All the redwood parks can be visited year-round.
Spring is the best time overall, especially from April through June. The woods are at their most lush and green, until Memorial Day the parks aren’t too crowded, and all the trails and roads are usually open. Huge pink rhododendron blooms appear in upland locations beginning in mid-May until as late as early July, depending on the location.
Summer is the most popular time to visit. The parks don’t get nearly as crowded as, say, Yosemite, but on a busy trail you might see a group of people every few minutes, and you may have to park a 5-minute walk from the trailhead.
Summertime weather is usually cool and dry. It almost never rains, but the sky near the coast is overcast by marine layer clouds in the morning and late afternoon. Everything is still pretty lush and green around Memorial Day and even the Fourth of July; by Labor Day, grassy areas have turned brown and the woods look a little dry and dusty.
Wildfires affected the redwood parks from July through October of both 2020 and 2021, producing weeks-long periods of wierdly orange, overcast skies and unhealthful air quality. At least twice, fires closed major highways for days at a time, making parks inaccessible or requiring hours-long detours. The 2022 fire season wasn’t as severe, although a large fire burned 20 miles from Redwood National Park for most of August and into September.
Fall is usually a nice time to visit, with clear warm days and maybe a little fall color from mid-October to mid-November. The first major rainfall ends the danger of fire sometime around mid-October. Rain is usually brief and mostly at night, so it’s pretty unusual to get rained on during a hike until about mid-November.
Winter can be a truly special experience if you’re willing to take a chance on the weather. The rain washes the dust off the foliage, the woods are filled with the sound of rushing creeks, and the sunshine has a soft, gentle quality. When it’s raining and the rain isn’t too heavy, the mist, fog, and drizzle can really enhance the scenery. Prairie Creek’s Drury Parkway is often closed in winter, but that’s actually a good thing if you’re hiking the trails near the parkway.
If highways 101 or 199 are closed, it can be difficult to get into or out of the parks. This has happened to me three times so far, about once every 30 trips. Snow can close the highways if the snow level gets below 2000 feet. Landslides seem to be the most common in January, February, and March, but a big one occurred in April 2017, closing the Highway 101 for 12 days and forcing a 3-hour detour to the Central Valley on narrow, twisty mountain roads. Google Maps doesn't show these closures; the best sources of information are the Caltrans Quickmap (select Options > Road Conditions > Full Closures) or local news websites.