If a leaf falls in the forest and nobody sees it, do you still have to rake it up? While autumn may be Minnesota’s favorite pastime, raking up the leaves that fall in our yards, definitely is not. Yet, most of us do it begrudgingly eventually, both to keep the leaves from smothering our grass, and to avoid the scorn of our neighbors.
What about the leaves on the driveway, sidewalk and road, though? In recent years, we’ve become aware that these leaves actually deliver a big dose of nutrients to lakes and rivers when they get washed into storm sewers in the streets. Is this a problem? After all, leaves are natural and fall into wooded lakes and streams all the time. In fact, part of the reason that lakes in central and southern Minnesota can grow more fish and bigger fish than those in northern Minnesota is because they receive more nutrients from natural sources like the leaves of deciduous trees.
The real problem is not that leaves contain phosphorus but that storm sewers make the contributing drainage areas for metro area lakes, rivers and streams so much larger. As a result, leaves from several square miles, instead of just a few acres, could end up in one little lake.
Consider the contrast between what happens to fallen leaves in a forest, versus those in a city neighborhood. In forests, earthworms and insects, fungi, and bacteria break down and decompose the leaves over time. As a result, some of the phosphorus and nitrogen that has been absorbed by trees is returned to the soil and helps to spur new growth in the spring.
In a city neighborhood, most of us remove the leaves from our yards in the fall so that they don’t smother and kill the grass. (That said, running over the leaves with a mulching mower during the fall is actually a good way to break them up and return nutrients to the soil – and it’s much easier to do than raking and bagging.) Leaves in the street begin to break down as well, but as they do, the nutrients are washed away into storm drains that connect to wetlands, lakes and rivers instead of being returned to the soil.
In freshwater systems where phosphorus is the limiting nutrient, algae and aquatic plants stop growing once the phosphorus runs out (nitrogen is the limiting nutrient in saltwater). Conversely, if stormwater pipes deliver twice as much phosphorus as normal to a lake, twice as much algae can grow. Many lakes in Washington County, including Lake St. Croix, suffer from too much phosphorus. The result is too much algae, which makes swimming and boating unpleasant, upsets the natural balance of aquatic ecosystems, and even causes toxic blooms on occasion.
So, what is a good, water-loving Minnesotan to do if you want to help protect your local lakes and the St. Croix River? If a leaf falls in the woods, by all means, let it be. But, if it falls on your driveway, sidewalk or street, rake it up for lakes’ sake!