Fertilizing is necessary when your lawn isn't getting enough nutrients from the soil alone to sustain healthy, dense growth — which for most maintained lawns means on a regular seasonal schedule rather than waiting for visible distress.

Signs your lawn is nutrient-deficient:

Yellowing or pale green color (often nitrogen deficiency)

Slow growth or thin, weak grass

Poor recovery after stress (heat, drought, foot traffic)

Lawn looks dull compared to neighbors' or previous years

When to fertilize (Ontario cool-season lawns):

Early spring — a light application to wake the lawn up as it comes out of dormancy, but don't overdo it or you'll push excessive top growth at the expense of roots

Late spring/early summer — a balanced feed to sustain growth heading into summer

Early fall (most important) — the single most valuable fertilizer application of the year; grass is storing energy in its roots for winter survival and spring green-up

Late fall — a winterizer fertilizer high in potassium strengthens roots and improves winter hardiness; applied just before the ground freezes