Weather risks: strategies to mitigate the risk of frost
Information for apple and tender fruit growers to assess and mitigate damage caused by frost.
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Apple and tender fruit growers in Ontario face many weather risks that can damage their trees and crops. It is important for growers to recognize the weather risks for their own location, and develop a strategy to reduce or eliminate the impact on their business operation. Frost is one of the main weather risks for tree fruit growers.
What it is
Frost damage occurs to the reproductive parts of flowers, when temperatures drop to or below 0°C. When the female parts (style or ovaries) are frozen, (appearing water-soaked and then dark), pollination and seed set cannot occur. If the male parts (stamen and pollen) are frozen, pollination and seed set might still occur if pollen from other flowers survives the cold event. Frost damage can also occur to the skin of the fruit when the outer tissues of the flower are frozen. This often appears as russeting, and if severe, may cause the disfiguration of the fruit. Early spur leaves can also be damaged by frost, which may affect early season crop nutrition, reaction to plant growth regulators like chemical thinners, and foliar nutrition and pesticide sprays.
When it occurs
Frost damage occurs mainly in two scenarios:
- When fruit bud development is pushed ahead of normal schedule by mild winter and/or early spring warming trends, or
- When below freezing temperatures occur in late spring after blossoms have opened in a normal sequence. Some crops may be more vulnerable to frost before the blossoms are fully open (e.g. Sweet cherries).
Frost damage should not be confused with winter injury, which happens from extreme cold or fluctuating winter temperatures during the dormant season.
Where it occurs
Frost injury can occur in any orchard that is at or near bloom. However, it tends to be worse on locations with level topography, or in low-lying areas of orchards. Buildings, windbreaks, woodlots and farm equipment (e.g. bins) can also block airflow and create frost pockets. Frost damage tends to be worse in orchards that are distant from moderating bodies of water (e.g. the Great Lakes).
What can you do
There are several strategies that may help avoid frost damage, or lessen the injury. None of these have proven 100% effective, and some have failed to protect from frost in some situations. Each should be assessed for its suitability to your individual site and business. In some cases, it would be wise to implement several strategies to protect your crop from more of the risk.
Mitigation strategies to consider:
- Production insurance (PI): Production insurance can be purchased well before bloom, and can give you peace of mind that at least some of your input costs will be covered. Your premiums will depend on the coverage you choose, your claim history, and the yield potentials of your orchards. Over time, your premiums can be reduced if you are lucky enough not to have claims. However, some growers struggle with the premium costs (especially in their start-up years or if they have claims) and the fact that PI is not intended to fully cover your loss, either in yield or price. There is also the problem of reduced coverage levels in the years after your crop is reduced, due to the effect of the loss on your long-term average yields. Also, spot loss insurance for frost is not available, so growers with multiple orchard sites may be penalized when good yields occur on the non-frosted sites.
- Selecting sites less susceptible to frost: Avoiding low-lying areas, analyzing the effect of buildings and windbreaks, and seeking sites with good air drainage or located near large bodies of water can help avoid frost. These are not available to all growers, but should be considered when choosing your orchard site.
- Thinning hedgerows or clearing forested areas: This may reduce the area of a frost pocket, or promote better air drainage. However, the benefit from wind protection through the season may be lost, making spraying more challenging and allowing soil movement and sand blasting. Generally thinning/clearing needs to be done in advance of the season. Prior to clearing forested areas or cutting trees in naturalized areas, landowners should first contact their local Municipal office to determine whether a permit to cut trees is required and to learn the requirements of local tree-cutting bylaws. Remember that bin piles may affect air movement.
- Selecting cultivars that bloom later: Generally early blooming cultivars are more susceptible to frost damage, simply because most frost events happening earlier in the season. In 2012, we noticed that Honeycrisp, Golden Delicious and Ambrosia tended to bloom later, and produce some crop even on frosted sites. This is not always feasible due to your climatic location, market demand, or length of growing season, but needs to be considered.
- Frost fans: Tower fans raise temperature of the blossoms by drawing down warm air from an inversion, and mixing it with cold air at ground level. Generally, the temperature at ground level can be raised by about 50% of the difference when warm air is trapped high above the orchard. Frost fans are installed in advance of the expected frost, and only protect about 10 acres. They are expensive to install and require a fuel source (propane, diesel, natural gas, gasoline) that must be replenish frequently (possibly daily) when in use. They only prevent frost if and when an inversion is present and there is enough temperature difference to raise temperatures above freezing. They cannot be operated in a wind (even a breeze). Neighbours do not like the oscillating noise, especially on clear, cold mornings, and the motors may be difficult to start in the cold. There are smaller, tractor-PTO mounted versions for much smaller areas, but these require continuous driving while temperatures remain below freezing.
- Cold air drains: These machines try to avoid frost by blowing cold air upwards, to keep air moving around blooms and prevent crystal formation. They require a barrier to direct cold air towards them (which may in itself create a frost pocket). Generally, at most they can only provide a few degrees of protection on about 10 acres. The current models are tractor PTO- or engine-driven, and can be moved between orchards. They are less expensive than tower fans. Again, they work best when an inversion is present.
- Frost protection with sprinklers: Applying low rates of water will cause ice formation around the blooms, because water releases small amounts of heat as it freezes, enough to protect the blooms. This technique has proven effectiveness in strawberries for many years. There is a cost for a solid-set irrigation system, including low-flow frost sprinklers. A large source of water is needed, because once started, the irrigation must continue until the sun comes up and melts the ice. If you run out of water, all will freeze. The weight of the ice build-up on trees may break some branches. The structure in trellised orchards may be a good candidate to install this system, especially if it can be used for irrigation or other uses.
- Burning hay or smudge pots: This technique covers the orchard in smoke, which may prevent frost formation at ground level. It is less expensive than frost fans, but requires a source of hay or fuel, and can be implemented the day of anticipated frost. At most, it may offer a few degrees of protection. Local burning regulations must be followed, and the smoke may irritate neighbours, workers and cause environmental issues and traffic concerns. This requires labour to ensure the fires burn as intended and do not escape.
- Orchard heaters: These could be placed through the orchard, but generally only a small amount of heat can be produced, and it would only protect a small area. In some situations, a heater may actually draw cold air down as air currents are affected.
- Products with frost protection claims: Scientists have tested numerous products over the years to protect against frost, including foliar zinc sprays, Pristine fungicide, and Promalin plant growth regulator. If ice-nucleating bacteria is present in the blossom (which makes ice form at higher temperatures), and if some products replace this bacteria, there may be some frost protection. But this is not always the case. Promalin may promote the formation of parthenocarpic fruit (without seeds), but a full crop is not likely to be set, and fruit would have limited shelf life with no seeds. The cost-potential benefit of these products needs to be closely analyzed.
Frost injury is not uncommon, and each year there are orchards in Ontario that have reduced yields due to frost. In some years only blossoms on lower branches are affected, while in other year, as in 2012, a large portion of the crop is lost. There are a variety of options available to mitigate frost injury in orchards. The effectiveness of each mitigation strategy varies depending on weather conditions, site conditions and economics. Chose the strategy(ies) that works best for your situation.