WateringTimesWatering plan, $19
Asclepias curassavica, WUCOLS IV

How often to water scarlet milkweed in California

In July in the Central Valley, established scarlet 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. Scarlet milkweed, also sold as tropical milkweed, is a tender perennial from Central and South America. Clusters of star-shaped flowers in red and orange or red and yellow top its narrow leaves. In mild gardens it can bloom nearly year round, self-sowing readily, and through winter its leaves can carry a parasite that harms monarch butterflies.

PerennialRated for 5 of 6 zones

How often to water scarlet milkweed, month by month

Your city
Plant width
2 ft
Your soil

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.

Every month, on your water days. The personal plan prints this for scarlet milkweed and the rest of your yard, on the days your water supplier allows.Get the plan for $19

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.

Which soil do you have?

Dig down a few inches, take a handful of moist soil and squeeze it.

  • Sandy: gritty, falls apart when you open your hand, will not form a ball.
  • Loam: forms a soft ball that crumbles when you poke it.
  • Clay: forms a firm, sticky ball and squeezes out into a ribbon between thumb and finger.

Still not sure? Pick loam. Most garden beds sit between sand and clay.

How to measure plant width

A shrub with its width marked from one side to the other and the circle under it shaded blue

Measure across the plant, from the tips of the leaves on one side to the tips on the other.

Water the whole area the plant covers, shown in blue. The gallons here cover that area.

A wider plant means more gallons each watering, not more waterings. While it is young, measure again each spring.

How to tell it needs water

  • Soil dry 2 inches down around scarlet milkweed in summer signals that it needs water
  • Leaves wilting and lower leaves yellowing in summer heat can point to a dry root zone
  • Stems yellowing and collapsing at the base in soil that stays wet point to poor drainage
  • Dense clusters of aphids with black sooty mold on the stems are a pest problem, not a water one
  • Leaves stripped down to the stems usually mean monarch caterpillars have been feeding, not that the plant is dry

How to water it

  • Water at soil level around the clump rather than spraying the leaves
  • Soak the top foot of soil, then let the top inch or two dry before watering again
  • Plant in full sun in fertile, well-drained soil, away from wild areas where its seedlings could spread
  • Mulch lightly around the base to hold moisture, keeping it back from the stems
  • Pull self-sown seedlings that come up in the watered area unless you want more plants

Through the seasons

A new scarlet milkweed needs steady moisture through its first summer. Established plants want regular water while they grow and bloom from spring through fall. In mild gardens it stays leafy into winter, so monarch experts advise cutting it to the ground in November and keeping it trimmed until spring. Growth then returns from the base. Rain usually covers the cool months on the coast and in the Central Valley. WUCOLS rates scarlet milkweed moderate water on the north-central coast and in the Central Valley. It rates low water on the south coast, in South Inland and in the low desert. WUCOLS has not evaluated it for the high desert.

In pots

Scarlet milkweed does well in a pot, which also keeps its seedlings from spreading. Check the mix an inch or two down every few days in summer and water when it is dry, letting excess drain from the holes. Empty the saucer afterward. Cut a potted plant to the ground in November and water sparingly through winter until spring growth begins. Clip seed pods before they split so the silky seeds do not blow into the garden.

1 Your climate zone six WUCOLS zones, each from its own reference city
2.67gal per sq ft in JulyModerate water use in the Central Valley zone
A year
17.11 gal per sq ft
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses
0.671.32.02.70.4JAN0.65FEB1.09MAR1.52APR2.08MAY2.48JUN2.67JUL2.39AUG1.8SEP1.18OCT0.56NOV0.34DEC0.671.32.02.70.4JFM1.52AMJ2.67JAS1.18OND
Gallons per square foot per month, Central Valley. Peak 2.67 in Jul, 17.11 a year.

Monthly water need by climate zone

Scarlet Milkweed: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.370.40.310.31–0.31
Feb0.530.650.320.35–0.37
Mar0.871.090.460.52–0.61
Apr1.181.520.560.64–0.78
May1.462.080.640.78–0.98
Jun1.612.480.690.92–1.12
Jul1.612.670.791–1.14
Aug1.492.390.740.93–1.04
Sep1.241.80.60.74–0.83
Oct0.871.180.460.53–0.6
Nov0.470.560.330.36–0.38
Dec0.310.340.260.27–0.27
Year1217.116.27.37–8.43

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 scarlet milkweed. Plants that drink alike can share one valve and one schedule.

