Previous PageTable Of ContentsNext Page

Plant density and planting date impacts on Pima cotton development

D. S. Munk

University of California Cooperative Extension, Fresno County, Fresno, California U.S.A.


Studies manipulating plant spacing and planting date were needed for the regional development of grower guidelines for cotton planting and replant decisions. Statistical methods were used to evaluate the impact of plant date and plant density variables on early- and mid-season crop growth parameters of Pima cotton. The impact of planting date on several crop growth parameters was much greater than the range of plant spacing variables evaluated in 1998 and 1999. Although plant vegetative growth was largely compensated for before late season, delayed plantings primary productivity limitations were observed through large sympodial branch number decreases. Plant density also impacted in-season growth parameters measured including plant height, vegetative node number, fruiting branch number and nodes above yellow flower. Late planted Pima cotton having high yield potential was heavily impacted by planting delays of less than 15 days.


Pima cotton, plant density, earliness, plant mapping, plant height, fruiting branch number.


Pima cotton, Gossypium barbadense, requires a significantly longer growing season than conventional Upland varieties. Because of the extended season of these varieties, information is needed to make grower recommendations on final plant populations, planting dates and making replant decisions. Information on early- middle- and late-season plant development has been used to identify limitations of varieties to a range of plant densities. Kittock (4) demonstrated final planting height was a key characteristic in determining optimum plant density in Pima cotton. Recent studies (2) have also demonstrated cultivars influences optimum plant spacing with more indeterminate varieties performing best at low densities. Buxton (1) found that fruit initiation was influenced by plant density in Upland cotton.


A randomised and replicated split plot design was used with planting date established as the main affect and plant density as the split plot. Planting dates were separated using 15-day intervals on five dates, with initial plantings on March 10 and March 11 for 1998 and 1999, respectively. Pima S7 plots were established on 4-1m beds 20 m in length. Plants were thinned to 2.5, 5.0, 10.0 and 15.0 plants/m2 at the 2 to 4 leaf stage, for each of the planting dates.

Agronomic management strategies were used that would minimise yield impact across all treatments. Irrigation events were timed equally for all plots using published leaf water potential guidelines for most stressed treatment. This assured treatments would be irrigated at or below water stress levels that may impact yield. Defoliation and harvest timing were optimised for whole plot yield and quality.


During cotton’s early development period, heat unit accumulation is highly correlated with node number in Upland cotton (3). These trials also demonstrated planting date was the primary factor determining plant height in early- and mid-season plant monitoring dates, figure 1. This relationship did not hold up season-wide and was reversed at the time plants reached cutout. This reverse trend in plant height is likely caused by the lower retention of bottom fruit and overall plant boll load on plants having an earlier planting date.

Plant height also responded to changes in plant spacing. With each decrease in 1998 density treatment, early-season plant height was reduced 1 to 2 cm while similar trends were observed in 1999. Plant height differences between density treatments were also reversed as the highest density treatment was found to be shorter than the low-density treatments in late July and August.

Figure 1. Mean plant height measured on three sample dates in 1998. Means are developed from a 20 plant sample for each of 5 planting dates and 4 densities.

Our evaluation of monopodial branch number prior to the initiation of sympodial branches found a steady numerical decline as the planting date was delayed. A decrease of 1.1 and 0.8 monopodial branches was measured from the earliest to latest planting date in 1998 and 1999 respectively. The impact of plant spacing on monopodial branch number was also significant and resulted in an elevated number of monopodial branches by 0.4 in 1998 and 0.5 in 1999. This earlier fruit set brought on by delayed planting and increased plant population may have contribute to the decreased plant height observed late in the season as early fruiting structures consume plant resources otherwise directed at vegetative growth.

Regardless of the evaluation year, Pima S7 had consistently lower sympodial branch numbers on early-, middle-, and late-season evaluations as planting date was delayed. Reductions of 2.5 and 4.6 fruiting branches were observed late August between earliest and latest plantings in 1998 and 1999 respectively. Small reductions in fruiting branch number across a wide variation in planting date can be linked to the lower spring and early summer temperatures that prevailed in the 1998 season. Sympodial ranch number trends were also noted in plant density, although the treatment separations were less robust. Low density plantings tended to have more fruiting nodes in both study years, averaging 0.6 more nodes on the low density plantings compared to the highest density plantings.


The manipulation of planting date and density demonstrated Pima cotton’s resilience in maintaining productivity under adverse field conditions. Pima S7 was able to partially compensate for delayed planting by decreasing monopodial branch number prior to the first fruiting branch and maintaining high vegetative growth rates late in the season. Increased plant spacing in the row resulted in earlier first fruit set, however, this only occurred at very low plant populations indicating it is not a desired tool for simultaneously increasing earliness and maintaining high yield. Due to the consistent reductions in fruiting branch number on delayed plantings, yield potential might be maintained under these circumstances with proportional increases in fruit retention on all fruiting branch sites, particularly second and third position sites. However, this is contrary to our study observations that found a measurable reduction in early fruit retention rate on late-planted cotton.


1. Buxton, D.R., Briggs, R.E., Patterson, L.L., and Watkins, S.D. 1977. Canopy characteristics of narrow-row cotton as influenced by plant density. Agron. J. 69:929-933.

2. Kerby, T.A., Cassman, K.G., and Keeley, M. 1990. Genotypes and plant densities for narrow-row cotton systems. I. Height, nodes, earliness, and location of yield. Crop Sci. 30:644-49.

3. Kerby, T.A., and Hake, K.D. 1996. Monitoring Cotton’s Growth. In: Cotton Production Manual. Pub. 3352. (U.C Div. of Agriculture and Natural Resources, S.J. Hake, et al) pp. 335-355.

4. Kittock, D. L., Selley, R.A., Cain, C. J. and Taylor, B.B. 1986. Plant population and plant height effects on Pima cotton lint yield. Agron. J. 78:534-38.

Previous PageTop Of PageNext Page