Hydrozone Yard Planner

A hydrozone is a group of plants, not a group of beds

A hydrozone is a set of plants that are watered together because they want the same thing (the same how-often and the same how-much) and therefore get their own valve on the irrigation clock. The grouping is hydraulic rather than botanical. Two shrubs at opposite ends of a garden can belong on the same station because they want the same interval, and two shrubs in the same bed can need separate stations because one is a desert native and the other is not.

The failure this tool exists to catch is the mixed border on a single valve. When a mesquite and a lantana share a station, one of two things happens and both are wrong: the station is set for the mesquite and the lantana sulks through every summer, or the station is set for the lantana and the mesquite is overwatered into a short life and a shallow root system. The fix is not a better timer setting. It is a second valve, and a plan that decides which plants are on it.

Why three zones, and why the third one has nothing in it

AMWUA's plant database carries a water-use field on every profile. Read across the 213 profiles behind this page, that field takes exactly four values: Very Low, Low, Low to Moderate and Moderate. Those are the published ratings, and this page prints them verbatim beside each plant name rather than collapsing them into something tidier.

The three zones are this site's grouping of those four published values, and the mapping is printed above the tool rather than left implicit. Everything AMWUA calls very low or low goes in the low zone, because those plants are genuinely rain-fed once established. Everything it calls low to moderate or moderate goes in the medium zone, because those want help through the hot months.

That leaves the high zone, and it is empty of starter plants deliberately. AMWUA publishes a low-water plant palette, and the highest rating in it is Moderate: the association's database carries no lawn, no vegetable crop and no citrus profile, because those are not what a low-water plant guide is for. There is no published rating to copy into the high zone. Rather than invent one, this tool leaves the zone for the reader to fill, shows no rating against anything placed there, and lists the kinds of planting that usually end up in it (a lawn, a vegetable bed, citrus, pots) as labels with no number attached.

Reading the ratings without over-reading them

Every rating on this page is AMWUA's own word for that species, and each name links straight to the profile it came from, so the check is one click rather than an act of trust. The mature height and spread, the sun exposure, the growth habit and the cold-hardiness figure on each row come from the same profile. In the whole starter list of 48 plants, 23 are rated very low and 12 are rated low to moderate or moderate. That proportion is itself the point: the desert palette is enormous, and the plants that need a second valve are a minority of it.

One field on those rows is worth calling out because it connects this tool to another one. The cold-hardiness figure is the coldest night the profile rates the plant for, and it is the same quantity the frost explorer measures from the opposite direction, counting what Sky Harbor has actually done. A lantana rated to 28 °F and a record showing that 9 November is the earliest the airport has ever frozen are two halves of one decision.

What the grouping does to the clock

This is the part with published numbers in it, and they come from University of Arizona Cooperative Extension Bulletin #41, Irrigating Native Southwest Trees and Shrubs. The publication is explicit that a drip system should run at least two hours for each irrigation, so that the water goes deep rather than often. It says that about every fifth irrigation the drip should run for 8 to 10 hours, which leaches away the salts that have accumulated around the plant, and salt accumulation is the specific problem of a yard watered little and often in a dry climate. It gives a small plant in a one-gallon container a watering twice a week once temperatures are over 100 °F, a daily or at-least-every-other-day case above 105 °F, and regular deep irrigation for the first one to two years after transplanting regardless of the size of the plant when it went in.

Notice what is missing from that list: any number of minutes per station, and any interval for a mature plant. That absence is the reason this tool has no minutes in it either. Minutes are set by the emitter flow rate, the soil and the size of the plant, and a page that printed a friendly-looking number would be inventing the one figure a gardener would actually act on. What the zone gives you is the ordering: the low zone runs least often and longest, the medium zone more often than the low, the high zone most often of the three.

Where the numbers stop

A rating belongs to a species, not to the plant in front of you. A velvet mesquite on a bare slope and a velvet mesquite sitting in a lawn are drinking different amounts, and AMWUA rates the species rather than the site. The figure also does not know your soil, which decides how long a run should be more than the plant does: Bulletin #41 gives a cubic foot of clay soil as holding 1 to 1 2/3 gallons of water against 1/3 to 2/3 of a gallon in sand, so the same two-hour run puts several times as much water past the roots in a sandy yard as in a clay one. And the frequencies above are the publication's small-plant case: they are given for container plants and for the first year or two after transplanting, which is exactly when a planting needs the most attention and exactly when a schedule matters least.

So the plan this tool produces is a plan about structure: which plants share a valve. The interval and the run time are then set by watching what the soil does, which is the part no page can do for you.