356
FLORIDA STATE HORTICULTURAL SOCIETY, 1958
Effect of Light on Development
The incident illumination has significant
effects on the development of avocado, and
the spacing of the grove will greatly influence
the ultimate form of the mature tree. For
example, seedlings grown indoors in dim light
develop long whip-like or vine-like stems,
with much elongated internodes, reduced
linear leaves, and few or no branches. At the
opposite extreme, avocado seedlings which
have developed under the high illumination
of the pacific slopes of Central America show
a compact bushy habit. It has been possible
to make a number of observations in Florida
and Cuba concerning the growth habit of
avocado in relation to exposure to light. Al
though many varieties show the tendency
when young to develop into tall slender py
ramidal trees, old individual trees in Cuba,
if not closely surrounded by other trees and
receiving more or less uniform illumination
on all sides, invariably present dense rounded
crowns; the pyramidal habit of the young
tree gives way to this more desirable form,
in which the crown is usually not over one-
third higher than broad, and frequently no
tend to have an upright growth; the lower
shaded laterals which do not assume upright
growth become unproductive and may even
tually die; such lateral buds on the lower
portions of the tree that sprout are unable
to prosper in the low light intensity which
reaches them.
With wider spacing (30 feet or more), the
growth rate of the main axis in relation to
the growth of the lateral branches becomes
proportionately reduced as the tree reaches a
height of 15 to 20 feet. The position of the
principal lateral branches tends to be more
open and gradually ascending. When these
trees have reached a height of between 25
and 30 feet, growth in height takes place
slowly and the crown becomes rounded from
the continued development of laterals, pro
vided the sides of the tree receive adequate
sunlight. In southern Florida, where the spac
ing in many avocado groves is somewhat
less than 25 x 25 feet, one observes consider
able contrast between the slender form of
the trees within the grove and the more
rounded denser crowns of the trees in the
border rows. In Cuba, the minimum spacing
higher than broad.
necessary to achieve desirably-shaped trees
without pruning is about 30 x 30 feet, with a
spacing of 35 x 35 or even 40 x 40 even
tinct spacings, some generalizations are pos
sible at the present time:
When conditions for the growth of the tree
are good and the spacing is close (under 25
feet) and the sides of the trees begin to
more
From studies of the development of unpruned trees of various ages planted at dis
shade or interfere with one another, further
horizontal growth is inhibited, but vertical
growth continues indefinitely, and the upper
portions grow beyond the practical reach of
the spray equipment. The lateral branches
desirable. Using a triangular planting
scheme rather than a square provides a more
efficient use of the planting area in relation
to the development of the trees.
BIBLIOGRAPHY
Chandler, W. H. Evergreen Orchards, pp. 208-228. Lea &
Febiger, Philadelphia. 1950.
Ruehle, G. D. "Cause and control of avocado scab."
Univ. Fla. Press Bull. 580. Dec. 1951.
Wolfe, H. S., L. R. Toy and A. L. Stahl. "Avocado pro
duction in Florida." Univ. Fla. Bull. 141. Dec. 1949 (as re
vised by G. D. Ruehle).
TEMPERATURE VARIANCE IN MICROCLIMATES OF
SOUTHERN FLORIDA
Taylor R. Alexander
University of Miami
Coral Gables
Microclimate refers to climatic conditions
within a few feet of the ground and therefore
is of extreme usefulness to plantsmen. This
is true for locating good planting sites, con
struction of shelters, planting of protecting
plants, and also for protection against cold
after a planting is made. Microclimates exist
within macroclimates. Weather Bureau re
cording techniques and records are adequate
for the latter but inadequate for the former.
When local areas are considered special tech-
ALEXANDER:
TEMPERATURE VARIANCE
niques and concepts are required to obtain
an accurate picture of climatic conditions as
they affect plants close to the ground in those
areas. The term "local area" as used in this
paper refers to plots ranging from a fraction
of an acre in size to considerable acreage,
particularly when the site is uniform. An
average size nursery or grove
would likely
have many microclimates. Slat sheds and
hedged garden areas are microclimates. One
of the best discussions
on
microclimatology is to be found in a book written in
Germany by Geiger (1). In this book the dis
cussion concerns the climate to a height of
about six feet above the ground. Ecologists in
this country are paying increasing attention
to this concept of climate and microclimatology is recognized as a special study.
Destructive frosts in the Coral Gables area
in November 1940 were observed by the
writer and as an ecologist, the significance
of the microclimate on the distribution and
occurrence of killing frost was immediately
apparent. Plans for a study of the influence
of microclimatic factors were made. However,
the war interfered and it was not until 1947
that a study was initiated. Since that time
a total of seventeen cold nights with antici
pated temperature drops to near freezing
have been studied. To do this, two stations
and study areas were set up, one on the Main
Campus of the University of Miami in Coral
Gables and the other on a private property
about one mile west of South Miami. Both
stations were on high pineland soils.
The main objective was to determine how
cold the air and objects near the ground
really become in southern Florida and to ob
serve the reaction of plants growing in the
vicinity to this cold. Differences in low tem
perature caused by shelters, natural plant
cover, position of the thermometer, and bare
and grass covered soil were also measured.
In this report only minimum temperatures
are reported since frost damage to plants is
commonly known.
school station also had a Taylor recording
thermometer to give a record of cold dura
tion.
All
equipment
Methods
was
regularly
checked
and standardized. Thermometers were placed
at the recommended height (3), four and one
half feet, in the instrument shelter. In the
immediate vicinity of the shelters small stakes
were driven to allow hanging an exposed
thermometer three feet above the ground.
