VALLEY — A hard frost in late spring 2017 killed the flower buds on fruit trees of every kind in the San Luis Valley, so very few trees produced fruit in 2017. The flowering of most crabapple trees was even cut short by the hard freeze. Prunus trees, which include peach, plum, cherry, and apricot are even more sensitive to the temperatures here. 

Many homeowners are excited by the prospect of planting fruit trees and growing their own fruit. The San Luis Valley is one of the colder areas of Colorado and stone fruit trees either do not grow well here or do not produce fruit most years. Peach trees generally do not survive at all. Many expensive peach trees die in the first year or two, even those that have been carefully covered in winter and we know of only one or two that have ever produced fruit. Apricot trees generally survive in the SLV, but most rarely produce fruit. 

The most common plum tree in this area is the native plum, which is not native to the SLV. It tends to become a very weedy shrub which occasionally produces a small, sour fruit. The one thing that these prunus trees have in common is that they are the over-wintering host of the green peach aphid, which is a pest of trees, flowers, vegetables, and many crops. 

While planting one of these prunus trees is unlikely to result in fruit production, it is very likely to produce a crop of aphids. Green peach aphids move from gardens, potato fields, and canola fields in the fall looking for a place to over-winter. They can also survive the winter indoors as adult aphids in homes or warm greenhouses. Outdoors, they are only able to over-winter in the egg stage on prunus trees and more eggs are produced in warm years. In spring, eggs hatch into stem mothers that feed on tree leaves, causing leaf curling. These aphids are born with daughters and granddaughters inside, so populations multiply rapidly. In addition to being a pest of home gardens, the aphids can be a significant pest in potato fields, where they damage plants and spread viruses. Green peach aphids spread more plant viruses worldwide than any other aphid. One thing that can be done to reduce the aphid hatch on these trees is to treat them with a dormant oil spray. Dormant oil is a safe treatment that is used in most orchards, including organic orchards because it protects the trees from several common pests. It is a mix of mostly water, with a small amount of mineral oil or even vegetable oil. The treatment simply coats the aphid eggs, which smothers them and prevents hatching. It doesn’t hurt beneficial insects, such as ladybugs. Potato growers fund a program to provide this service to prunus tree owners in the Monte Vista, Center, and Del Norte areas free of charge. Call Agro Engineering at (719) 852-3705 for more information on this service.   

A quarantine that restricts the planting of most prunus trees in the SLV has been implemented through the Colorado State Dept. of Agriculture for those reasons.  Luckily, Colorado has many fruit orchards in warmer parts of the state. Local peaches, plums, and apricots are readily available at farmers markets and grocery stores. 

The good news is that it is still possible to grow fruit in the San Luis Valley. A number of varieties of apple trees grow well in the SLV and some produce fruit most years. 

Currants, chokecherries, raspberries, and strawberries also grow well here and none of these are winter hosts of the green peach aphid. Other types of berries and fruits have been planted in recent years that may provide new options as well.

Rather than planting a tree that isn’t suited to this area, you can easily prevent aphid outbreaks and insecticide use by reducing the green peach aphid’s ability to reproduce. Please plant a nice apple tree in the San Luis Valley and enjoy a nice crop of fruit without harboring a pest.