>g. Crops MR 71-1
JUL 11 1972
PREDICTION OF MATURITY IN VEGETABLE CROP0
by I.F.A.S. Univ. of Florida
Thomas G. Hart
County Agent Vegetable Crops Production and Handling Session
January 20-21, 1971
Predicting maturity in vegetable crops is not a new subject. Work
was started on this topic about 40 years ago. Through the years, most of
the work has been relatively restricted to sweet corn and English peas.
The primary reasons for this are threefold.
(1) Most important Is the fact that English peas and sweet corn are
both subject to rapid sugar degradation or conversion and moisture loss
after these crops attain their prime quality of maturity. When prime
quality maturity is reached, these crops must be harvested and processed
rapidly or two Important constituents (sugar and moisture) are lost and
the quality of these vegetables quickly becomes poor. This is particularly
true for the fresh and frozen product, although very Important for the
canned product too.
(2) A very large portion of English peas and sweet corn Is marketed
and consumed In some processed form as compared with many other vegetables
of which a higher percentage are sold as fresh products.
(3) Plantings, and more importantly although related, harvests,
must be scheduled In order that processing plants are operating at full or
optimum capacity for a definite period of time. Profits diminish quickly
w'en processing plants are either underworked, too overworked or not worked
WHY SHOULD FLORIDA VEGETABLE GROWERS BE INTERESTED IN MATURITY PREDICTION?
It Is felt that being able to predict the maturity of many vegetables,
not only peas and sweet corn, will become more and more important In Florida
for the following reasons;
1. Labor shortages If growers are able to predict the harvesting
schedule of their vegetables in advance, it will be easier for them to plan
and arrange for labor well in advance of actual needs and thereby avoid, in
part at least, a lack of sufficient labor when needed.
2. Mechanical harvesting prediction of maturity via scheduled
plantings and harvests is a must In order to have sufficient acreage ready
to warrant the use of mechanical harvesters, but not more acreage ready for
harvest at any one time than the harvesters can handle, and, to plan for the
scheduling and use of harvesters, trucks, labor, etc., over a definite pre-
planned period of time.
3. Production of better quality produce harvesting produce at a
predetermined maturity, by a system such as heat unit summation which will
be discussed presently, definitely helps to maximize harvesting at prime
quality. This is primarily why the system has been employed on sweet corn
and English peas mentioned earlier. However, such a system need not be
limited to only these crops and would have merit on practically all vegetable
crops, even multi-harvested crops. With an ever Increasing quality-conscious
public, harvesting vegetables at the time of their prime quality should put
growers In a more competitive position for marketing and marketing at higher
4. Keener Competition through Cost Reduction -
(a) Prediction of maturity permits growers and processors to
order supplies when he needs them. Thus, one can minimize inventories and
concomitantly therefore, maximize on the efficiency of warehouse space
utilization. By the same token, allied industries have found it useful in
scheduling the manufacture and delivery of containers and supplies.
(b) timing of Insect and disease spray programs can be done more
scientifically rather than by traditional calendar schedules or waiting for
the pests to manifest themselves.
(c) the orderly and well managed execution of planting and
harvesting schedules permits growers to cope well with these operations on
the one hand, and, to avoid gluts and hold prices high on the other hand.
HOW TO PREDICT MATURITY OF VEGETABLES
A simple, widely used, quite accurate method is known as the Linear
Heat Unit System. This system utilizes the principle that a given vegetable,
or more appropriately, a specific cultivar of that vegetable, requires a
definite amount of accumulated heat, with certain limitations, to reach
maturity. It is not an exceedingly precise tool, but a useful tool.
Some definitions concerning the jargon of this system seem appropriate
before proceeding further:
A. All of the following terms are synonomous and can be used
interchangeably. Some authors prefer one or more over others:
dd = degree days = dd summation = heat unit summation = heat
summation heat sum.
B. dh degree hours = degree days x 24. This expression is seldom
C. How is a degree day or daily heat sum determined?
high temp. + low temp. base temperature, or mean temperature
for a particular 24 hour day base temperature. A thermograph or max-min
thermometer can be used for this purpose.
WHAT IS A SASE TEMPERATURE?
Base temperature Is commonly defined as the lowest temperature at which
the physiological processes of plant development will proceed. This Is a
cumbersome definition and has been criticized by certain workers as not very
A much better definition for base temperature is that temperature,
which, when used in a linear heat unit system, gives rise to the least
variation in heat unit summation over the temperature range that is normally
experienced in the phase of crop development Involved and when several to
many planting dates are compared.
A hypothetical example shown below, using the above definition shows
correct and incorrect selection of BASE TEMPERATURE:
Days to 45v 550 35u
Mean Temperature Maturity H.U.S. H.U.S. h.U.S.
