How often to water arnica in California
In July on the north-central coast, established arnica in loam needs a deep watering about every 9 days: about 1.5 gallons each time for a plant 2 feet across, about 1.1 gallons a week. Arnica is a low, clump-forming perennial from the meadows and mountains of Europe. It makes a rosette of downy leaves topped by bright yellow-orange daisies in summer. It wants moist but well-drained, humus-rich soil and dies back to the ground for winter. It is harmful if eaten and can irritate skin, so handle it with gloves.
How often to water arnica, 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 an inch or two down around the rosette signals that arnica needs water during spring and summer growth
- Leaves wilting over dry soil in afternoon sun can indicate thirst in a plant that prefers moist ground
- Leaves yellowing and the crown softening in soggy ground can indicate overwatering or poor drainage
- Ragged holes in the leaves point to slugs or caterpillars rather than a watering problem
- Top growth disappearing in winter is normal dormancy, not a sign of drought
How to water it
- Water arnica at the base of the rosette with drip or a watering can rather than overhead
- Moisten the top 6 inches of soil, then water again once the surface inch has dried
- Work compost into the bed so the soil holds moisture yet drains freely
- Mulch lightly around the clump to keep the root zone cool and evenly damp
- Extend watering outward as the spreading roots widen the clump
Through the seasons
Keep new plants or divisions evenly moist until they root in. Established clumps take deeper watering through spring and summer, while the rosette grows and flowers open from late spring into summer. Cut the stems down after flowering, and expect growth to die back for winter. Winter rain covers the dormant crowns on the north-central coast. WUCOLS rates arnica moderate water on the north-central coast. WUCOLS has not evaluated it for the Central Valley, the south coast, the high desert or the low desert. It marks South Inland as not suited to it.
In pots
Arnica grows well in a container, which also keeps its spreading roots in check. Use a pot with drainage holes and a humus-rich mix that holds moisture but drains. Check the mix an inch down every few days in summer and water when it feels dry. Water until excess runs out, then empty the saucer. Move the pot into afternoon shade on hot days if it dries fast. In winter the top dies back, so water only enough to keep the mix from drying completely.
- A year
- no figure
- Reference
- Sacramento climate table
- Counts
- Plant need, before sprinkler losses
WUCOLS lists Arnica as not evaluated for the Central Valley zone, so no monthly figure is computed here.
Monthly water need by climate zone
Arnica: gallons per square foot per month, by WUCOLS climate zone
| Month | N. Coast | C. Valley | S. Coast | S. Inland | Hi Desert | Lo Desert |
|---|---|---|---|---|---|---|
| Jan | 0.37 | – | – | – | – | – |
| Feb | 0.53 | – | – | – | – | – |
| Mar | 0.87 | – | – | – | – | – |
| Apr | 1.18 | – | – | – | – | – |
| May | 1.46 | – | – | – | – | – |
| Jun | 1.61 | – | – | – | – | – |
| Jul | 1.61 | – | – | – | – | – |
| Aug | 1.49 | – | – | – | – | – |
| Sep | 1.24 | – | – | – | – | – |
| Oct | 0.87 | – | – | – | – | – |
| Nov | 0.47 | – | – | – | – | – |
| Dec | 0.31 | – | – | – | – | – |
| Year | 12 | – | – | – | – | – |
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 arnica. Plants that drink alike can share one valve and one schedule.
Lilyleaf LadybellsAdenophora liliifolianot rated, Central Valleyno Central Valley rating
Frilly BergeniaBergenia ciliatanot rated, Central Valleyno Central Valley rating
Cabbage on a StickBrighamia insignisnot rated, Central Valleyno Central Valley rating
Basket PlantCallisia fragransnot rated, Central Valleyno Central Valley rating
Forest SedgeCarex sylvaticanot rated, Central Valleyno Central Valley rating
Umbrella DracaenaDianella ensifolianot rated, Central Valleyno Central Valley rating
Rusty FoxgloveDigitalis ferrugineanot rated, Central Valleyno Central Valley rating
Big-Flowered FoxgloveDigitalis grandifloranot rated, Central Valleyno Central Valley rating
Woolly FoxgloveDigitalis lanatanot rated, Central Valleyno Central Valley rating
Sunset FoxgloveDigitalis obscuranot rated, Central Valleyno Central Valley rating
Male FernDryopteris filix-masnot rated, Central Valleyno Central Valley rating
Alpine Sea HollyEryngium alpinumnot rated, Central Valleyno Central Valley rating
Purple SpurgeEuphorbia dulcisnot rated, Central Valleyno Central Valley rating
Queen of the MeadowFilipendula ulmarianot rated, Central Valleyno Central Valley rating
DropwortFilipendula vulgaris (Filipendula hexapetala)not rated, Central Valleyno Central Valley rating
GentianGentiana scabra procumbensnot rated, Central Valleyno Central Valley ratingTurn this into minutes on your water days
Every city page has a calculator that takes arnica, 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 arnica need per day?
In July on the north-central coast, established arnica needs about 0.2 gallons a day for a plant 2 feet across and about 0.7 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 arnica change across California?
Yes. WUCOLS rates arnica separately for each of the six California climate zones: moderate water in the North-Central Coastal zone, not evaluated in the Central Valley zone, not evaluated in the South Coastal zone, not suited to this region 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.
Is arnica poisonous?
Yes. It is harmful if eaten and can cause skin irritation, so wear gloves when dividing or cutting it. The flowers and roots carry the toxic compounds. Keep it away from areas where children play.
Why is my arnica flopping over?
Its single flowers sit on stems up to 2 feet tall, and wind can push them over. Stake taller stems in exposed spots. Cutting the stems back before flowering keeps them shorter and can bring a second bloom.
Does arnica spread?
Yes. Its roots can spread and take over more space than intended. Divide the clump in spring or fall every few years to keep it vigorous and in bounds.
Can you overwater arnica?
Yes. It likes moist soil but needs good drainage, and a soggy crown can rot. Let the surface dry slightly before watering again, especially while it is dormant in winter.
When does arnica bloom?
Its yellow-orange daisies, 2 to 3 inches across, open from late spring into summer, one to a stem. Cut the stems down once flowering is finished. Keep the soil evenly moist while buds form.
More plant watering guides
Sources
- WUCOLS IV plant water use classification for Arnica montana, 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