How often to water giant red lobelia in California
In July in the Central Valley, established giant red lobelia in loam needs a deep watering about every 14 days: about 1.5 gallons each time for a plant 2 feet across, about 0.8 gallons a week. Giant red lobelia is a towering Chilean perennial with a woody base, felted gray-green leaves and purplish red stems. Spikes of tubular, bright red flowers open from mid to late summer into fall. It comes from sandy and gravelly soils near the coast, so it wants sharp drainage. It is harmful if eaten and can irritate skin.
How often to water giant red lobelia, 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 3 inches down at the edge of the clump in summer means it is time for a deep soak
- Large felted leaves that hang limp into the morning over dry soil point to a thirsty root zone
- Lower leaves yellowing while the soil stays wet for days point to poor drainage rather than thirst
- Ragged holes in young leaves and slime trails point to slugs, not to a watering problem
- Foliage dying back in the cool season is normal for this deciduous perennial, not a sign of drought
How to water it
- Water at the base of the clump with drip or a slow hose, keeping spray off the felted leaves
- Soak the root zone about a foot down, then let the top few inches dry before the next soak
- Plant in sandy or gravelly soil that drains freely, like the coastal slopes of its native Chile
- Spread mulch around the clump but keep it clear of the woody base
- Widen the watered area as the clump spreads by rooting stems and self-sown seedlings
Through the seasons
Water a new plant through its first summer, until its roots spread well beyond the planting hole. Established clumps need less, and the closest soil checks belong from midsummer through the fall bloom. Its foliage is deciduous, and the stems can be cut back once flowering ends. Winter rain covers the cool months on the north-central coast, in the Central Valley and on the south coast. WUCOLS rates giant red lobelia low water on the north-central coast and low water in the Central Valley. It rates moderate water on the south coast. WUCOLS has not evaluated it for South Inland, the high desert or the low desert.
In pots
Giant red lobelia can reach 4 to 8 feet tall, so only a very large, heavy container will hold it upright. Use a gritty mix that drains freely. Check it 2 inches down every few days in summer, and more often during hot inland spells. Water until it runs from the drainage holes, then empty the saucer. Set the pot in a sheltered spot out of strong wind, in sun or light afternoon shade. After flowering, cut the stems back and let the mix dry further between winter soakings.
- A year
- 6.84 gal per sq ft
- Reference
- Sacramento climate table
- Counts
- Plant need, before sprinkler losses
Monthly water need by climate zone
Giant Red Lobelia: gallons per square foot per month, by WUCOLS climate zone
| Month | N. Coast | C. Valley | S. Coast | S. Inland | Hi Desert | Lo Desert |
|---|---|---|---|---|---|---|
| Jan | 0.15 | 0.16 | 0.78 | – | – | – |
| Feb | 0.21 | 0.26 | 0.81 | – | – | – |
| Mar | 0.35 | 0.43 | 1.15 | – | – | – |
| Apr | 0.47 | 0.61 | 1.4 | – | – | – |
| May | 0.58 | 0.83 | 1.61 | – | – | – |
| Jun | 0.64 | 0.99 | 1.74 | – | – | – |
| Jul | 0.64 | 1.07 | 1.98 | – | – | – |
| Aug | 0.6 | 0.95 | 1.86 | – | – | – |
| Sep | 0.5 | 0.72 | 1.49 | – | – | – |
| Oct | 0.35 | 0.47 | 1.15 | – | – | – |
| Nov | 0.19 | 0.22 | 0.84 | – | – | – |
| Dec | 0.12 | 0.14 | 0.65 | – | – | – |
| Year | 4.8 | 6.84 | 15.5 | – | – | – |
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 low water in the Central Valley, like giant red lobelia. Plants that drink alike can share one valve and one schedule.
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 giant red lobelia, 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 giant red lobelia in summer?
In July, established giant red lobelia in loam needs a deep watering about every 14 days in the Central Valley and about every 7 days on the south coast. 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 giant red lobelia need per day?
In July in the Central Valley, established giant red lobelia needs about 0.1 gallons a day for a plant 2 feet across and about 0.4 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 giant red lobelia change across California?
Yes. WUCOLS rates giant red lobelia separately for each of the six California climate zones: low water in the North-Central Coastal zone, low water in the Central Valley zone, moderate water 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.
Is giant red lobelia poisonous?
Yes. It is harmful if eaten, for people and for pets, and it can irritate skin. Wear gloves when you cut it back or pull seedlings, and plant it away from where pets chew.
How tall does giant red lobelia get?
It usually reaches 4 to 8 feet tall and 3 to 5 feet wide on upright stems. Give it a sheltered spot out of strong wind. Leave room for the clump to widen over time.
Does giant red lobelia spread?
Yes. It self-sows and can spread by rooting stems to form small colonies. Pull unwanted seedlings while they are small. Removing spent spikes before seed ripens cuts down on new plants.
When should I cut back giant red lobelia?
Cut it back after flowering ends. Its foliage is deciduous, so a bare clump in winter is normal. Water it in winter only when rain fails and the soil is dry several inches down.
Does giant red lobelia need full sun?
It takes full sun or part shade, and cool sun suits it best. Shaded soil dries more slowly, so check it before each soak.
Is every plant sold as giant red lobelia the same?
Not always. A related Chilean shrub, Lobelia excelsa, has been sold under this name. It is woodier, with reddish orange flowers from spring to early fall. Check the label and the bloom season.
More plant watering guides
Sources
- WUCOLS IV plant water use classification for Lobelia tupa, 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