According to some reports, Colorado weather in 2014-2015 has resulted in the deaths of 80% of our cherry, plum and peach trees. How did this happen? Does it make sense to replant? And if so, how can we reduce future losses and increase fruitful successes? This article will focus on cherry trees.
The Colorado Front Range is well known for our normally difficult gardening conditions: low day-time humidity, large day-night temperature variations, abrupt weather changes, many mild winter days with occasional extreme cold, some extremely early snows (Sept) and late snows (May/June), occasional major wind storms and common hail storms.
But in 2014, a long, mild fall kept plants green and growing into November when temperatures dropped from the 60s to well below zero in 2-3 days. When plants have not gone dormant, the water in their cells freeze, and the ice crystals rupture the cells, damage tissues and damage or kill fruit buds. If cold is more gradual, a layer of cells forms between the leaf stem and the branch, cutting off sap to the leaf. Then as the green chlorophyll diminishes, the less dominant pigments of yellow, orange, red and brown create the fall colors. Soon after, the leaf falls. If the cold occurs suddenly, the corky layer has not had time to form between the leaf stem and the branch, the leaves flash freeze and eventually fall, leaving openings into the tree through which fungal diseases can enter. This can cause serious die-back of branches and even death. This occurred around 1990 when large numbers of Siberian Elms died and died-back. And it happened again because of the November, 2014 deep freeze.
Boulder County Extension Agent Carol O’Meara, reported that the combination of the November flash freeze, followed by the Mother’s Day snow, followed by a very cool, wet May, caused damage and stresses that favored cytospera canker disease and left many fruit trees dead, damaged or fruitless.
According to the 2014 Climate Change in Colorado report, Colorado temperatures have already warmed by 2 degrees F. over the last 30 years, and could warm 2 ½ to 6 ½ degrees more by 2050. This could lead not only to drier conditions, but also to extended warmer Autumns, which could trick trees into delaying dormancy.
With these weather issues in mind, does it make sense to plant fruit trees like cherries here on the Front Range of Colorado? Of course I’m prejudiced because I have a plant nursery, but I would say, yes! Historically there have been a lot of orchards in Denver, Boulder, Lyons, Loveland and Ft. Collins. The 1935 census showed 516,000 cherry trees growing in Colorado, with one orchard in Loveland being the biggest cherry orchard west of the Mississippi. Drought, late spring freezes, 41degrees below zero in 1951, and a shortage of canning supplies caused by WWII destroyed the pie cherry industry in Colorado. However, as many of us know, tart cherries are often very productive in our area, providing wonderful eating and healthy benefits for us.
So which cherry varieties should we be growing and how should we cultivate them?
First, it is good to discuss cherry rootstock, as most cherry trees are produced by grafting in order to duplicate a named variety. It was found that Prunus mahalab was a better rootstock than Prunus mazzard, because P. mahalab produced faster growth, was more tolerant of alkaline soils and is better adapted to drier conditions. Some wilder forms of tart cherry can be reproduced by suckers, but these form thickets. In the old days some cherry orchards grew trees from seed.
The most common variety of sour cherry (Prunus cerasus) grown for the pie-cherry industry in the 30s and 40s was Montmorency. It is still very popular and usually successful. It is a cross between a sweet cherry and Nanking Cherry (Prunus tomentosa). Montmorency is very productive, very flavorful, highly valued today for its anti-inflammatory qualities, the most popular with birds (they know), and hardy to zone 4. It grows 12’ high by 15’ wide. Many Montmorency trees died or died back in the 2014 deep freeze. However it is still a good tree, and since stone fruit trees are rarely strong and healthy for more than 15 years or so, it is good to replant a new one every 10 years, anyway.
Bali Cherry is my current favorite. It is a Canadian variety, hardy to zone 3, and it did not die last year. The fruit is sweeter than Montmorency, and the tree grows to 12’. North Star Cherry is a natural dwarf to only 6’-8’ and is the easiest to net by tying the netting to the trunk which prevents birds from getting trapped. It is a good tasting tart cherry and is hardy to zone 3 or 4. Meteor is a tart cherry, zone 4, that blooms one week later than Montmorency. Mesabi is a cross between a sweet and a tart cherry. It grows 10’ high and 15’ wide, and is hardy to zone 4. These tart cherries are all self-fertile, so they do not need a pollinator. They are hardier than sweet cherries, have fewer diseases and are more tolerant of hot summers and harsh conditions.
Most Sweet Cherries, Prunus avium, are only hardy to zone 5, bloom earlier and are therefore more vulnerable to late frosts and seldom fruit here. Most sweet cherries on the Front Range died last year. Many new varieties are being bred so there is hope that one may be hardy and successful here.
There is a relatively new Canadian bush type cherry that may be useful here. The most often available are Carmine Jewel and Crimson Passion. They are hardy to zone 2 or 3, have full-sized fruit that is classified as sour, but is said to taste sweet. They grow on a 6’ bush, and are self-fertile but more productive when pollinated by Nanking Cherry.
There is also a selection of Nanking Cherry called ‘Orient’ that has a flavor superior to the species and with fruit that is slightly smaller than a tree cherry.
Cherries do best in well-drained (not wet) sandy loam soils with some organic matter, like compost. They prosper when fertilized in the fall with an organic fertilizer, and watered once a week, especially when the fruits are maturing. They appreciate some pruning but do not remove more than 15-20% yearly. You can lighten clay soils to improve their performance, by adding expanded shale to the planting mix 10-20% by volume.
If you are replanting, it best not to plant in the old cherry location. Stop watering in October to encourage dormancy, then water right before a cold period because moist soil holds heat better and the extra humidity results in the air cooling more slowly. Planting near a building, a big rock or on the north side of a building can moderate temperature swings.
Remember that home-grown fruit is fresher, can be left to ripen on the tree, which makes it sweeter, and often comes in quantities that can be shared or bartered. If a 10-20 year-old fruit tree dies in an extreme weather event, it is likely that if you plant another, it could live as long or longer. But if Climate Change does bring more extremes, pay attention to the varieties that survive and thrive in these extremes. This is the best opportunity to select the new plant variations that have adapted best.
P.S. Regarding our climate, don’t be fooled by references to The Colorado Freeze, which was established in 2014 with the hope of bringing women’s tackle football back to the Denver area.