Frost not major risk?

Press, Volume CXII, Issue 33094, 8 December 1972, Page 9

Frost not major risk?

Critics of the establishment of a tomatogrowing industry on the Canterbury Plains base much of their criticism on the frost susceptability of tomatoes in relationship to the incidence of late frosts, which by tradition can be expected up to Show Day.

Theoretically it can be shown that a frost can occur in any month of the year at Lincoln College, but less is known of what constitutes a critical tomato freeze.

In 1968, a grass minimum temperature of minus 5 deg. Centigrade on November 23, which was the lowest November temperature recorded since 1963, caused the temperature over the tomato crop to register minus 1.7 deg., with no apparent detriment, although frost damage was recorded on other crops. Since that date no further frosts have

occurred, while tomatoes have emerged in the paddock and the frost susceptibility of direct sown tomatoes remains to be assessed. On November 17 this year, however, a very favourable condition for late frost developed following a dry, cold front during the day, which was followed by clearing skies towards evening. Armed with 15 grassminimum thermometers instead of the one in 1968, it was hoped to establish the limits of tomato susceptability for both transplants and direct sown crops. Alas, Canterbury was saved by an unexpected increase of cloud at 10 p.m., which persisted sufficiently to prevent a major frost. Temperatures which had been at freezing on the grass as early as 9 p.m., rose again until finally falling to minus 2.5 deg., later in the night as the cloud once more cleared. Some interesting information was, however, collected. Over the grass headlands of the tomato paddock, temperature fell to minus 2.2 deg., while only 12 feet away over the bare dry soil a minimum of 0.8 deg. only was recorded. In the

middle of the paddock, 80 feet from grass, the temperature had risen to 1.1 deg., while over bare soil that had received irrigation several days previously the temperature failed to fall below 2.2 deg. What does this mean as far as tomato production is concerned? Due to the fact that the soil in November is warm, the temperature fails to drop because of a transfer of heat from the soil at greater depths to the surface. This process of transfer is facilitated by the soil being moist, and because moist soil absorbs greater quantitites of heat. Any- ' thing that insulates the soil from the clear sky above prevents this beneficial heat loss and so temperatures fall to much lower levels at the surface. Grass, for instance, in November offers a very dense insulation as do peas, and the temperature above such vegetation is much lower than over the bare soil, with consequently much greater frost frequency and severity. On November 17, as a consequence of these various factors, there was a temperature difference of 4 deg. over a distance of only a few feet. When such late frosts occur, tomato plants are only a few inches high at the most, with two leaves which are characteristically divided. That is, they do not offer much of a radiation surface and consequently are enveloped by the air mass over the paddock. A dock leaf, however, offers a wide leaf area for radiation similar to potatoes and one could expect a localised cooling over such a plant. In fact the temperature over the leaf of an isolated dock in the middle of the tomato paddock fell to 0.3 deg. (compared to 1.1 deg. over the bare soil around it). One would, therefore, expect frost damage to occur more readily over susceptable crops with large leaves, such as potatoes and beans, or where the paddock is covered with vegetation such as peas, where temperatures would be more ' closely related to those experienced over grass.

For this reason one can confidently expect that direct-drilled tomatoes in November will be the least likely crop to suffer damage by frost, being subjected to a less severe frost on a given night than potatoes, peas in flower, and even cereal crops in ear.

Bearing in mind the 1968 survival of tomatoes at minus 1.7 deg. on the plant itself, and using the probabilities of a given temperature occurring derived by K. Young, of Lincoln, based on 30 years of records, it can be stated with reasonable certainty that a frost exceeding this level (minus 1.7 deg. after November 12) may occur about one year in 20. It should be borne in mind that such a frost might kill tomatoes, but certainly would damage other traditional crops more, the severity being dependent on the length of freeze experienced. Fortunately for Canterbury, the evening of Friday, November 17, was not the time to find this out and it looks as if a definite proof will have to wait for another year, but the evidence accumulating would indicate that late November frosts are the least of the worries influencing a decision on whether Canterbury is a suitable area for a direct-sown tomato industry.

This item on observations on the risk of a late frost to tomatoes has been written by R. A. Crowder, of the horticulture department at Lincoln College.