Now is the perfect time to plant roses in the Flagstaff area. We still have two months before our first frost, and rose roots will continue to grow until the rose becomes dormant and the ground freezes.
Winters in Flagstaff at our 35 degrees of latitude and 7,000 feet elevation are cold and long. A rose must be cold hardy enough to survive our winters without so much branch die off that it will be unable to increase in size every year.
Rose hybridizers have developed roses suited to a variety of climates. The genetics of a rose are set from the time it sprouts from a rose seed, or grows from a leaf bud or cutting. More water, fertilizer and mulch will not enable a cold-tender rose to survive winters.
You can reliably purchase plants that are suited to your zip code. By the 1960s the United States Department of Agriculture (USDA) had developed a map of the United States which documented the average minimum winter temperatures taken from a vast number of weather stations in every area of the country. The 2012 update of this USDA Hardiness Zone Map detailed 13 zones — 1 being the coldest, in Alaska, and 13 being the hottest, in Puerto Rico. Zones 2-10 occur in the continental United States. These zones are noted on the tags of plants sent to nurseries.
The cold USDA Zones of Hardiness that Flagstaff falls into are 5b-6a. The most common zone ranges on the tags of the plants are usually Zones 5-9 or Zones 6-9. Allowing for the possibility of a really severe winter, never buy a rose that is not cold hardy to Zone 5! That number is easily remembered. It is the number of fingers on your hand. Immediately put back any rose that is cold hardy only to Zone 6. It is not bred for our mountain climate, will decline and die.
Also be sure to double check the cold zone range on the rose tags. Mistakes are sometimes made. Use the search feature on your cell phone. In the Search box put in “rosa,” a space, then the name of the rose and press Search. A half dozen websites of nurseries selling this rose will appear. Open at least four of these to be sure each says this rose is cold hardy to Zone 5. If the majority of those websites do not agree that the rose is cold hardy to Zone 5, put the rose back.
Secondly, you need to always plant each rose deeply. To withstand our cold temperatures you need to plant the waist of the rose, where the canes meet the roots, two inches below the level of the ground.
Many modern roses are budded, where a leaf bud from the rose that is being grown is grafted onto the top of a robust rootstock. If this area where the budded rose plant originated freezes beyond its tolerance, the budded plant will die and you will have lost the rose variety you bought. The rootstock will then send up its own canes, and rootstock rose blooms are not glamorous.
So be sure to plant the rounded bud union two inches below the level of the ground. Before you mulch in the fall, be sure there are still two inches of soil over this vital area of your roses.
Determining the cold hardiness of a rose then buying only roses that are cold hardy to Zones 5, 4, 3 or 2 are the secrets to growing great roses in Flagstaff.