Rush milkweed

Year-round

Rush milkweed is a desert plant native to the low deserts of southeastern California, Arizona, Nevada and northwest Mexico, not to San Francisco's coastal chaparral and oak woodland ecoregion. It can be grown here as a garden plant with full sun and fast drainage, but San Francisco's cool, foggy, Mediterranean climate genuinely differs from the hot, dry desert this species evolved for, so expect slower growth, fewer blooms and less of its classic leafless summer dormancy than in Arizona or the low desert.

• Give it your sunniest spot. San Francisco's persistent summer fog cuts real sunlight even on clear-looking days, and this plant needs full sun to perform anywhere near what it does in the desert.
• Drainage matters more here than heat does. San Francisco gets roughly 21 to 22 inches of rain a year, almost all of it between November and March, a wet season this desert species never evolved to sit through, so raise the bed or work in coarse grit if the soil holds water.
• This is a monarch and queen butterfly host plant, and every part of the genus Asclepias is considered toxic if eaten by people or pets, so plant it where kids and animals are not likely to chew on it.
• Do not expect Arizona-style performance. In its native low desert this plant drops its leaves and stands as bare green stems through summer heat. In San Francisco's mild summers it will likely hold more foliage, grow smaller, and bloom less heavily.

Its narrow, upright, rush-like form and long bloom window read as tidy and low-maintenance, which most HOA-conscious neighbors respond well to, though it is not a locally native look, so it is worth checking any plant list your HOA maintains before planting.

Summer

San Francisco summers are cool and often fogbound instead of hot and dry, per NOAA/NWS data showing average summer highs of only 60 to 70 degrees under persistent morning stratus. That is a genuinely different environment than the low desert this plant is built for, so growth and bloom will likely be more modest here than in Arizona or Nevada.

• Full sun still matters most in summer. Gardens in San Francisco's sunnier pockets away from the immediate coast will get noticeably more bloom than gardens under the thickest fog belt.
• Water sparingly, if at all, once established. This plant thrives on very little water even in real heat, and San Francisco's cool summer air means it needs even less.
• If caterpillars strip the leaves, that is normal for a monarch and queen host plant and is not a sign of a problem.
• Do not expect the bare, leafless summer look this plant shows in the desert. In San Francisco's milder, foggier summers it is more likely to hold its foliage instead.

Fall

September and October are typically San Francisco's warmest, sunniest weeks of the year, according to NOAA/NWS climate records, which note that the three hottest days on record in San Francisco all fell in September and October. This stretch is when rush milkweed will look closest to its native desert self here.

• This is the best planting window of the year for this species in San Francisco. Warmer, drier, sunnier weather gives new roots the closest thing to desert conditions before winter rain returns.
• Ease off water as the season progresses. Rain typically returns by November, and this plant does not want to head into winter with wet roots.
• Prune only spent or damaged growth. This is a low-input plant that generally does not need seasonal cutting back.
• Enjoy the best bloom and nectar activity of the year during San Francisco's warm fall stretch, before the cooler, wetter months slow things down.

Fall's warm, sunny stretch is when this plant looks its fullest and most deliberate, a good season to let it be the visible, tended feature of a bed for HOA-conscious curb appeal.

Winter

Winter is the highest-risk season for rush milkweed in San Francisco, not from cold but from wet roots. Over 80 percent of the city's roughly 21 to 22 inches of annual rain falls between November and March, per NOAA/NWS records, while frost stays rare in most neighborhoods. This plant is built for a place that gets almost no winter rain, so soggy soil is the real threat here.

• Do not water. Winter rain alone is more moisture than this plant sees in a typical year across its native desert range.
• Confirm the bed drains between storms. If water stands for more than a few hours after rain, raise the planting area or add coarse material rather than relying on this plant to tolerate it.
• If a rare hard frost does hit, expect at most minor stem dieback rather than the plant's normal desert leaf-drop response. Wait until spring growth appears before deciding what to cut.
• This is the season to leave the plant alone. It is naturally low-input, and winter is not the time to add water or fertilizer.

Spring

San Francisco's rain year runs heavy through winter and tapers by spring, per NOAA/NWS climate data. Plant or divide rush milkweed once the heaviest rains ease, typically by April, so new roots are not sitting in soggy soil while they establish.

• Plant from containers in April or May, after the winter rain pattern has eased but before San Francisco's dry season fully sets in.
• Check for winter dieback first. Hard frost is uncommon in most of San Francisco, which sits in USDA zone 10a to 10b, but if any stems blackened over winter, wait for new growth to confirm which wood is alive before cutting anything back.
• Skip fertilizer. This species is adapted to low-nutrient desert soil and does not need feeding to get going.
• As flowers open, watch for monarch and queen butterfly caterpillars on the stems. This is a known host plant for both, so leave any caterpillars you find.

Spring planting keeps the bed looking intentional and tended during the season neighbors notice new plantings most, which supports an HOA-friendly first impression.