There has been considerable interest in protecting almonds from frost
because of the frequency with which they are subjected to such conditions
by reason of their early blossoming habit. The use of orchard heaters
in some districts has become a common and successful practice. It is
possible to economically control several degrees of frost for three or
four nights or possibly more by a judicious use of heaters.
The best type of heater for almond orchards has not been thoroughly
worked out as yet, but the reservoir heaters of the Bolton or Hamilton
type have been commonly and successfully used. The former are commonly
known as “pots.” At least 75 one or two-gallon heaters per acre are
necessary to control temperatures as low as 27 or 28 degrees F. Probably
not over 100 pots to an acre would be needed at most. Double the number
of pots should be placed around the outside row of the orchard to afford
the necessary protection.
The gravity of oil best suited for orchard heating is between 20 and 25
degrees Beaumé. It is essential that it be as free from impurities, such
as sulphur, as possible. During the spring of 1917, orchard heating was
done very extensively in some districts of the state while the trees
were approaching full bloom. As the season progressed it became apparent
that some serious injury had been done by the heating, for the bulk of
the blossoms fell off and the leaves turned yellow in streaks as if
burned by an acid. The appearance was exactly as if the particles of
soot settling on the pistils of the flowers and on the young leaves had
absorbed sulphur dioxide gas (a product of oil combustion where sulphur
is contained in the oil) and that the dews uniting with it to form
sulphurous acid had done the damage. Had the heating been done later and
only after the trees had passed full bloom, it is possible that a much
heavier set of nuts might have been secured, since the small fruits,
where they had formed, seemed to have been uninjured, only the pistils
of the flowers having been affected, probably preventing pollination and
fertilization of the ovules.
The time of heating is therefore a very important point. Almond blossoms
become progressively more tender to frost as they advance in development.
With their petals on they are not nearly as tender as they are after the
petals have fallen. They reach their most tender stage after the calyx
shucks have fallen from the young expanding fruits before they are the
size of a pea. Ordinarily, orchard heating before the trees have passed
full bloom is a waste of time and material and is often injurious. The
most needed time is for a period of two or three weeks after the bulk of
the petals have fallen, unless temperatures below 28 or 29 degrees are
encountered before that time.