WateringTimesWatering plan, $19
Ruellia simplex, WUCOLS IV

How often to water Mexican petunia in California

In July in the Central Valley, established Mexican petunia in loam needs a deep watering about every 20 days: about 8.8 gallons each time for a plant 4 feet across, about 3.0 gallons a week. Mexican petunia is a shrubby, woody-based perennial with long, narrow, dark green leaves and violet trumpet flowers that each last a single day. It blooms from summer into fall and stays evergreen where winters are mild. It grows anywhere from boggy pond edges to dry beds once established. That range cuts both ways, since it spreads by rhizomes and seed, most of all in moist ground.

PerennialShrubRated for 6 of 6 zones

How often to water Mexican petunia, month by month

Your city
Plant width
4 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 Mexican petunia 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 1.5 feet 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 3 inches down in the middle of the clump means it is ready for a deep soak
  • Narrow leaves drooping late in the day, with dry soil below, show the clump has run short of water
  • A flush of seedlings and new shoots beyond the clump is a cue to water less, since it spreads most in moist ground
  • Bare, spent stems after a round of bloom call for a trim rather than extra water to bring on new flowers

How to water it

  • Water at the base of the clump with drip or a slow hose, wetting the soil under the outer stems
  • Soak until water reaches about a foot down, then let the top few inches dry before the next soak
  • Mulch the bed to slow drying, and pull seedlings that sprout in the mulch while they are small
  • Keep its irrigation off neighboring beds and open ground, where stray seeds and rhizomes take hold fastest in moist soil
  • Plant in fertile, well-drained soil in full sun for the best flowering, knowing it also takes boggy ground

Through the seasons

Water a new Mexican petunia regularly through its first summer, until its rhizomes have spread through the bed. After that it handles dry spells well. It flowers from summer into fall, so that is when soil checks matter most. In mild-winter gardens the leaves stay green, and the plant needs little extra water once winter rain arrives. Winter rain covers the cool months on the coast and in the Central Valley, but not in the deserts. WUCOLS rates Mexican petunia moderate water on the north-central coast and in South Inland. It rates low water in the Central Valley, on the south coast, in the high desert and in the low desert.

In pots

Mexican petunia grows well in a container on a patio, and a pot keeps its rhizomes from roaming. A dwarf cultivar suits a smaller pot. Check the mix 2 inches down every few days in summer, and daily during South Inland or desert heat. Water until it drains from the holes, then empty the saucer. Ripe pods fling seed in all directions, so trim spent flower stems or choose a sterile cultivar. Growth slows in winter, so let the mix dry a little more between soakings.

1 Your climate zone six WUCOLS zones, each from its own reference city
1.07gal per sq ft in JulyLow water use in the Central Valley zone
A year
6.84 gal per sq ft
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses
0.631.31.92.50.16JAN0.26FEB0.43MAR0.61APR0.83MAY0.99JUN1.07JUL0.95AUG0.72SEP0.47OCT0.22NOV0.14DEC0.631.31.92.50.16JFM0.61AMJ1.07JAS0.47OND
Gallons per square foot per month, Central Valley. Peak 1.07 in Jul, 6.84 a year.

Monthly water need by climate zone

Mexican Petunia: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.370.160.310.780.30.31
Feb0.530.260.320.870.370.37
Mar0.870.430.461.30.580.61
Apr1.180.610.561.610.760.78
May1.460.830.641.950.950.98
Jun1.610.990.692.291.141.12
Jul1.611.070.792.511.221.14
Aug1.490.950.742.331.121.04
Sep1.240.720.61.860.860.83
Oct0.870.470.461.330.580.6
Nov0.470.220.330.90.370.38
Dec0.310.140.260.680.260.27
Year126.846.218.418.518.43

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 low water in the Central Valley, like Mexican petunia. Plants that drink alike can share one valve and one schedule.

LavenderLavandula spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyRosemaryRosmarinus officinalislow water, Central Valley1.07 gal/sq ft in JulyBougainvilleaBougainvillea & spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyAgapanthusAgapanthus orientalislow water, Central Valley1.07 gal/sq ft in JulyGeraniumsPelargonium x hortorum and cvs.low water, Central Valley1.07 gal/sq ft in JulyOlive TreesOlea europaea ssp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyCrape MyrtleLagerstroemia spp., hybrids & cvs.low water, Central Valley1.07 gal/sq ft in JulyLantanaLantana camara ssp. & 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 JulyAloeAloe spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulySucculentsEcheveria spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyJunipersJuniperus 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 July
Minutes for your sprinklers

Turn this into minutes on your water days

Every city page has a calculator that takes Mexican petunia, 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 Mexican petunia in summer?

In July, established Mexican petunia in loam needs a deep watering about every 20 days in the Central Valley, about every 27 days on the south coast and about every 9 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 Mexican petunia need per day?

In July in the Central Valley, established Mexican petunia needs about 0.4 gallons a day for a plant 4 feet across and about 1.7 gallons a day for a plant 8 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 Mexican petunia change across California?

Yes. WUCOLS rates Mexican petunia separately for each of the six California climate zones: moderate water in the North-Central Coastal zone, low water in the Central Valley zone, low water in the South Coastal zone, moderate water in the South Inland zone, low water 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 Mexican petunia invasive?

It spreads by rhizomes and by seed, and it has escaped gardens in parts of the southeastern United States. The Florida Exotic Pest Plant Council lists it as a Category One invasive species. It is hard to remove once established, so plant it only where it can be contained.

Are there sterile Mexican petunias?

Yes, some cultivars such as 'Purple Showers' are sterile and set no seed. They still spread by rhizomes, so the clump widens over time. Dwarf forms and pink or white flowered types are also sold.

Can Mexican petunia grow in standing water?

Yes. It thrives as a marginal water plant at pond edges and in boggy soil. It is also most invasive in moist areas, so a pond edge that drains toward wild land is a poor spot for it.

Why do Mexican petunia flowers last only a day?

Each flower opens for a single day and then drops. New buds keep coming from summer into fall. Cutting back stems after a flush of bloom encourages more flowers.

Does Mexican petunia attract butterflies?

Yes, the violet flowers attract butterflies, and the long bloom season feeds them from summer into fall. Deer seldom damage the plant.

Why do seeds shoot out of my Mexican petunia?

The bean-like pods split open explosively when ripe and throw seed in all directions. The seeds can persist in the soil. Trim off spent flower stems before the pods mature to cut down on seedlings.

More plant watering guides

StonecropAdam's NeedleLavenderRosemaryBougainvilleaAgapanthusGeraniumsOlive Trees

Sources