Nearby bare areas were made in the grass
cover so thermometers could be placed one
inch above the bare soil and a few feet away
another thermometer could be placed in the
grass. The use of exposed thermometers at
ground level follows recommendations given
by Hansen and Farrall (2).
Observation and Discussion
In
Table
1. data are presented without
reference to station, date, or grassy or bare
soil as the object is to show that thermometers
in shelters, four and a half feet above ground
may give readings that are much higher than
those at ground level in the same area. Tem
peratures are given to the nearest whole num
ber. It should be remembered that the ground
level reading is not considered a true air
temperature as is the case inside the shelter.
However, the ground level temperature is a
good indication of the temperature of objects
in that location.
Table 1. Representative Recorded Minima (F°)
Standard Shelter:
37 41 36 36 29 39 49 43 43 38 42 32
Ground Level:
24 30 22 30 24 32 40 31 33 34 38 26
Differences:
13 11
14
6
5
7
9 12 10
4
4
6
In Table 2. data are presented that were
obtained by using exposed thermometers, one
at ground level and one three feet above the
ground. No reference is made to date or sta
tions as the object is to show that consider
able difference may occur in the lowest three
feet of air.
Table 2.
Three feet:
Ground:
Differences:
Each station had a standard Weather Bu
reau cotton-region instrument shelter (3) and
an adequate supply of maximum and mini
mum registering thermometers and standard
laboratory type centigrade thermometers. The
357
Representative
'
Recorded
Minima
(F°)
36 28 32 32 37 39 43 30 34 38
33 22 28 27 30 34 40 27 32 34
3645753324
In Table 3. data are tabulated that were
read from exposed thermometers, one placed
one inch above bare ground and the other
a few feet away placed in short grass cover.
These data show that bare ground areas are
warmer than grass or mulch covered areas.
358
FLORIDA STATE HORTICULTURAL SOCIETY, 1958
Table 3.
Representative Recorded Minima
(F°)
Bare ground:
28 45 33 41 27 33 51
Grass covered:
21 40 30 33 22 31 48
Differences:
Temperature
7
inversion
5
3
on
8
5
frost
2
tective measures are anticipated. Overhead
trees at one station caused a ground level
3
nights
is
known to cause the coldest air to be next to
ground as heat in the environment is lost
continuously by radiation. Among factors that
increase the frost hazard related to inversion
are to be found the following: (1) lack of
natural or artificial protection such as trees
or buildings; (2) evaporation of water from
surfaces; (3) low relative humidity; (4) ab
sence of wind; and (5) clear sky. These fac
tors working together and without the advent
of new cold air can cause a seventeen degree
drop between 7:00 p.m. and sunrise the next
morning on frost-prone nights. For example,
in one instance the shelter box thermometer
was 46°F at 7:00 p.m. and 29°F at 7:00 a.m.,
a drop of 17°F. However, the ground level
thermometer at this location registered 24 °F
at 7:00 a.m. Ground frost had formed at this
station at 11:00 p.m. when the shelter box
thermometer registered 34°F.
Since many of the plants grown in southern
Florida are frost sensitive and have vulner
able parts near the ground, the placement
of exposed thermometers near the soil is ad
vantageous. This is true in spite of the fact
they probably register lower than true air
temperature. In many instances the probabil
ity of frost forming was indicated by these
thermometers far ahead of the ones placed
three feet above the ground.
Thermometers should be carefully placed
in a local area. At one station, an east-west
slope of only three inches in two hundred
feet was the apparent reason for the four de
gree difference between minima in favor of
the thermometers near the top of the slope.
Dependence on a single thermometer, care
lessly placed, can lead to disaster where pro
minimum that was three degrees higher than
a nearby open area.
Grass and weeds are a factor in a micro
climate. The reasons involved when plants
survived in bare areas, but not in nearby
grassy areas, are well known. Heat is lost by
evaporation of water from plant surfaces in
greater amounts than from bare ground. The
plants and their accumulated litter are poor
conductors of the heat in the soil and prevent
radiation of heat from the soil to the air im
mediately above the plants. Accordingly, the
grassy layer becomes colder than the air above
it and begins to withdraw heat from this air
while a nearby bare area may still be radiat
ing heat from the soil to the air above and
warming it. Reference to Table 3 shows the
differences can be considerable.
The data presented can be used as a guide
to analyzing a specific microclimate for low
temperature possibilities. One should keep in
mind that no two microclimates are likely
to be alike. Wind drainage, spacing of plants,
moisture, etc., can nullify or increase expect
ed differences. Also it is very apparent that
no two cold fronts act exactly alike and the
variations that occur are many and sometimes
extreme. However, the perfect radiation frost
night can be expected to develop minima
that will correlate in general with those pre
sented. This exact knowledge of microclimate
coupled with regular Weather Bureau fore
casts on macroclimate can put the problem
of low temperatures on a scientific basis in
southern Florida.
LITERATURE
CITED
1. Geiger, R. The Climate Near the Ground. Cambridge,
Mass.: Harvard University Press. 1950.
2.
Hansen, C. M. and Farrall, A. W. Protection of Toma
toes from Frost Damage by Use of Radiant Energy. Mich.
Exp. Sta. Quarterly Bull.: Vol. 31, No. 3, 332-342. 1949.
3.
Young, F. D. Frost and the Prevention of Frost Dam
age. U. S. Dept. of Agric. Farmers' Bui. 1588. 1947.