Persian Walnuts in the Denver Metro
As I understand, black walnuts were recommended for planting in the Denver metro until the arrival of Thousand Cankers disease. However, Persian walnuts are supposed to be rather more resistant to this disease, and the Carpathian strain is considerably more hardy than other Persian types. So I was considering planting some, but noticed that even before the arrival of Thousand Cankers, Persian as opposed to black walnuts were not recommended for planting here. Why is this?
Arapahoe County Colorado trees and shrubs
There are several reasons why most nuts trees are not recommended for the Front Range: our erratic early/late freezes and soil conditions are the two biggest limiting factors.
Juglans regia (aka Persian walnut, common walnut, English walnut, Carpathian walnut or Madeira nut) is hardy to Zone 3, however, it normally breaks dormancy in mid-April (or earlier in warm springs such as 2018) — which is well before our last frost date. The late spring freezes and severe weather we experience will adversely affect any nut production and kill new growth.
Juglans regia requires deep, fertile, well-drained soil with a pH between 6 and 7, and a texture of medium loam to sandy or silt loam texture. Most soil texture here is clay, requiring amending to improve fertility and drainage. Our soils also tend to be in the high alkaline range of over 7, but a soil test would be able to determine that.
Like fruit trees, nut trees have a chilling requirement, meaning there must be a certain number of hours below freezing in order for fruit production. Walnut trees have a long chilling requirement of more than 1,400 hours per year. In the case of our very erratic winter temperatures with extended periods of warmer than normal weather, this results in no nut production.
Information from CSU states that although Juglans regia appears to have an intermediate resistance to thousand cankers disease, it has been observed to be attacked in sites where large numbers of walnut twig beetles are present. It is also important to note that Juglans regia is sometimes grafted onto rootstock of other Juglans that is susceptible to thousand cankers.
Not knowing where you received information about Juglans regia not being recommended for planting here, the Denver Botanic Gardens' Navigator indicates they have three; two at York Street, one at Chatfield. You might consider contacting the DBG to obtain their observations on the health and nut production of these specimens.
This is a helpful link to the Front Range Recommended Tree List: