ways to help ensure that the phosphorus is recycled into the biosphere. Not much of anything escapes the grasp of the forest soil down to the underlying aquifer, such is its excellence as a biotic filter. A properly maintained stand of turfgrass comes second on that list, believe it or not.