DahliasDahlia spp.moderate water, Central Valley2.67 gal/sq ft in JulyFernsNephrolepis cordifolia (SHADE IN DESERT)moderate water, Central Valley2.67 gal/sq ft in JulyDayliliesHemerocallis spp. & cvs.moderate water, Central Valley2.67 gal/sq ft in JulyBegoniasBegonia & ssp. & cvs. (shade in the desert)moderate water, Central Valley2.67 gal/sq ft in JulyCalla LiliesZantedeschia aethiopica SHADE IN DESERTmoderate water, Central Valley2.67 gal/sq ft in JulySageSalvia officinalis and cvs.moderate water, Central Valley2.67 gal/sq ft in JulyBicolor MonkshoodAconitum cammarummoderate water, Central Valley2.67 gal/sq ft in JulyGarden MonkshoodAconitum napellusmoderate water, Central Valley2.67 gal/sq ft in JulyCarpet BugleAjuga reptans (shade)moderate water, Central Valley2.67 gal/sq ft in JulySnowdrop WindflowerAnemone sylvestrismoderate water, Central Valley2.67 gal/sq ft in JulyMountain SandwortArenaria montanamoderate water, Central Valley2.67 gal/sq ft in JulySwamp MilkweedAsclepias incarnatamoderate water, Central Valley2.67 gal/sq ft in JulyOrange MilkweedAsclepias tuberosamoderate water, Central Valley2.67 gal/sq ft in JulyGreater MasterwortAstrantia majormoderate water, Central Valley2.67 gal/sq ft in JulyHardy BegoniaBegonia grandismoderate water, Central Valley2.67 gal/sq ft in JulyWax BegoniaBegonia semperflorensmoderate water, Central Valley2.67 gal/sq ft in July

Similar plants that need less water 16

Rated lower than scarlet 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 JulyGeraniumsPelargonium x hortorum and cvs.low water, Central Valley1.07 gal/sq ft in JulySalviaSalvia greggii ssp., cvs. & hybridslow water, Central Valley1.07 gal/sq ft in JulyIrisesIris germanicalow water, Central Valley1.07 gal/sq ft in JulySucculentsEcheveria spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyRussian SagePerovskia spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyYarrowAchillea millefolium (non-native hybrids)low water, Central Valley1.07 gal/sq ft in JulyMountain AlyssumAlyssum montanumlow water, Central Valley1.07 gal/sq ft in JulySilver SageSalvia argentealow water, Central Valley1.07 gal/sq ft in JulyPurple MulleinVerbascum phoeniceumlow water, Central Valley1.07 gal/sq ft in JulySilvery YarrowAchillea clavennaelow water, Central Valley1.07 gal/sq ft in JulyFern Leaf YarrowAchillea filipendulinalow water, Central Valley1.07 gal/sq ft in JulyCalifornia Native YarrowAchillea millefolium (CA native cultivars)low water, Central Valley1.07 gal/sq ft in JulyWoolly YarrowAchillea tomentosalow water, Central Valley1.07 gal/sq ft in JulyPerennial HollyhockAlcea rosea cvs.low water, Central Valley1.07 gal/sq ft in JulyMountain Lady's MantleAlchemilla alpinalow water, Central Valley1.07 gal/sq ft in July
Minutes for your sprinklers

Turn this into minutes on your water days

Every city page has a calculator that takes scarlet 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 often should I water scarlet milkweed in summer?

In July, established scarlet milkweed in loam needs a deep watering about every 5 days in the Central Valley, about every 18 days on the south coast and about every 14 days in the South Inland zone. Sandy soil holds less water, so waterings come closer together. Clay stretches them. Check the soil before each watering and skip one after a soaking rain.

How much water does scarlet milkweed need per day?

In July in the Central Valley, established scarlet 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 scarlet milkweed change across California?

Yes. WUCOLS rates scarlet milkweed separately for each of the six California climate zones: moderate water in the North-Central Coastal zone, moderate water in the Central Valley zone, low water in the South Coastal zone, low water in the South Inland zone, not evaluated in the High and Intermediate Desert zone, low water in the Low Desert zone. The monthly table above shows how the same rating translates into different gallon amounts as reference evapotranspiration changes.

Is scarlet milkweed bad for monarchs?

It can be in mild areas, where its leaves stay green through winter and build up a parasite that harms monarchs. It may also disrupt their migration. Cut it to the ground in November and keep it trimmed through winter, or replace it with a California native milkweed.

Is scarlet milkweed sap poisonous?

Yes. The plant is toxic if eaten in quantity, and the milky sap can irritate skin and eyes. Wear gloves when cutting it back, and keep pets from chewing it.

How do I get rid of aphids on milkweed?

Spray them off with a forceful stream of water or pick them off by hand. Avoid insecticides, which harm monarch eggs and caterpillars. Sooty mold usually weathers away once the aphids are gone.

Should I remove scarlet milkweed seed pods?

Clipping pods before they split keeps the silky seeds from drifting through the garden. The plant self-sows readily and can turn weedy. Removing pods does not change its water needs.

Can scarlet milkweed survive frost?

It is tender and often grown as an annual where winters freeze. In mild gardens it survives and regrows from the base after a winter cutback.

More plant watering guides

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Sources