Increasing plant population densities is a useful technique for raising yield and potential profits in brassicas. For high-density cole crop production to be successful, however, nitrogen applications should increase to accommodate increased nutrient demands. The use of high-density populations has certain disadvantages. Broccoli and cauliflower yields per unit area normally increase with closer planting densities but are associated with smaller head size. While this may increase the numbers of heads and the total yield, maturity is often delayed and the quality is reduced (Salter and James, 1975).
Manipulating resource use by altering drilling or planting dates is exemplified by the work of Siomos (1999) (Tables 5.8, 5.9 and 5.10) who studied Pak Choi (B. rapaL. Chinensis group) grown under unheated plastic in Greece, in three periods: December/January; January/March; and March/May. Only when the temperatures and light incidence were high (period 3) did increased plant density (ten plants increased to 16.7 plants/m2) raise yield; here individual fresh weight fell but the total number of plants increased. Changing the planting date and within-row plant spacing had little effect on dry matter, total soluble solids and fibre content.
Nutrient requirements alter with changes to crop density and spacing. In the southeastern USA, vegetables are usually planted on raised beds (Parish, 2000), with either single or double rows to each bed. Double rows offer higher yields per unit area, but may be difficult to maintain physically because of the erosion of the sides of the bed caused by localized heavy rainfall. Beds also provide advantages of quicker and earlier soil warming and allow the use of mechanically guided cultivation such as steerage hoeing. In some areas, beds promote the avoidance of soil-borne pathogens such as P.
brassicae, the causal agent of clubroot disease, because the soils are drier and
Table 5.8. Seeding, transplanting and harvesting dates for Pak Choi cv. Troy F.
Days from
Trans- Sowing to Trans- Sowing
Growing Sowing planting Harvesting trans- planting to to
period date date date planting harvesting harvesting
1 25 Oct 1993 3 Dec 1993 27 Jan 1994 39 55 94
2 23 Dec 1993 28 Jan 1994 28 Mar 1994 36 59 95
3 25 Feb 1994 30 Mar 1994 14 May 1994 33 45 78
Results for Greece were similar to those from The Netherlands and Australia for
glasshouse/polythene-grown Pak Choi; after May, the crops tend to bolt under protection.
After Siomos (1999).
Table 5.9. Effect of the growing period and plant spacing on yield (kg/m2) of Pak Choi cv.
Troy F1.
Growing period
Plant spacing (cm) 1 2 3 Mean
15⫻40 5.14cd 5.63c 12.66a 7.81A
25⫻40 4.02d 5.71c 9.29b 6.30B
Mean 4.58C 5.67B 10.98A
Means followed by different letters are significantly different at the 0.05 level of probability (Duncan’s multiple range test).
Comparisons between means are differentiated by upper-case letters.
After Siomos (1999).
this inhibits the movement of primary zoospores towards the host root hairs.
In areas of moderate rainfall, the bed structure is retained and greatest yields came from multiple row plots.
Beds are especially useful for rapidly maturing brassicas such as the leafy greens which have become popular for both processing and fresh markets, such as mustard (B. juncea), turnip (B. rapa) and collard (B. oleraceaAcephala group). Growing six rows on 2 m wide beds proves very effective, producing higher yields compared with fewer rows on narrower beds.
In cauliflower and broccoli crops, results show that with increasing rectangularity of spacing, i.e. between-row spacing divided by within-row spacing, crop yields are decreased (Chung, 1982). This indicates that it is more advantageous to grow crops in a square formation rather than in a rectangular pattern (Salter et al., 1984; Sutherland et al., 1989). Modification of plant population densities is used to control cauliflower curd weight. A number of studies demonstrate that curd weight decreases with increasing plant density (Dufault and Waters, 1985; Singh and Naik, 1991).
Commercially, spacing varies substantially depending on location, genotype and husbandry systems. For example, in Europe, summer cultivars require
much smaller spacing compared with overwintered types. In Western Australia, wider spacing is the norm, thus between-row spacing of 0.75–0.80 m and, for most cultivars, a within-row spacing of 0.40–0.50 m and two rows of cauliflower per planting bed are used. Resultant curd size varies in the range from 0.5 to 2.0 kg. Recent field experiments in Western Australia (Stirling and Lancaster, 2005) demonstrated that plants grown in a four-row configuration produced significantly (P= 0.007) higher total yields than control plants grown in a two-row configuration (Fig. 5.1). Within the four-row configuration, a significant (P= 0.019) linear trend was observed, with yield falling by 0.3 t/ha for every 0.01 m increase in within-row plant spacing.
