Muir Woods National Monument fills a deep, narrow canyon on the southwestern side of Mount Tamalpais in Marin County. Its old-growth coast redwoods create a microclimate of their own. The canyon's shape and the trees' ability to catch water from the marine layer both matter. Along with the thick canopy overhead, they make the forest floor cooler, damper and darker than almost anywhere else in the Bay Area. Visiting Muir Woods calls for different expectations, and different clothes, than most other trips around the region.
The Redwood Microclimate
Coast redwoods do more than live in the local weather, because they help shape it. These trees evolved along a coast where summer brings fog rather than rain, and they make good use of it. As fog drifts through the forest, tiny droplets collect on the needles and branches, merge into larger drops and fall to the ground. This fog drip works like a light, steady rain during the dry months, and in a foggy summer it can add up to several inches of water.
As a result, Muir Woods stays moist and green all summer, even without rain. Ferns and redwood sorrel carpet the ground under the canopy, and the air feels noticeably damper than on the sunny hills above. The tall trees also block most of the sunlight, so the canyon floor stays dim and cool through the day. In every season, the forest feels like a separate world from the towns a few miles away.
Summer: Cool and Foggy
Summer is when the microclimate matters most to visitors. On a typical summer afternoon, inland Marin towns such as Novato may reach the upper 80s or 90s. Farther north in Sonoma County, Petaluma and Santa Rosa can be hotter still. On the floor of Muir Woods, temperatures usually stay between the mid 50s and mid 60s. The marine layer flows into the canyon from the ocean side, the redwoods shade the ground and the damp soil keeps the air cool.
That makes Muir Woods a good escape during heat waves. When much of the North Bay is near 100°F, the main trail along the canyon floor may sit around 60°F in deep shade. The other side of that coin is that the canyon can feel cold to visitors who arrive in shorts. On foggy summer mornings, the floor can sit in the low 50s, and the constant shade keeps it from warming much later in the day. Bring a fleece or light jacket, even in July.
Fall and Winter
Fall often brings the clearest days of the year. As the marine layer weakens in September and October, more sunlight reaches the forest floor and slants through the canopy in long golden beams. Mornings with a thin, lifting fog produce the classic Muir Woods photographs, with rays of light cutting through the mist between giant trunks. Afternoons are still cool, usually in the low to mid 60s, but the light is at its best.
Winter brings rain as the main source of water. The canyon gets roughly 40 inches in a typical year, far more than San Francisco, because storms from the Pacific are forced up the slopes of Mount Tamalpais. In winter, Redwood Creek runs full, banana slugs cross the trails and the forest takes on a deep green, misty look.
Winter is also when coho salmon and steelhead trout return from the ocean to spawn in Redwood Creek, usually from December through February after heavy rains. Spotting them in the creek is a rare treat. Crowds are smaller in winter, and trails away from the main boardwalk can feel almost empty, though rain gear is a must.
Spring in the Redwoods
Spring is a quieter, gentler season in the canyon. From February through April, the creek still runs strong from the winter rains, and the forest floor comes alive with small wildflowers. Western trillium, with three white petals that fade to pink, blooms along the trails in late winter and early spring. Redwood sorrel, a low plant with clover-shaped leaves, sends up pale pink flowers a little later.
Spring weather swings between late storms and mild, bright days. A sunny afternoon in March may reach the low 60s on the canyon floor, while a rainy one can stay in the 40s. By May, the summer marine layer starts to return, and gray, foggy mornings become common again. Spring weekdays are often less crowded than summer weekends, which makes it a good time for a slow walk through the forest.
What to Expect and How to Dress
The most common mistake visitors make is dressing for the weather somewhere else. A warm, sunny morning in downtown Mill Valley or at the Marin Headlands says little about the canyon floor. Expect it to be roughly 10 degrees cooler, with little direct sun. The National Park Service recommends layers, and the advice holds in every season. A light jacket and a base layer that dries quickly will serve you well year-round.
Trails can be damp from fog drip even when it has not rained, so wear shoes with good grip. In winter, roots and wooden steps get slick, and the creek-side paths can be muddy for days after a storm.
Visiting Muir Woods requires an advance reservation for parking or the shuttle. The park's trails connect to the larger Mount Tamalpais State Park network, which climbs out of the canyon to open ridges with very different weather. Up there, it is often sunny and warm above the fog, with views of the bay and the ocean. A loop from the canyon floor to the ridge shows the full range of Marin's microclimates in one hike, from cold, misty forest to warm, open grassland.
