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University of Florida
AGRICULTURAL RESEARCH AND EDUCATION CE :TL:
Research Report CF 80-3 H UiV 1979
EFFECT OF PLANTING DATE ON THE PERFORM CE OFJ PS LETwTCEIE
ON TWO CENTRAL FLORIDA ILS
J. M. White and V. L. G C. .,
Assistant Horticulturist and Horticulturist, respectivei"-""
Additional index words, temperature, flowering, bolting, Lactuca sativa L.
Ten plantings of 'Valmaine' and 'Florida 1978' cos lettuce were planted in
1977 and 1978 on mineral and organic soils at three week intervals. Marketable
yield was significantly higher on mineral soil for 'Valmaine' as compared to
'Florida 1978'. Marketable plant weights (size) were significantly higher on
organic soil. More plants bolted (developed seedstalks prematurely) when grown
on mineral soil than on organic soil under conditions favorable to bolting.
Bolting was significantly correlated to mean air temperature of the growing
season which was slightly higher at the mineral soil site. Higher yields were
obtained when planted between September 20 and larch 7. Yield of plantings made
after March 7 was markedly depressed due to excessive bolting.
'Valmaine' probably is the standard cos variety used in Florida, but is
susceptible to lettuce mosaic virus (LMV). One method of preventing losses
due to LIV is to select cultivars resistant to this disease. 'Florida 1978',
developed by V. L. Guzman and T. A. Zitter, is a breeding line resistant to
LMV and to Bidens mottle virus (BlIV). Florida 1978 was selected for
characteristics equal to or better than Valmaine, including resistance to pre-
mature bolting (seedstalk development). During field trials in the fall of
1976, Florida 1978 was observed bolting less than Valmaine.
Bolting in lettuce has been shown to be influenced by temperature and
photoperiod (2,3,4,5). Favorable lettuce growing conditions include sunny
days, uniformly cool nights, a good water supply, and a monthly mean
temperature between 12.8 and 15.6C (1).
The purpose of this study was to investigate the effect of planting date
and soil type on the yield and premature seedstalk development of these two
AREC-Sanford and AREC-Belle Glade, respectively.
Valmaine and Florida 1978 were hand seeded in central Florida into mineral
(1Myakka fine sand) and organic (Lauderhill muck) soil every three weeks on ten
planting dates starting in the fall (September 20, 1977), The two locations are
approximately 45 kilometers apart. Plot size was 18.6 m on the mineral soil
and 22.3 m' on the organic soil. Treatments at each location were arranged in a
randomized block design with six replications. One-half of each plot was har-
vested for yield and size data when the lettuce was estimated to be mature. The
remainder of the plot was examined four times (every third day) to record bolt-
Marketable yields were higher on mineral soil when the lettuce was planted
between November 1 and January 3 (Table 1). Bolting was a major problem when
planted on September 20 or after January 3. Bolting was significantly corre-
lated to the mean temperature of this growing period. Plantings of lettuce
could continue until February 14 on the organic soil with good yields and lower
incidence of bolting (Table 1). The mean temperature was slightly lower at the
organic soil site for all of the growing periods except the Iarch 28 planting.
The mean temperature on organic soil would have been lower for the 'larch 28
planting had it not been for the longer period of time the lettuce was in the
field before 100% bolting occurred.
Overall, lettuce grown in organic soil had significantly higher plant
weights (767 vs. 696 g) but significantly lower total marketable yield (215 vs.
262 q/ha) than in mineral soil. This was in part due to some freeze damage
and slightly cooler and drier growing conditions which caused stand problems
on the organic soil. Averaging all planting dates, 'Florida 1973' had
significantly higher average plant weights (770 vs. 686 g) and total
marketable yields (272 vs. 199 q/ha) than 'Valmaine'. Regression analysis
indicated a highly significant negative correlation between mean temperature
during the growing period and the number of days to harvest for both soils
(locations) and for both cultivars (Table 2). There was a highly significant
positive correlation between the percentage of bolted plants and the mean
temperature during the growing period for both soils and 'Valmaine' and 'Florida
1978'. Significant negative correlations were found between the number of days
to harvest and the percentage of bolted plants on mineral and organic soil
for both cvs.
