The nature of frost damage to flowers and fruitlets
- At freezing temperatures cells are destroyed in the flower.
- Usually the ovule and style are more susceptible to freezing damage than the pollen (Lommel and Greene, 1931).
- Also, temperatures just above freezing inhibit the fusion of the nuclei in the embryo sac, resulting in only the egg and not the endosperm being fertilised (Konstantinov, 1960).
Research conducted at Long Ashton Research Station in the 1960s (Williams, 1970b) showed that pre-blossoming frosts could have a very damaging influence upon potential fruit set of Cox apples.
- When opening, these flowers appeared undamaged but internal studies showed arrested or abnormal ovule development, leading to an inability to set fruits.
- Further work is needed to enable growers to forecast potential problems caused by frosts that occur prior to flowering.
Poor weather conditions shortly before, during and shortly after flowering of apples can cause significant reductions in fruit set and harvestable yield.
- Cold temperatures, especially frost, are the most damaging, although consistent strong winds (>10 mph) will inhibit bee foraging and reduce pollination.
- Hail, although unusual at flowering time in the UK, can also, if severe, damage blossoms and young fruitlets.
Two types of frost occur in UK orchards – radiation frosts and wind frosts.
Radiation frosts
- These occur on cloud-free, dry and relatively wind-free nights when the soil, trees and surrounding vegetation lose heat by long wave radiation.
- Cold air builds up from ground level and soon reaches the height of the lowest flowering branches on modern slender spindle trees.
- On sloping sites this cold air flows away down the slope and damage can be avoided by facilitating the free flow of this cold air down the slope.
- Windbreaks must be positioned so as not to impede this air flow.
- In foggy, misty conditions or where there is significant cloud cover this long wave radiation heat loss is reduced and frost damage is less frequent.
Wind frosts
- These occur when air at temperatures below freezing is carried onto the orchard site by winds of varying strengths.
- They are much more common on hilly sites or close to the coast.
- They are best avoided by appropriate choice of site and by the provision of suitable windbreaks.
There are several possible strategies for reducing the worst effects of poor weather conditions. These are:
- Avoid sites at high altitudes or the tops of exposed hills where wind frosts are a likely occurrence.
- Ensure flows of cold air down slopes and out of orchards in times of radiation frosts.
- Do not plant windbreaks which impede this flow.
- Orchard management aids to reducing frost damage.
- Installation of a frost protection system.
- Protecting the trees within canopy structures.
- Use of chemical sprays to trees aimed at providing some protection from frost.
Orchard management aids to reducing frost damage
Soil management
If frost damage is to be reduced, the soil should be kept free of weeds and be uncultivated, i.e. firm and be moist down to approximately 15cm.
- Most heat loss during radiation frosts (50%-80%) is from the soil surface.
- To help in avoiding damage from radiation frosts, keep soil surface free of weeds and grass, keep soil compact and moist.
- Although covering the surface during the day with materials aimed at increasing heat absorption and then removing these in the evening is beneficial, it is also much too labour intensive.
Tree management
- Although contrary to modern systems of management, growing apple trees taller can reduce radiation frost damage.
- A difference of only 30 cm in height in the tree canopy can, on occasions, mean a 1 or 2ºC difference in temperature.
Protecting the trees within canopy structures
The low profit margins usually associated with apple production have deterred growers from protecting trees from frost damage using canopies or polythene tunnels. Also, it is not easy to provide the required amount of heat rise (1-2oC) under covering materials of low cost.
- On cold nights of radiation frosts, radiant heat is lost from the soil in the infra red end of the spectrum.
- To be effective in reducing this heat loss, therefore, the covering material should have low transmittance to infra red long wave radiation.
- Trials conducted at East Malling in the 1970s showed that whilst polyvinyl chloride (PVC) exhibited low infra red transparency, polyethylene and polypropylene showed high transparency (Hamer et al., 1973) and were, therefore, of little value in this respect.
Trials attempting to reduce frost damage using covers of plastic ‘Rokolene’ netting, erected 3m above ground level, were not very successful (Hamer, 1974).
- Firstly, it was necessary to only put the netting in position when frosts were expected, as otherwise light levels to the trees were reduced excessively.
- Secondly, although the netting reduced the heat loss from the soil to the surrounding air (by 20% to 30%) and flower buds at 90cm from ground level were slightly warmer than those on trees without nets, at 210cm above ground there were no benefits. This was attributable to the net itself lowering the air temperature at this level.
Protecting apple trees from frost or other poor weather conditions at the time of flowering, by enclosing them within plastic or other structures, is not considered to be economic in the UK.
- The costs of applying and removing covers is prohibitively high.
- Leaving trees under covers for extended periods of time causes problems of low light levels, atypical growth, poor fruit quality and reduced flower production.
- Polythene covers provide almost no protection from frost, as they are permeable to long wave radiation. They may prevent damage from wind desiccation, however.