How often to water swamp milkweed in California
In July in the Central Valley, established swamp milkweed in loam needs a deep watering about every 5 days: about 1.5 gallons each time for a plant 2 feet across, about 1.9 gallons a week. Swamp milkweed is an herbaceous perennial from the central and eastern United States that grows wild in wet places along streams, ponds and bogs. Clusters of pink to rose-purple flowers feed many pollinators, and its leaves feed monarch caterpillars. It grows up to 5 feet tall in sun or part shade and adapts to drier ground in moist clay or loam.
How often to water swamp milkweed, month by month
Type your city above and the days between waterings and the gallons appear here, worked out from your city's climate table. Or read the figures by climate zone further down.
How this is worked out: established roots fill the top 1 foot of soil, and you water again when that soil has dried halfway. Loam soil holds about 1.5 inches of water per foot. Water the whole area the plant covers, and skip a watering after a good soaking rain.
How to tell it needs water
- Soil dry 2 inches down beside the stems in summer means swamp milkweed is ready for water
- Leaves wilting in afternoon heat can indicate the soil has dried too far
- Flowers fading early and lower leaves yellowing can follow a dry spell
- Leaves stripped to the stems are usually monarch caterpillars feeding, not a water problem
How to water it
- Keep the soil evenly moist, watering at the base with drip or a hose
- Wet the root zone 6 to 8 inches deep and water again before it dries out
- Plant in full sun to part shade in moist clay or loam
- Use it beside a pond, a rain garden or a low, damp spot
- Leave monarch caterpillars to feed on the leaves
Through the seasons
Keep a new plant moist through its first summer. Established clumps still like damp soil, though they cope with drier sites in moisture-holding clay or loam. Flowers open in mid-spring and continue into early fall, then pods split to release seeds that float on the wind. Stems die back in winter and regrow in spring. In the Central Valley, WUCOLS rates swamp milkweed moderate water. WUCOLS has not evaluated it for the north-central coast, the south coast, South Inland, the high desert or the low desert.
In pots
Swamp milkweed can grow in a large, deep pot, which suits a patio pollinator garden. Use a mix that holds moisture well. Water when the surface just starts to dry, and soak until water runs from the base. In summer, a pot in full sun may need water every day. A saucer can help keep the mix moist for this wet-soil plant, but empty it if water sits for days. In winter, after the stems die back, water only lightly.
- A year
- 17.11 gal per sq ft
- Reference
- Sacramento climate table
- Counts
- Plant need, before sprinkler losses
Monthly water need by climate zone
Swamp Milkweed: gallons per square foot per month, by WUCOLS climate zone
| Month | N. Coast | C. Valley | S. Coast | S. Inland | Hi Desert | Lo Desert |
|---|---|---|---|---|---|---|
| Jan | – | 0.4 | – | – | – | – |
| Feb | – | 0.65 | – | – | – | – |
| Mar | – | 1.09 | – | – | – | – |
| Apr | – | 1.52 | – | – | – | – |
| May | – | 2.08 | – | – | – | – |
| Jun | – | 2.48 | – | – | – | – |
| Jul | – | 2.67 | – | – | – | – |
| Aug | – | 2.39 | – | – | – | – |
| Sep | – | 1.8 | – | – | – | – |
| Oct | – | 1.18 | – | – | – | – |
| Nov | – | 0.56 | – | – | – | – |
| Dec | – | 0.34 | – | – | – | – |
| Year | – | 17.11 | – | – | – | – |
How this is worked out: the WUCOLS water rating for each zone times that zone’s climate for the month. A dash means WUCOLS does not rate the plant there. The figures are what the plant uses, before sprinkler losses.
Plants with the same water needs 16
Rated moderate water in the Central Valley, like swamp milkweed. Plants that drink alike can share one valve and one schedule.
DahliasDahlia spp.moderate water, Central Valley2.67 gal/sq ft in July
FernsNephrolepis cordifolia (SHADE IN DESERT)moderate water, Central Valley2.67 gal/sq ft in July
DayliliesHemerocallis spp. & cvs.moderate water, Central Valley2.67 gal/sq ft in July
BegoniasBegonia & ssp. & cvs. (shade in the desert)moderate water, Central Valley2.67 gal/sq ft in July
Calla LiliesZantedeschia aethiopica SHADE IN DESERTmoderate water, Central Valley2.67 gal/sq ft in July
SageSalvia officinalis and cvs.moderate water, Central Valley2.67 gal/sq ft in July
Bicolor MonkshoodAconitum cammarummoderate water, Central Valley2.67 gal/sq ft in July
Garden MonkshoodAconitum napellusmoderate water, Central Valley2.67 gal/sq ft in July
Carpet BugleAjuga reptans (shade)moderate water, Central Valley2.67 gal/sq ft in July
Snowdrop WindflowerAnemone sylvestrismoderate water, Central Valley2.67 gal/sq ft in July
Mountain SandwortArenaria montanamoderate water, Central Valley2.67 gal/sq ft in July
Orange MilkweedAsclepias tuberosamoderate water, Central Valley2.67 gal/sq ft in July
Greater MasterwortAstrantia majormoderate water, Central Valley2.67 gal/sq ft in July
Hardy BegoniaBegonia grandismoderate water, Central Valley2.67 gal/sq ft in July
Wax BegoniaBegonia semperflorensmoderate water, Central Valley2.67 gal/sq ft in July
English DaisyBellis perennismoderate water, Central Valley2.67 gal/sq ft in JulySimilar plants that need less water 16
Rated lower than swamp milkweed in the Central Valley. Worth a look for a new bed or a lawn replacement.
