Drip irrigation in dry air and wind
How to water vegetables and xeric plantings on separate zones, check depth instead of the clock, and avoid a universal inch-a-week rule.
Dry air and wind take water before a plant does. A sprinkler at noon on a June day along the Front Range can lose a dismaying share of what it throws, to evaporation and to drift onto the sidewalk.
Drip and soaker lines put water at the soil, on a calm schedule, and they let you water the tomato without watering the penstemon. That separation is the design. The hardware is the easy part.
There is no honest single number, in inches per week, that fits a windy week in Pueblo and a cool week in the Gallatin Valley. Evaporation, soil, and the plant’s stage all move the need.
Local water utilities and extension fact sheets publish seasonal lawn schedules for their own systems. Use those for turf. For vegetables and new plantings, use a probe and a plant, and treat any printed inch as a starting hypothesis you are required to check.
Zones that match the plants
Split the system by water need, which Denver Water’s xeriscape thinking calls hydrozones. At least three are worth plumbing if you are building from scratch.
Vegetables and annuals. Steady moisture in the root zone while they are sizing. Miss a week in July and beans flower into the void, carrots crack, and tomatoes split when the next irrigation hits. Drip on a regular rhythm, then adjust.
New trees, shrubs, and perennials, for the first one or two summers. They are not drought-tolerant yet. A separate line, or a diligent hose, keeps them alive until the roots are out of the planting hole.
The plant pages assume this establishment period. A “drought-tolerant” label that dies in August of year one was not a lie. It was a schedule you skipped.
Established low-water plantings. Deep, infrequent soakings, then a long gap. Many of these plants should be allowed to look a little spare in a dry August. If they share a valve with the lettuce, you will either drown them or starve the lettuce.
A lawn, if you keep one, is a fourth zone. It wants a different head, a different time of day, and a different season. Buffalo grass and blue grama, covered in water-wise lawns, are not on the Kentucky bluegrass clock even when they are the lawn.
Depth is the measurement
A screwdriver, a piece of rebar, or a soil probe tells you more than the timer. After an irrigation, the tool should push into moist soil through the root zone you care about: six to eight inches for a lettuce bed, a foot or more for a tomato in midsummer, deeper for a young tree if you are watering the basin and not just the trunk.
If it stops in dust at three inches, you are coloring the surface. If it squelches at two inches and the clay below is dry, you are sealing the top. Clay needs cycle and soak: run the zone until water starts to sit, wait an hour, run it again, and repeat until the depth is right. Then lengthen the days between irrigations until the probe says the root zone is drying.
Morning is the right time. Leaves dry, wind is often lower, and you can see what happened. Evening watering in a cold frame or under a dense canopy lingers and feeds disease. Midday sprinklers in a chinook are a donation to the air.
Mulch changes the interval. A bed under a mulch that stays put, discussed in mulch and wind, can go longer between waterings than bare soil.
Check anyway. Mulch can also hide a dry bed. The surface looks finished while the soil below is powder.
Drip layouts that survive a season
Put lines on the surface where you can see them, under mulch if the mulch is not so coarse that it lifts the emitters off the soil. Burying thin drip tubing in clay is how you lose it to a shovel and to roots that plug emitters.
Use pressure regulation and a filter. Western water is often hard, and emitters plug. Open the ends of the lines and flush them a few times a season.
Emitters should sit near the plant, moved outward as a shrub grows, not left in a tight circle at the trunk forever.
A tree watered only at the trunk grows a lazy root system and then falls over in the first real wind. Widen the basin each year. For vegetables, a line every crop row, or two lines on a four-foot bed, beats one line down the middle that leaves the edges in the rain shadow of your own irrigation.
Soaker hoses are drip’s less precise cousin. They are fine on a short bed. They water unevenly on a long run and on a slope. On a slope, put the supply at the top, use emitters rather than a soaker if you can, and watch the bottom of the bed for a bog.
Freeze protection is a Western chore. Drain the system before hard freezes.
A line left full in October becomes a line of ice and splits, and in May you discover it as a dry bed and a high water bill. Blow it out or open the low points.
Winter watering of trees and evergreens, on a mild day when the hose can run, is a separate and real need in dry winters. A drip system that is drained cannot do that job. A hose and a basin can.
New plantings versus August
Seedbeds need frequent, shallow moisture until germination. Carrots, which the Colorado Vegetable Guide says can take three weeks, are the extreme. A mister or a gentle overhead spray is legitimate for that period even if the rest of the garden is on drip. Once the row is up, switch to the drip and mulch the shoulders so the surface does not crust.
Containers and raised beds dry faster than flat ground, especially when the air is moving. They may need water on a day the native bed does not. Probe them separately. A tall bed in wind is a different zone pretending to be part of the vegetable zone.
In a dry August, priority goes to plants that are sizing fruit or that were planted this year. Established native grasses and penstemons can wait. A vegetable garden cannot.
If a municipal restriction cuts days, water the food and the new trees on the allowed day to the full depth, and let the ornamental gravel garden coast. That is the point of having built it as a low-water zone.
Checklist
- Put vegetables, new woody plants, established xeric plantings, and lawn on separate valves.
- Probe after you water. Add time or cycles until the root zone is wet, then stretch the interval.
- Water in the morning.
- Filter and regulate drip, and flush the lines.
- Move emitters off the trunk as plants grow.
- Drain the system before a hard freeze.
- Use overhead water only for germination, then return to drip.
- In a restriction, water food and first-year plants first.
Rain barrels are a small supplement, not this system. Colorado’s residential allowance, summarized by the state engineer and by CSU Extension, is generally two barrels totaling no more than 110 gallons, from a residential roof, for outdoor use on the same property.
That will fill a few containers. It will not carry a tomato bed through August. Other states write different rules. Do not build a cistern from a Colorado barrel article.
Sources
- Denver Water, Xeriscape principles, including efficient irrigation and separate turf zones. The 2017 Water Wise Landscape Handbook lists the seven principles.
- CSU Extension, Colorado Vegetable Guide, on low precipitation and on keeping carrot seedbeds damp.
- CSU Extension, Rainwater Collection in Colorado.
- Colorado Division of Water Resources, rain barrel guidance.
- CSU Extension, Iron Chlorosis of Woody Plants, on overwatering and chlorosis.