Frost injury in Heartnut walnut orchards

Japanese walnuts are also known as 'heartnut' walnuts and have been growing in managed orchards in southern Ontario for a number of years. A few heartnut orchards are now in full production after thirty to forty years of growth. As a horticultural commodity, tree nuts produced in managed orchards share similar production challenges as fruit trees, and frost injury is a good example.

Heartnuts grow well in deep fertile loam to clay loam soil, in areas having a moderated climate, which is similar to other orchard commodities. In moderated regions, heartnuts are cold tolerant to Ontario winters with tolerance that is comparable to hardy apple culitvars.

In spring, bud break in heartnuts occurs later than other tree fruit, which helps to protect new shoots from frost injury. Late bud break however, may not be delayed enough in regions that are prone to spring frost. Experience is showing that heartnuts are very sensitive to spring frost injury after bud growth has started.

From recent experience it may be more suitable to locate heartnut orchards in areas that have optimum moderation from spring frost, such as in peach and apricot regions. Providing adequate air drainage is also important to help prevent the formation of frost pockets, which can form along dense windbreaks or low spots in the orchard.

In spring of 2007 and again in 2009, heartnut walnuts have been damaged by late frost that occurred after bud break. Frost injury in the spring of 2007 killed early vegetative buds and blossoms that were open, which eliminated most of the heartnut crop for 2007. Frost injury occurred again this past spring and damage is now visible on vegetative stems as black and brown leaves and stems. Unlike 2007, the flowers this year were not fully open when the frost occurred, it may not have damaged the flowers.

It is still too early to know if traces of frost injury that appeared on the open flowers in late May will affect pollination and nut development in heartnuts. Nut crop development will be observed as the 2009 season progresses.

Currently, most heartnut orchards in Ontario are small plantings of one to three acres in size where growers are still testing the potential for crop yield, ease of management and market interests. Wind machines that are installed in many Niagara vineyards, protecting existing crops against frost damage, have also prevented frost injury in adjacent heartnut orchards. In the future, if markets continue to grow and heartnut orchards become significantly larger, the cost of wind machines may be justified.

Figure 1. Frost injury on leaves and leaf stems has a burned appearance.
Frost injury occurred most frequently on trees near the dense white cedar windbreak. Windbreaks consisting of spruce trees allow better air drainage for orchards.

Figure 2. Flower cluster of heartnut walnuts with a trace of darkened frost injury below the red pollen receptorsa.
Heartnuts, like other tree nuts are pollinated primarily by wind, less by wild insect activity.