Most lawn calendars are written for a national audience and split the year into four even seasons. Portland's climate really only has two hinge points that matter most — early fall and mid-spring — with everything else supporting them.
Late Winter (Feb–March)
Growth is minimal. This is a good window for a first light mow once the ground firms up, and for pruning any spring-flowering shrubs after they bloom, not before.
Spring (April–May)
Growth accelerates fast with warming temperatures and steady rain. Weekly mowing typically starts here, along with a light spring fertilizer application and a bed mulch refresh.
Early Summer (June)
Growth is still strong before the dry stretch sets in. A good window for weed treatment while the lawn is actively growing and can recover quickly.
Peak Summer (July–August)
The dry season. Mow higher, water deeply but less often if you irrigate, and hold off on heavy fertilizing — this is the lawn's stress period, not the time to push growth.
Early Fall (September–October) — The Big One
This is the single most important window of the year for a cool-season Portland lawn: core aeration, overseeding, and the heaviest fertilizer application of the year all happen here, while soil is still warm and rain has returned.
Late Fall (November)
Leaf clearing becomes the priority as deciduous trees drop heavily. A final light feeding can help color hold through the coming wet months.
Winter (December–January)
Mowing largely stops. This is planning season — a good time to book aeration or a fertilization program for the year ahead before fall booking fills up.