WateringTimesWatering plan, $19
Ambrosia psilostachya, WUCOLS IV

How often to water western ragweed in California

In July on the north-central coast, established western ragweed in loam needs a deep soak only about once a month or less: about 1.5 gallons each time for a plant 2 feet across. Western ragweed is a weedy native perennial found throughout California in roadsides, dry fields and disturbed patches of chaparral and coastal sage scrub. It spreads by rhizomes and stands 8 inches to about 3 feet tall. Its pollen may trigger allergies. Experienced native nurseries treat it as a sign of a disturbed site rather than a garden plant.

PerennialCalifornia nativeRated for 4 of 6 zones

How often to water western ragweed, 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 western ragweed 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

  • Plants spreading fast after summer watering point to too much water for this weedy native
  • Leaves dropping in late summer drought is normal stress dormancy
  • New shoots popping up across the bed come from spreading rhizomes
  • A site full of western ragweed often signals disturbed soil

How to water it

  • Give it no regular water, since it thrives on neglect
  • Avoid planting it in gardens, especially near allergy sufferers
  • Dig out rhizomes where it is not wanted
  • Restore disturbed ground with other natives
  • Mow or cut it before it flowers to reduce pollen

Through the seasons

This native needs no help from irrigation. It drops its leaves under late summer drought and regrows when rain returns. Watering only encourages its spread by rhizomes, so leave it dry or dig it out before it flowers. Western ragweed gets a very low water rating from WUCOLS on the north-central coast and the south coast. South Inland and the low desert give it a low water rating. The Central Valley and the high desert have no WUCOLS evaluation for it.

In pots

Western ragweed is not a plant to grow in pots or gardens. If a seedling shows up in a container, pull it before it flowers so its pollen does not spread. If you keep one for study, use a sandy mix, water only when the mix is dry, and let it drain fully. Never let it sit in water. Let it rest dry in late summer, when it drops its leaves and rests until rain returns.

1 Your climate zone six WUCOLS zones, each from its own reference city
Not ratedWUCOLS does not rate Western Ragweed for the Central Valley zone
A year
no figure
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses

WUCOLS lists Western Ragweed as not evaluated for the Central Valley zone, so no monthly figure is computed here.

Monthly water need by climate zone

Western Ragweed: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.04–0.080.31–0.31
Feb0.05–0.080.35–0.37
Mar0.09–0.110.52–0.61
Apr0.12–0.140.64–0.78
May0.15–0.160.78–0.98
Jun0.16–0.170.92–1.12
Jul0.16–0.21–1.14
Aug0.15–0.190.93–1.04
Sep0.12–0.150.74–0.83
Oct0.09–0.110.53–0.6
Nov0.05–0.080.36–0.38
Dec0.03–0.070.27–0.27
Year1.2–1.557.37–8.43

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 not evaluated in the Central Valley, like western ragweed. Plants that drink alike can share one valve and one schedule.

Lilyleaf LadybellsAdenophora liliifolianot rated, Central Valleyno Central Valley ratingArnicaArnica montananot rated, Central Valleyno Central Valley ratingFrilly BergeniaBergenia ciliatanot rated, Central Valleyno Central Valley ratingCabbage on a StickBrighamia insignisnot rated, Central Valleyno Central Valley ratingBasket PlantCallisia fragransnot rated, Central Valleyno Central Valley ratingForest SedgeCarex sylvaticanot rated, Central Valleyno Central Valley ratingUmbrella DracaenaDianella ensifolianot rated, Central Valleyno Central Valley ratingRusty FoxgloveDigitalis ferrugineanot rated, Central Valleyno Central Valley ratingBig-Flowered FoxgloveDigitalis grandifloranot rated, Central Valleyno Central Valley ratingWoolly FoxgloveDigitalis lanatanot rated, Central Valleyno Central Valley ratingSunset FoxgloveDigitalis obscuranot rated, Central Valleyno Central Valley ratingMale FernDryopteris filix-masnot rated, Central Valleyno Central Valley ratingAlpine Sea HollyEryngium alpinumnot rated, Central Valleyno Central Valley ratingPurple SpurgeEuphorbia dulcisnot rated, Central Valleyno Central Valley ratingQueen of the MeadowFilipendula ulmarianot rated, Central Valleyno Central Valley ratingDropwortFilipendula vulgaris (Filipendula hexapetala)not rated, Central Valleyno Central Valley rating
Minutes for your sprinklers

Turn this into minutes on your water days

Every city page has a calculator that takes western ragweed, 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 western ragweed in summer?

In July, established western ragweed in loam needs a deep watering about once a month or less 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 western ragweed need per day?

In July on the north-central coast, established western ragweed needs about 0.0 gallons a day for a plant 2 feet across and about 0.1 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 western ragweed change across California?

Yes. WUCOLS rates western ragweed separately for each of the six California climate zones: very low water in the North-Central Coastal zone, not evaluated in the Central Valley zone, very 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.

Should I plant western ragweed?

Native plant growers do not recommend it for gardens. It is weedy and its pollen may be an allergen. Choose other natives.

Is western ragweed native to California?

Yes, it is a native perennial common throughout California. It grows in roadsides, dry fields and disturbed areas. It can indicate a site in trouble.

How does western ragweed spread?

It spreads by rhizomes and can colonize disturbed ground. Dig out rhizomes to control it. Do not water it.

Does western ragweed cause allergies?

Its pollen may be an allergen. Cut or remove it before it flowers. Wear a mask when handling flowering plants.

Why does western ragweed lose its leaves?

It is stress deciduous and drops leaves in summer drought. That is normal. It regrows after rain.

More plant watering guides

Bolivian Mountain CoconutJapanese PhotiniaCommon ReedPinelliaEastern White PineYellow Wood PodocarpusMyrtle-leaf MilkwortSilverweed

Sources