Is there anything besides fertilizer that can be used to feed flowers in planters and window boxes?
Those fertilizers that allow you to grow big petunias are the same ones that will cause a proliferation of noxious weeds and algae to bloom in your lake when they eventually make it to the water. It doesn’t really matter whether the fertilizer is synthetic or organic, if it helps your garden grow, it’ll help your lake weeds grow, too. Planters on or near docks are especially problematic if you allow fertilizer to run through them into the lake.
Your best solution is to skip the need for fertilizers altogether by choosing to use only native plants at the cottage. Use black-eyed Susans and purple coneflower in your planters, and pearly everlasting and wild strawberry for window boxes. Add compost to the soil when planting, be vigilant about watering, and deadhead regularly to encourage blooms all season. You’ll create a garden that’s easier to care for, looks fabulous, and helps keep your lake happy.