Olives can be planted virtually year round if winter temperatures do not fall below -5ºc. Although most parts of Nelson & Marlborough fall into this category, every year young olive trees are killed by frost in pockets around the country. Sometimes by extreme or sudden low temperatures, but more often than not the trees are already suffering from lack of irrigation, pests or harsh pruning. So it is advisable to give young trees the best chance over winter.
Newly planted trees are the most vulnerable. Last winter in some southern regions early hard frosts led to deaths in young trees and damage to older plants where temperatures dropped to -10-12O c. In the Waihopai valley frost damage was often associated with severe winds as well as freezing temperatures.
There are a number of things you can do to reduce frost damage, especially if your grove is in a heavy frost area. Methods, that may result in an increase of temperature by a couple of degrees, often the difference between tree health and damage.
Watering: As with other orchard and vine crops, keeping your soil moist but not overly wet will reduce frost damage. A tree carrying sufficient moisture uses it for protection against fast freezing and thawing. Moreover, the tree will be in better health and under less stress if irrigation has been monitored before and during winter. If very cold weather is expected, irrigate as moisture helps to keep the ground warm. Be careful not to over do it though as overly wet feet and olives do not mix.
Slashing: Keeping the weeds and grass between your rows as short as possible will reduce frost damage as bare ground absorbs more heat during the day and radiates it at the cold of night and early morning. Longer vegetation traps cold air and does not allow it to flow away from the grove.
Mulching: In frost prone groves, spring is the best time to mulch after the frost risk has passed. This allows your grove to receive the benefits of mulch during the warmer months and to leave the ground bare for the following winter. Some reports show that dense mulch, even weeds and grass right by the tree up to 60cms from the trunk, protect it from frost at winter. Whilst probably true, the competition of the weeds is detrimental to the tree's growth and health. Better mulch is pea or barley straw, acting as an insulator. Even stones that are often more of a nuisance can be turned into an advantage by stacking them around the base to retain heat and also deter pests such as rabbits from attacking the plant.
There are a few other ways to assist your plants, although little research has yet been done to clarify the benefits for olives. These include mounding of the trees to raise them to a warmer elevation and reduce water logging in areas of heavy clay soils. A plastic tree guard can cause a warmer microclimate around the lower trunk where frosts are coldest. However, never allow them to be sealed at the base as this will restrict air movement having the opposite effect. The tape or staple that has held the tree to the stake should also be removed as this may trap water against the young stem and form ice.
Careful choice of varieties is another important criteria. Many Tuscan cultivars are proving their worth in hardiness such as Leccino, Frantoio, Pendolino and Moraiolo, together with Manzanilla, Picual, Picholene and Kalamata. Research shows that olives build up a tolerance to frost and several light frosts are an advantage prior to a hard frost in building up tolerance.
Early signs of frost damage include; leaf drop, inward curling leaves, splitting of the bark, and dead tissue. When early frosts damage the fruit of late maturing varieties, this generally causes the fruit to shrivel, although a degree of this does not appear to affect the quality of the oil.
If trees are damaged by frost, it is best to wait until early summer before pruning. By this time you will know how hard the trees have been hit and which ones will re-shoot. For trees re-shooting from the base, wait until the tallest sucker in the bush is 250-300mm tall then select one leader and tie it to the stake. The other shoots should then be removed once they are a suitable size, this may be after the following winter.
Application of sufficient irrigation and fertilisers will assist in keeping the trees healthy as they regain their strength in the warmer months following.
Frosts will always be with us in New Zealand, as they are in Italy or France, but with the right choice of cultivar and some care of the above procedures, there is no reason not to achieve a high survival rate.
