Lawn Care or Lake Care? The Trouble with Spring Fertilizers

May in Southern New Hampshire is a glorious thing. The peepers are in full chorus, the loons are back on territory, and suddenly every lakefront lawn looks like it could use a little help after the long winter. Garden centers know it too, which is why this is the month their shelves fill up with bright bags of lawn fertilizer promising a greener yard by June. Before you toss one in the trunk, though, it’s worth pausing to think about where all that green really ends up.

Here’s the uncomfortable truth: the same nutrients that make your grass grow also make lakes sick. Lawn fertilizers are built around nitrogen and phosphorus, and our lakes and ponds are extraordinarily sensitive to both. A healthy New Hampshire lake runs on very small amounts of phosphorus, often measured in parts per billion. When even a little extra washes in from shoreline properties, it acts like rocket fuel for algae and aquatic weeds.

The path from your yard to the lake is shorter than most people think. Spring rain and lawn sprinklers carry dissolved nutrients across driveways, down storm drains, and through the thin strip of soil between a lawn and the water. Frozen ground from a late frost makes it worse, because nothing soaks in. By the time you notice the grass looking lush, the phosphorus that did the work is often already in the water.

 The good news is that a healthy lawn and a healthy lake are not mutually exclusive. Start with a simple soil test through UNH Cooperative Extension; most New Hampshire lawns already have plenty of phosphorus and don’t need any more. In fact, state law prohibits applying phosphorus fertilizer to established lawns except in very specific circumstances, and it’s banned outright within 25 feet of any surface water. If you do fertilize, choose a phosphorus-free product (look for the middle number on the bag to be a zero), apply it sparingly, and never right before a rainstorm.

 Better still, let the shoreline do what it wants to do. A buffer of native plants and unmowed grass along the water’s edge will catch runoff before it reaches the lake, shade the shallows, and give turtles and frogs a place to live. It’s the cheapest water quality tool a lakefront owner has, and it doesn’t come in a bag.

 A lush green lawn can be a point of pride, but a clear, swimmable lake is the reason most of us chose to live here in the first place. This May, before the spreader comes out of the shed, ask the honest question: is this bag going to feed my grass, or feed the weeds in Canobie Lake? Our lakes are counting on us to know the difference.