The bolting results obtained in this study were not as expected and were
different from those obtained in a study in 1976. Both cvs. planted had zero
bolting when harvested on December 22, 1976. When continued 26 days later,
83% of the 'Valmaine' plants had bolted while only 15% of 'Florida 1978' had
bolted. In other tests, including this one, these results could not be dupli-
cated. Temperature during the growing season, day length, and the seed lot
do not explain this difference. One factor which was not examined was the age
of the seed since seed from the same lot was used in all of the tests.
Photoperiod has an effect on bolting of leaf lettuce (2). Bolting of cos
lettuce in Florida appears to be conditioned more by temperatures than by photo-
period. High fall temperatures with decreasing photoperiod (September 20
planting) promoted bolting. In the spring (March plantings), increasing photo-
period with high temperatures also promoted bolting.
In general, both Valmaine and Florida 1978 are similar in their response
to seasons when planted after September 20. Mineral soils are less suitable
for coo lettuce production when growing under hot conditions. Best yield and
less bolting occurs in plantings after September 20 and before March 7,
particularly in organic soils.
1. Knott, J. E. 1955. Vegetable Growing (5th ed.). Lea and Febiger,
Philadelphia, PA. p. 353
2. Rappaport, L. and S. H. Wittwer. 1956. Night temperature and photoperiod
effects on flowering of leaf lettuce. Proc. Amer. Soc. Hort. Sci. 68:279-282.
3. and 1956. Flowering in head lettuce as influenced by seed
vernalization, temperature, and photoperiod. Proc. Amer. Soc. Hort. Sci.
4. Thompson, H. C. and J. E. Knott. 1933. The effect of temperature and photo-
period on the growth of lettuce. Proc. Amer. Soc. Hort. Sci. 30:507-509.
5. Tincker, H. A. II. 1933. The influence of the length of day on lettuce.
Gard. Chron. 93:404-405.
Table 1. Response of cos lettuce to planting date and soil type in central Florida.
% Bolted plants
Mean Mkt. yield (q/ha)z Avg. plantwt. (g)z at harvest + 12 days
Planting date- Days to temp Florida Florida Florida Florida
and soil harvest (OC) Valmaine 1978 Valmaine '1978 Valmaine 1978 Valmaine 1978
Sept. 20, 1977
Oct. 11, 1977
Nov. 1, 1977
Nov. 22, 1977
Dec. 13, 1977
Jan. 3, 1978
Jan. 24, 1978
20.9 38 gh
19.8 90 defg
16.7 114 def
16.3 125 de
14.1 340 b
13.8 61 fg
14.2 400 ab
13.7 49 g
14.2 416 a
14.1 69 efg
14.8 329 b
14.3 192 c
15.8 129 d
14.9 251 c
147 ef 590 de 731 de
111 ef 617 cde 617 f
121 ef 749 b 731 de
171 ef 681 bed 672 ef
315 d 867 a 799 bcd
160 ef 663 bed 776 cde
397 bcd 913 a 854 abe
203 ef 613 cde 749 cde
459 ab 981 a 958 a
110 f 595 de 726 de
376 bed 749 b 726 de
400 bed 731 bc 913 ab
568 de 549 f
568 de 931 a
Table 1. cont'd.
Days to temp
Mkt. yield (q/ha)z
Avg. plant wt.(g)z
+ 12 days
Feb. 14, 1978
March 7, 1978
March 28, 1978
0 g Of 0 g
436 abc 572 de 844 abcd
0 g Of
345 cd 513 e
ZMean separation within columns by Duncan's Multiple Range Test, 5% level.
Table 2. Correlation coefficient r values for average mean temperature during
the growing season, the number of days cos lettuce were grown, and
two soil types (locations.).
Mean temp days
Soil type Valmaine 1978
Mineral -.93** -.93**
Days % bolts
% bolts-mean temp
*'**Correlation significant at the 5% (*)
and 1% (**) level.