AgapanthusAgapanthus orientalislow water, Central Valley1.07 gal/sq ft in July
GeraniumsPelargonium x hortorum and cvs.low water, Central Valley1.07 gal/sq ft in July
SalviaSalvia greggii ssp., cvs. & hybridslow water, Central Valley1.07 gal/sq ft in July
IrisesIris germanicalow water, Central Valley1.07 gal/sq ft in July
SucculentsEcheveria spp. & cvs.low water, Central Valley1.07 gal/sq ft in July
Russian SagePerovskia spp. & cvs.low water, Central Valley1.07 gal/sq ft in July
YarrowAchillea millefolium (non-native hybrids)low water, Central Valley1.07 gal/sq ft in July
Mountain AlyssumAlyssum montanumlow water, Central Valley1.07 gal/sq ft in July
Silver SageSalvia argentealow water, Central Valley1.07 gal/sq ft in July
Purple MulleinVerbascum phoeniceumlow water, Central Valley1.07 gal/sq ft in July
Silvery YarrowAchillea clavennaelow water, Central Valley1.07 gal/sq ft in July
Fern Leaf YarrowAchillea filipendulinalow water, Central Valley1.07 gal/sq ft in July
California Native YarrowAchillea millefolium (CA native cultivars)low water, Central Valley1.07 gal/sq ft in July
Woolly YarrowAchillea tomentosalow water, Central Valley1.07 gal/sq ft in July
Perennial HollyhockAlcea rosea cvs.low water, Central Valley1.07 gal/sq ft in July
Mountain Lady's MantleAlchemilla alpinalow water, Central Valley1.07 gal/sq ft in JulyTurn this into minutes on your water days
Every city page has a calculator that takes swamp milkweed, your sprinkler type and the month, and gives the minutes per water day for that supplier's schedule. Or put it in a zone of the personal watering plan and get all twelve months printed.
Questions people ask
How much water does swamp milkweed need per day?
In July in the Central Valley, established swamp milkweed needs about 0.3 gallons a day for a plant 2 feet across and about 1.1 gallons a day for a plant 4 feet across. Give it in deep waterings spread days apart rather than a little every day, so the whole root zone gets wet.
Does watering for swamp milkweed change across California?
Yes. WUCOLS rates swamp milkweed separately for each of the six California climate zones: not evaluated in the North-Central Coastal zone, moderate water in the Central Valley zone, not evaluated in the South Coastal zone, not evaluated in the South Inland zone, not evaluated in the High and Intermediate Desert zone, not evaluated in the Low Desert zone. The monthly table above shows how the same rating translates into different gallon amounts as reference evapotranspiration changes.
Does swamp milkweed need wet soil?
It grows wild in wet places but adapts to moist clay or loam. Keep its soil evenly damp. It suits rain gardens and pond edges.
Is swamp milkweed good for monarchs?
Yes, its leaves are a food source for monarch caterpillars, and many pollinators visit the flowers. Expect some chewed leaves. Plant enough to share.
Is swamp milkweed poisonous?
It has low severity poison characteristics. Keep pets and livestock from eating it. It is a plant for pollinators, not for grazing.
What pests bother swamp milkweed?
Aphids and milkweed beetles are common but rarely serious. Milkweed bugs can damage the seed pods. Most of these insects need no treatment.
How tall does swamp milkweed grow?
It can reach about 5 feet tall and 2 to 3 feet wide. Give it room at the back of a bed. Deer tend to leave it alone.
More plant watering guides
Sources
- WUCOLS IV plant water use classification for Asclepias incarnata, the University of California survey California agencies use for landscape plan reviewRead August 23, 2026 on wucols.ucdavis.edu
- Model Water Efficient Landscape Ordinance, adopted text and appendices, effective 2025-01-02 · monthly reference evapotranspiration per city, Appendix CRead August 23, 2026 on water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Water-Use-And-Efficiency/MWELO-Rulemaking/Final-MWELO-Text-and-Appendices_20250103.pdf
- UC Statewide IPM Program, irrigation scheduling using evapotranspirationRead September 30, 2026 on ipm.ucanr.edu/PMG/GARDEN/ENVIRON/evap.html
- Shaw and Pittenger, Landscape Irrigation System Evaluation and Management, UC Cooperative Extension, 2009Read September 30, 2026 on ucanr.edu/sites/default/files/2013-02/160836.pdf
- Pittenger, Water Management in Gardens and Landscapes, UC Riverside, 2017Read September 30, 2026 on ucanr.edu/sites/default/files/2017-03/258022.pdf