600 120 1800 600 3000
650 90 1800 900 2700
750 60 1800 1200 2400
800 51 1803 1288 2318
It is obvious that 450 Is the correct base temperature for this hypo-
thetical example since there exists the least amount of variation In the
heat unit summation (H.U.S.).
HOW IS BASE TEMPERATURE DETERMINED?
Dr. Charles Y. Arnold discusses this in great detail In reference #1
cited below. The concepts are somewhat difficult to comprehend and can only
be appreciated and understood well by actually studying his paper and then
working some examples out. Let It suffice here to say that two mathematical
derivations are considered very acceptable and give accurate base temperatures.
These are listed, but not elaborated upon to any extent, below:
1. Determination of base temperature by coefficient of variation:
CVdd = Sd 100 where Sd = Sdd
xd 3t tb
CVdd coefficient of variation In degree days.
Sd = standard deviation In days.
;d mean number of days required for development in the entire
series of plantings.
Sdd standard deviation in degree days.
xt = mean temperature for the entire series of plantings upon
which Sdd Is based.
tb the base temperature.
2. Determination of base temperature by x Intercept method: Values
from a series of plantings are used to calculate a regression equation in which
x (mean temperature) Is the Independent variable and y (mean rate of
development 100 ) is the dependent variable.
no. days planting to harvest
It Is very Important to use the correct base temperature. A tb of
500 is often used for sweet corn, but this is erroneous. Dr. Arnold has
demonstrated that 42.7o for 1954 and 39.80 for 1955 were the appropriate
base temperatures for sweet corn grown in Illinois. Furthermore, Dr. Arnold
shows that selection of too high a base temperature, which Is commonly done,
gives greater errors than selecting too low a base temperature.
3. Correct base temperature can also be determined by trial and error
(subtracting out assumed tb) until the one tb is found that gives the least
variation in H.U. summation among a series of plantings. This is cumbersome
though and the mathematical computations are much easier and quicker once
familiar with these methods.
LIMITATIONS OF HEAT SUMMATION
Asurplus or deficiency of a number of factors can Impose errors
on maturity prediction by heat summation:
1. Light -
(a) Insufficient light, i.e., unusual or prolonged cloudiness
or overcast conditions.
(b) photoperiodic responses and interaction with temperature.
2. Water droughts or floods.
3. Nutrition fertilizer imbalances, deficiencies, toxicities or
soluble salt problems.
4. 'Temperature itself -
(a) higher summations will occur in warmer parts of growing
season compared with cooler parts of growing season.
(b) higher summations will occur In warm years than cold years.
(c) higher summations occur in southern latitudes than northern
(d) higher summations occur at lower altitudes than higher
(e) frost damage
1. weeds either as competition for light, water and nutrients
or due to toxic levels of herbicides
2. Insects -
3. diseases -
4. wind damage -
5. variety effects different cultivars of the same vegetable
possess intrinically different heat summation indices for
*********SLIDE ON PEA TYPES AND VARIETIES*********
HOW DOES ONE BEGIN TO USE THE LINEAR HEAT UNIT SYSTEM?
A. Obtain from the weather bureau the long-term average daily mean
temperature records for the area.
B. From these, construct a curve of accumulated heat units above the
base temperature for a normal season from planting through harvest.
C. Plot actual daily mean temperatures for the particular growing
D. Plot actual daily heat unit summations for the particular growing
*********SLIDES ON TEMP. AND H.U. DATA*********
E. Using Peas as an example of formulating planting schedules:
1. Generally, about 30 degree days will accumulate each day at
2. Thus, If a grower Is to harvest 5 acres of peas per day and
wants to plant 25 acres he would proceed as follows:
a. Plant first 5 acres then wait until 30 heat units have
accumulated (about 3 to 5 days) before planting next 5
b. After planting second 5 acres, wait until 30 heat units
have accumulated again before planting third 5 acres, ad
3. Presumably then, the 25 acres should be able to be harvested
efficiently and at prime quality in 5 successive days at harvest-time. Prime
quality of peas has been attained when tenderometer readings give a value of
HOW TO CORRECT ESTIMATED FORECASTS FOR MATURITY PREDICTION
A. Once a heat summation (H.U. or dd) has been established that is
necessary to mature a particular cultivar, grown in a particular area, on a
certain soil type, growers will have a good objective guideline measurement
to follow. In most cases, or at least for warm season crops, collection of
H.U. data should probably not commence until the day of crop emergence or
at the cracking stage. However, this H.U. value will vary from year to year,
e.g., it will be higher in warmer than normal years and lower in cooler than
**********SLIDE TOMATOES 3 YRS.**********
3. From temperature data and heat unit data curves discussed previously,
Increase date of maturity prediction (greater no. of days to harvest) accord-
ingly when current season H.U. curve deviates lower than (below the) normal
or long range average H.U. curve. Decrease date of maturity prediction
(lesser no. of days to harvest) accordingly when current season H.U. curve
deviates higher than (above the) normal or long range average H.U. curve.
