An Evaluation of Dormant Oil Phytotoxicity on Six Species of Woody Ornamentals

Materials and Methods

Late in February of 1987, 100 dormant saplings representing 6 species of common woody ornamentals were moved from outside into a precooled greenhouse. Included in the 4 deciduous and 2 evergreen varieties were a pear cultivar (Pyrus calleryana 'Bradford'), sugar maple (Acer saccharum), Japanese maple (A. palmatum),European hornbeam (Carpinus betulus), Scots pine (Pinus sylvestrls, and red pine (P. resinosa).Plants were 3-4 years old and included both single and multiple-stemmed forms. Following introduction into the greenhouse, all were thoroughly watered and allowed a week to acclimate prior to any experimental treatment. Subsequent watering was usually twice weekly, with a single application of time released fertilizer (20-20-20) two weeks after plants were introduced into the greenhouse. Plants were retained in their original 5" and 6½" diameter plastic pots, being neither repotted nor pruned. Since facilities to artificially refrigerate the greenhouse were not available, temperatures were maintained as low as possible by the use of 1) a large blower fan bringing in and circulating outside air, 2) heavy gauge translucent polyethylene sheeting to filter and reflect direct sunlight, and 3) white washed walls to reduce both incident and reflected radiation.

The horticultural oil being evaluated for dormant phytotoxicity was superior oil Sunspray 6E, a product of Sun Refining and Marketing Company, Philadelphia, PA. The refining specifications of this product are:

It is interesting to note that on the new label recently approved for this product by the Environmental Protection Agency, only the first two properties are specified, even though it is the distillation characteristics of this oil that separate it from other products on the market (3).

Using a set of hand sprayers adjusted to deliver a uniform, fine mist, all plants were sprayed to the point of runoff. Oil was diluted with distilled water to concentrations by volume of 2%, 4%, 6% and 8% while control plants received only a distilled water spray. Daily fluctuations in air temperature were recorded on a Taylor Instrument Company thermograph, itself being periodically checked for accuracy against a Tel-Tru rotary stem thermometer. Plants were usually evaluated on an every-other-day basis for 11 weeks. Replications per treatment were as follows: Japanese maple, European hornbeam, and Scots pine had 4 each, pear, sugar maple and red pine had 2 each.

Results and Discussion

Sunspray 6E was applied to dormant saplings at up to 4 × the usually recommended concentration. Treatment results are given in Table 1. It appears from these data that, at least for these six species, overwintering buds were not damaged by dormant oil applications at concentrations of up to 8%. This is well above the usual recommendation of 2%. No abnormal developmental differences were observed between terminal and lateral buds, although a few of the terminal buds had been winter killed through lack of watering prior to being brought to the greenhouse. With up to 10× magnification, no morphological changes were observed in any of the buds that failed to open. The bud scales did not flare apart and there was no indication that the oil had solvent properties, dissolving the resins sealing the bud scales and facilitating dessication damage. Final bud break percentage did not seem to be influenced in any way by the oil.

Since it became apparent that Sunspray 6E did not damage dormant buds, the second question we asked was whether any of the treatments delay or retard bud break? Table 2 gives the number of days following treatment to first bud break. The average number of days is given in parentheses.

Only in the pear species did bud opening seem to be slightly slowed by the oil, and only then at the highest concentration. This result in pears may well represent an abberation due to the small number of replications available, since a similar response was not seen in any of the other species. The same possible cause, that of few replications, is suggested for the (apparent) delay in sugar maple bud break at only the 6% oil concentration. All plants of both conifer species initiated shoot growth nearly synchronously within a day or two of each other. A similar response was seen in the Japanese maples with no determinable differences between oil-treated plants and controls. These findings indicate that Sunspray 6E did not significantly retard either lateral or terminal buds from breaking winter dormancy and initiating growth.

Neither did we observe any inhibition or damage to the actual foliage of any test plant as it developed. Evaluation of conifer shoot elongation by the end of the study, as given in Table 3, is representative of this finding.

In general, as the oil concentration increased, average shoot length also showed a slight increase. The cause(s) and/or validity of this apparent stimulation of growth is unknown.

Throughout the course of this study, careful attention was paid to possible mechanical damage to bark or bud scales that might have been caused by the oil treatment. None was observed at any concentration in any plant species. No splitting, blistering, cracking or lenticel enlargement had occurred by the end of the study or upon inspection several months later when the saplings were established in an outdoor planting. No twig dieback was observed. Even though plants in the greenhouse were regularly subjected to severe daily temperature fluctuations as shown in Table 4, there was no indication that Sunspray 6E caused either mechanical or physiological damage to any plant part. It is worth noting that test plants were first sprayed when the air temperature was 85°F, the top of the "permissible" range suggested on the product label.

In summary, this study has shown that the superior oil Sunspray 6E is apparently free of any serious phytotoxic side effects when applied to these six species of dormant woody ornamentals. Oil concentrations well above the levels usually recommended and wide fluctuations in daily air temperature did not result in observable damage to any plant part or subsequent foliage. The plants' vigor and normal spring development schedule did not seem to be affected. Foliar phytotoxicity and efficacy in controlling arthropod pests will be evaluated in a report in preparation.