Here we offer a summary of month-by-month growing conditions in the Flagstaff area. For advice about gardening tasks throughout the year, see Resources for Gardeners on The Arboretum at Flagstaff website.
Remember: The average growing season in the Flagstaff area is 103 days. This means that on average, temperatures drop below 32° more than 250 nights a year although they are much less likely to do so after the second week of June. See Freeze and Frost Summary.
- January: Winter storms from the Pacific coast pass over northern Arizona. Between storms, the snow quickly sublimates (evaporates) to leave the ground bare in sunny areas. Without an insulating layer of snow, soil and plants are vulnerable to freezing, frost-heaving, and desiccation. Mulching soil and dormant plants lessens the chance of freezing or frost-heaving. Beware of a possible “January thaw,” a brief period of several warm and sunny days early in the year. Supplemental watering may be needed now and then throughout the winter.
- February: Winter storms from the Pacific coast continue to pass over northern Arizona. Conditions resemble those in January.
- March: Historically, March is the snowiest month of the year. Yet some perennials do emerge, especially on southern exposures. Fruit trees may begin to leaf out or perennials may sprout, making them vulnerable to freezes. Providing cover—eaves, cloches, floating covers—over plants lessens the likelihood of damage.
- April: The temperature of the air during the day warms up long before the soil becomes warmer. Cold soil inhibits the germination and growth of many plants. Yet small spring blooms appear, especially in places sheltered from bright sun and wind.
- May: Days are warm, dry and windy. Supplemental watering will likely be needed in exposed areas. Postpone planting nursery stock in the ground until the rains begin in July. However, planting in containers works fine at this time of year.
- June: June 9 is the average last overnight frost—32 degrees or lower. Records from 1950 to 2013 show that the latest was July 8, 1955. June has the longest, sunniest days but it can drop below freezing overnight. It’s also very windy, with little if any precipitation. Supplemental watering will likely be needed in most areas. Sunny, dry, windy, and potential freezing are very difficult conditions for seedlings or nursery transplants. Wait until the rains begin in July to establish perennials or transplant nursery stock.
- July: Days are warm but the wind gradually diminishes. Summer rains typically begin this month. Mornings are generally clear with thunderstorms developing in the afternoons. Protect vulnerable plants from runoff and possible hail.
- August: Summer rains continue. Mornings are generally clear with thunderstorms developing in the afternoons. Protect vulnerable plants from runoff and possible hail. Records from 1950 to 2013 show that August 22, 1968, was the earliest initial fall frost—32 degrees or lower. August 23, 1968, was the earliest initial fall freeze—28 degrees or lower.
- September: Summer rains end; humidity drops. September 24 is the average date of the first frost of the fall season—32 degrees or lower.
- October: Days become progressively cooler and drier, and there are more hours of darkness than of sunlight. October 5 is the average date for the first freeze of the fall season—28 degrees or lower. Records from 1950 to 2013 show that October 14, 1981 was the latest initial fall frost—32 degrees or lower. October 27, 2010, was the latest initial fall freeze—28 degrees or lower.
- November: Successive winter storm fronts begin to arrive from the Pacific coast. Between snowstorms, the snow usually sublimates, leaving the ground bare especially in sunny area.
- December: Successive winter storm fronts continue to arrive from the Pacific coast. Between snowstorms, the snow will likely sublimate and leave the ground bare in sunny area.