Uniformity of curd maturation improved when the number of plant rows per bed was increased from two to four (Table 5.11). The majority of curds from plants grown in four rows were removed in the first two harvests, with only a small proportion of curds remaining at the final harvest. An increase in the uniformity of mature curds was identified in plants grown in four rows, spaced at 0.40, 0.45 and 0.50 m.
Curd weight decreased significantly (P < 0.001) when the planting configuration was altered from two to four rows (Table 5.12). Within the four- row configuration, there was a significant (P= 0.011) linear effect of plant spacing on curd weight, which decreased with increasing plant density.
Average curd weight decreased by 4.1 g for every 0.01 m decrease in plant spacing. There was a significant (P= 0.003) decrease in the average diameter of all curds harvested per treatment when plants were grown in four rows compared with two (Table 5.12). Uniformity of curd maturity improved when the row number per bed was increased from two to four. This is an important consideration for cauliflower producers as it has a major influence on variable costs. Crops that mature in unison require fewer harvests, thereby substantially reducing labour and machinery costs.
In Minnesota, USA, as cauliflower populations were increased from 24,000 to 72,000 plants/ha with nitrogen rates held constant at either 112 Table 5.10. Effect of growing period and plant spacing on mean daily growth increments (g/m2/day) of Pak Choi cv. Troy F1.
Growing period
Plant spacing (cm) 1 2 3 Mean
15⫻40 93.4c 95.4c 281a 156.6A
25⫻40 73.0c 96.8c 206.5b 125.5B
Mean 89.2B 96.1B 243.8A
Means followed by different letters are significantly different at the 0.05 level of probability (Duncan’s multiple range test).
Comparisons between means are differentiated by upper-case letters.
After Siomos (1999).
0 5 10 15 20 25 30 35 40
0.40 m 0.40 m 0.45 m 0.50 m 0.55 m 0.60 m
Yield (t/ha)
Total yield Marketable yield
*
*
a b
Fig. 5.1. Illustration of total and marketable yield of cauliflower (Brassica oleracea var. botrytis) cv. Summer Love produced by plants spaced at 0.40, 0.45, 0.50, 0.55 and 0.60 m. *Two row treatment data. Bars indicate the least significant difference between all treatments (5%) = 5.2 (a) and 7.0 (b) (Stirling and Lancaster, 2005).
or 224 kg N/ha, marketable curd weight decreased in a linear manner at any population. Increasing the nitrogen rate to 112 kg/ha or higher, reduced cull production at 24,000 plants/ha, but not at populations of 36,000 or higher.
Cauliflower yields were optimal at 24,000 plants/ha and 112 kg N/ha based on considerations such as reduced cull production, satisfactory curd weights and transplant economy (Dufault and Waters, 1985).
Studies in Minnesota, USA, showed that as broccoli cv. Southern Comet (B. oleracea var. italica) populations were increased from 24,000 to 72,000 Table 5.11. Percentages of cauliflower (Brassica oleraceavar. botrytis) curds cut at several harvest dates relative to plant spacing.
Spacing between
Number of rows plants within Harvest Harvest Harvest Total
per 1.6 m bed row (m) 1 (%) 2 (%) 3 (%) (%)
2 0.40 10.79 74.59 14.62 100.00
4 0.40 31.97 65.24 2.78 100.00
4 0.45 34.74 61.72 3.54 100.00
4 0.50 24.53 73.06 2.41 100.00
4 0.55 22.81 69.88 7.31 100.00
4 0.60 25.32 67.63 7.04 100.00
LSD between all treatments (5%) 10.87 15.16 14.46 LSD between 4-row treatments only 12.16 11.88 11.21 LSD = Least significant difference.
From Stirling and Lancaster (2005).
plants/ha at nitrogen rates of 112, 168 or 224 kg/ha, head weight decreased linearly. Increasing the nitrogen rate from 56 to 224 kg/ha at any population increased broccoli head weight and marketable yields, and decreased cull yields. Broccoli yields were highest at 72,000 plants/ha and 224 kg N/ha.