The U.S. spends more than $5 billion a year on fossil-fuel-derived fertilizers that leak chemicals into the ground and accelerate the release of nitrous oxide—a greenhouse gas. Try alternatives, from old-fashioned compost to grass clippings, which contain about 4% nitrogen. More adventurous gardeners use a homemade fertilizer mix that includes seaweed extracts for potassium and fish proteins and oils for nitrogen. Or go native and embrace wildflowers and indigenous grasses. Weeds are a matter of taste.