Fall Watering: Keep Going Until the Ground Freezes

Watering usually stops on the first cool week of September. The air changed, so the season feels over, and the hose goes back on its hook. The soil has not caught up yet, and neither have the roots.

The air cools before the soil does

Soil holds summer heat well after the air lets go of it. University of Illinois Extension describes the fall pattern this way: "In the fall, soils hold onto heat longer while the above ground temps may be dropping." That warm soil is where root growth is still happening. Illinois puts new fall plantings on water "as needed up to the first hard frost," because the root systems need time to establish new roots in the soil and gradually go into dormancy as colder winter temperatures arrive.

Roots also stay awake longer than the rest of the plant. University of Minnesota Extension notes that roots "do not become dormant in the winter as quickly as stems, branches and buds" and are "less hardy than stems." The part of the plant still working in September is also the part least able to take a hard winter after a dry fall.

The stopping point is your own ground freezing

There is no national date for this. Iowa State University Extension gives the rule as a condition instead: for trees and shrubs planted this season, "a thorough watering every 7 to 10 days (in dry weather) should be sufficient 3 to 4 weeks after planting. Continue watering until the ground freezes."

That travels because it moves with you. If your ground freezes in early October, you have about a month of watering left. If it freezes in December, you have three. On the Gulf coast it may not freeze at all this year, in which case the watering season does not close, it just slows down with the weather.

Anything planted in the last few weeks needs more attention than that. Iowa State starts newly planted trees on water "every day for 3 or 4 days and then gradually reduce the frequency of watering," reaching the 7 to 10 day rhythm about three or four weeks after planting.

How deep, and how often

Depth does more than frequency. Colorado State University Extension asks for water down to about 12 inches for trees, applied "to many locations under the dripline and beyond if possible," and gives a size-based amount: "Apply 10 gallons of water for each diameter inch of the tree. For example, a two-inch diameter tree needs 20 gallons per watering." For shrubs it gives "5 gallons two times per month for a newly planted shrub," 5 gallons monthly for small established shrubs under 3 feet, and 18 gallons monthly for large established shrubs over 6 feet.

Those numbers come out of a dry-winter western climate with long stretches of bare, snow-free ground, so read them as a picture of how much water a soak actually takes rather than as a national schedule. The transferable part is the shape: a slow, long application over a wide area, not a quick pass with a nozzle.

Evergreens are the ones that pay for a dry fall

A deciduous shrub going into winter dry is a problem you may not see. An evergreen going into winter dry usually shows it. University of Minnesota Extension describes the mechanism: "Winter sun and wind cause excessive foliage water loss while the roots are in frozen soil and unable to replace lost water. This results in desiccation (drying out) and browning of the plant tissue."

Minnesota's own fall sequence for evergreens is a taper and then a soak: "Keep evergreens properly watered throughout the growing season and into the fall. Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up." Those months are written for Minnesota, so shift them to your own freeze date rather than borrowing the calendar. Where fall has been dry, Minnesota also advises watering "heavily before the ground freezes to reduce frost penetration."

The browning shows up in spring, months after the September that caused it, which is why it usually gets blamed on the winter.

Dry-winter regions do not stop at the freeze

In much of the arid west, the watering job continues through winter on the mild days. Colorado State warns that "injury or death to parts of plant root systems can result from long, dry periods during fall and winter," and sets two conditions on winter watering: "Water only when air temperatures are above 40 degrees F," and "monitor weather conditions and water during extended dry periods without snow cover, about once or twice each month." Water early enough in the day that it soaks in before night. Colorado State also flags which perennials are worth the trip: "Winter watering is advisable with late-planted perennials, bare root plants, and perennials located in windy or southwest exposures."

If your winters bring regular snow cover or rain, this section is not your job. If they bring bare, frozen, sunny ground for weeks at a time, it is.

Mulch can wait a while

Mulching the moment you plant works against fall root growth. Illinois Extension: "Fall plants don't need to be mulched right away and can benefit from the sun warming the soil. Mulching can wait until night-time temps are getting around 32F." Iowa State puts fall-planted perennials under several inches of straw or pine needles in late fall, which is the same timing said differently.

What to do this week

Find your average first freeze date, then count the weeks left. Put anything planted this season on a soak every 7 to 10 days in dry weather, and keep established trees and shrubs on a longer version of the same. Give evergreens one heavy watering before the ground closes up. Leave the mulch bag in the garage until nights get near freezing.