This Is done every day or every few days season-long.
C. Sweet corn, e.g., can be further checked and adjusted at silking
time, since it has been established that it requires about 500 H.U. or dd
above 500 F. from 80% silking until prime maturity (72% kernel moisture
content) Is reached.
D. About 420 H.U. or dd after 80% silking, field samples can be taken
and tested for kernel moisture content and final harvest-date predictions
made at this time.
GU.. A FOR PREDiCYiNG MATURI
Useful, only when
but not whe
_ LEMS OF USING MEAN SOIL Tc
Since these situations would give a maturity date considerably delayed
from what one would predict on the basis of mean soil temperature during the
first 2 weeks after planting, such predictions based on soil temperatures are
not considered reliable or useful.
*********SLIDE CORN, AUNG*********
Certain small soil temperature differences have a very marked effect
on tomato seedling emergence.
In California, it is recommended that tomatoes not be direct field
seeded until soil temperature at a 2 inch depth taken between 11 A.M. and
12 noon reads at least 570 F. for 3 consecutive days.
It Is also recommended In California that the spacing of planting
schedules be based on something other than merely calendar dates.
A successive planting could be made after the previous crop has
emerged or reached the first incipient true leaf stage. This will allow for
an orderly and continuous harvest and processing or marketing schedule free
of voids or excessive gluts. Again, the number of acres to plant at any
given planting is dictated by the number of acres one will be able to
harvest per day at harvest-time. How many harvesters will be operating?
Capacity of harvester? DD or H.U. accumulating daily at harvest-time?
'.rA'*****SL "'-' ..*. ,
In the development of Machine Harvest tomato cultivars at the Bradenton
Station here in Florida, still another approach is being-taken. Different
maturity indices are being built into similar M.H. cultivars. Thus, with
some simple arithmetic with regards to acreage, harvesting capacities, etc.,
all of the cultivars used could be planted at the same time, but would be
harvested successively as each one matured.
Days from planting to harvest -4
USEFULNESS OF HEAT SUMMATION FOR PREDICTING SPRAY PROGRAMS
As mentioned earlier, any cost cutting devi e should put a grower in
a more competitive position. Spraying daily or every 7 to 10 days may, at
times, be costly, and in fact, may be unnecessary. Waiting until the vermin
has stricken may be too late to eliminate significant damage. One useful
method for predetermining when certain pest problems are likely to occur
Is again that of heat summation.
It might be termed, a bit of objectively planned prevention Is worth
lots of cure.
POSSIBLE LIMITATIONSOF APPLYING THE LINEAR HEAT UNIT SYSTEM
IN CERTAIN AREAS OF FLORIDA DURING CERTAIN TIMES OF THE YEAR
Florida has certain unique aspects concerning its climate, particularly
as regards growing winter vegetables in South Florida. For example, sweet
corn grown in other regions of the country is actually grown under a climatic
However, winter-grown sweet corn in Florida usually takes place under a
climatic regime of:
This is quite a different set of conditions and does deviate drasti-
cally from the climatic conditions under which the linear heat unit system
of predicting maturity is normally used. Nevertheless, it may still work
well even for winter-grown vegetables in South Florida and should be tried.
1. Arnold, C. Y. 1959. The Determination and Significance of the Base
Temperature in a Linear Heat Unit System. Proc. Amer. Soc. Hort. Sci.
2. Arnold, C. Y. 1960. Maximum-Minimum Temperatures as a Basis for
Computing Heat Units. Proc. Amer. Soc. Hort. Sci. 76: 682-692.
3. Aung, L. H. et al. 1968. Effect of Temperature on Maturity of Sweet
Corn, Zea mays rugosa. Proc. Amer. Soc. Hort. Sci. 92: 516-522.
4. Seaston, H. L. 1955. Scheduling Plantings and Predicting Harvest
Maturities for Processing Vegetables. Food Tech. April 1955: 202-209.
5. Sims, W. L. et al. 1968. Mechanized Growing and Harvesting of Processed
Tomatoes. Univ. of Cal. Agric. Ext. Ser. AXT-232.
6. Warnock, S. J. 1970. Tomato Heat Unit Accumulation at Various Locations
in California. Hort Sci 5(5): 440-441.
7. Warnock, S. J. and Isaacs, R. L. 1969. A Linear Heat Unit System for
Tomatoes in California. J. Amer. Soc. Hort. Scl. 94: 677-678. 1969.