Frost Damage


Frost Damage to Young Trees

(Adapted from the October 1995 Olives Australia Newsletter)

The late harsh frosts which blasted much of Australia's east coast in early Spring had various effects on newly planted olive groves. The coldest newly planted groves which have been reported to us were in Queensland's  Granite Belt which recorded three consecutive mornings to minus 9 Celsius. There were a number of groves  planted with young trees which had only been in the ground 4 - 12 weeks through the winter and therefore hadn't grown much.

Damage throughout North New South Wales and Queensland varied  from trees in a couple of orchards having 'tip-burn' and therefore having to reshoot and start again, to groves which had very little or no damage at all. The orchards all received the minus 8-9 degrees Celsius mornings.

The main factor which determined the amount of damage a grove  received was the availability of water. Those groves which were not irrigated were the worst hit. Another  interesting factor is that the groves which had too much water were also badly damaged. The explanation for this seemingly strange situation  follows.

If you place a tray with 1 millimetre of water and another with  10 millimetres of water in the freezer at the same time, which will freeze  first? The 1 millimetre tray , due to its having a thinner layer of water to  freeze. It is the same in a plants leaf. If the plant is not irrigated and  therefore is lacking water, then there is very little water in the leaves. When the bad frost arrives, the water in the leaf freezes very quickly. At the point  of freezing, the water expands and literally smashes the leaf and even stem tissue.

The situation with overwatered trees works in exactly the same way. Overwatering rots the  roots of the tree and therefore water cannot be taken from the soil through the  roots (because they are rotted off) to the leaves. The leaves are therefore low in water just like the non-irrigated tree and the freezing process is the same.

We understand that there are a number of other factors involved  with the damage caused by extreme frosts, however, proper irrigation techniques  will reduce the damage significantly.

Rejuvenation of Young Frost Damaged  Trees

(Adapted from the October 1995 Olives Australia Newsletter)

A number of enquiries have been coming in regarding the growth habits of frost affected trees. The trees at our nursery are fully exposed to  frost throughout the winter months, however, severe frosts like those received in many parts of Australia during early Spring when a lot of soft, new growth had occurred, are entirely unpredictable.

If you were unfortunate enough to have the severe frosts in  your area and if your young trees were effected, then the following information  will be of use.

If the tip has been damaged to the point where it will not grow  again, then the tree will reshoot from live buds lower down the stem. Allow  these buds to shoot and grow into branches approximately 20cm (8") long. While  these shoots are growing they will be promoting root growth under the ground. When you finally choose the most vigorous of these new shoots as the main trunk for your tree, and remove the others, the roots which have been building up will then cause your newly chosen trunk to really take off.

Remember to keep your plants irrigation at an optimum level, that is, not too wet or too dry. This is the main key to reducing the effects of extreme frosts.

Cold Hardiness of  Olives

(Adapted from the October 1995 Olives Australia Newsletter)

The following brief notes are extracts from a 12 page  Californian paper titled, "Freeze damage and cold hardiness in olives: findings from the 1990 freeze."

In December 1990, California USA received a severe, fast hitting freeze which brought temperatures down as low as minus 22.8 degrees Celsius (-9F), and temperatures remained below freezing day and night for five  days! It struck fast and the olive trees weren't  properly acclimatised to it. The different olive varieties came through as  follows:

Ascolano and Mission were the hardiest. The mature trees were  cut to the ground at -20 Celsius but reshot in Spring.

Barouni and Sevillano were killed at around -18  Celsius.

Arbequina, Azapa, Kalamata, Picual, Salome and Verdale did not fare as well as the previous varieties.

Manzanillo and Nevadillo were worst hit in this devastating  freeze.

Summing up, all mature olives in good health will handle -10 Celsius without any real stress.  The 1990 California freeze situation was a freak and may never be experienced in Australia.