Soil Heating Techniquesfor Forcing Outdoor White Asparagus

into Production

G. ParoussiT. V. KoutsosE. Paroussis

ABSTRACT. Hot water was circulated in two pipes placed above (AC),beside (BC), and under (UC) the crowns of the asparagus (Asparagusofficinalis L.) hybrid cvs. ‘Dariana’, ‘Larac’ and ‘Steline’ to force theminto production in winter. An unheated bed of each cultivar was used ascontrol. Raised beds were covered with black plastic mulch to ensurewhite spears and weed control. The heating system was activated inmid-January for the 1998 harvesting season, mid-December 1998 for the1999 harvesting season and early January for the 2000 harvesting sea-son. In 1998 and 2000 harvesting seasons, early yield of A grade spearsin the UC treatment was significantly different from controls, and the ACand BC treatments which were similar. The total yield of the UC treat-ment was significantly different from the control and the AC and BCtreatments only in 1998. The UC treatment improved the quality of theproduct in 1998 and 1999 since “B” grade yield, as a percentage of “A”grade yield, appeared to be lowest. ‘Dariana’ consistently had higher to-tal yield than ‘Larac’ and ‘Steline’ in all production years. In 1999,‘Dariana’ yielded more early “A” grade spears than ‘Larac’ and ‘Steline’.[Article copies available for a fee from The Haworth Document Delivery Ser-vice: 1-800-HAWORTH. E-mail address: <getinfo@haworthpressinc.com>Website: <http://www.HaworthPress.com> 2002 by The Haworth Press, Inc.All rights reserved.]

G. Paroussi and T. V. Koutsos are affiliated with the National Agricultural Founda-tion, Agricultural Research Center of Macedonia-Thrace, 570 01 Thessaloniki, Greece.

E. Paroussis is affiliated with the National Agricultural Foundation, Soil ScienceInstitute of Thessaloniki, 570 01 Thessaloniki, Greece.

Journal of Vegetable Crop Production, Vol. 8(1) 2002http://www.haworthpressinc.com/store/product.asp?sku=J068

2002 by The Haworth Press, Inc. All rights reserved. 3

KEYWORDS. Asparagus officinalis L., off-season production, heatingtechniques

INTRODUCTION

Asparagus (Asparagus officinalis L.) is a very important crop for the Greekeconomy with more than 90% of the white asparagus production exported tothe European Union (E.U.) countries. The production of white asparagus be-gins in April and extends for a 7-9 week period. The demand is increasing es-pecially for an early high quality product. Factors affecting quality and earlinessare cultivar, use of plastic covering of ridges (Makus and Gonzalez, 1991), andheating (Pill et al., 1993; Krug, 1996; 1999a). The plastic covering of ridgescan advance the harvest season two weeks, the maximal, in Greece. Whereas,by heating the soil, a very early (off-season) production of white asparagus canbe achieved.

Fern “ripening” (middle degree of yellowing) under Macedonia (division ofGreece) conditions occurs about October 15. After fern cutting, temperaturesfrom 0-15°C during November and December act to retard crown develop-ment (Krug, 1998). However, crowns might sprout if exposed to at least 25°Cincubation following a cold treatment (Krug, 1996; 1999b).

The objective of this study was to investigate the efficacy of heat dissemina-tion in the soil through variously placed pipes on earliness, quality and totalyield of outdoor white asparagus production.

MATERIALS AND METHODS

Three hybrid cultivars, ‘Dariana’, ‘Larac’ and ‘Steline’, were tested usingthree methods of heating the soil. Two heating pipes (2.54 cm diameter) wereplaced in different positions in relation to the asparagus crowns as follows:(1) above the crowns (AC), (2) beside (one pipe at each side) the crowns (BC),(3) under the crowns (UC), and (4) an unheated bed, as control (C). The hotwater circulating in the plastic pipes originated from a central heating systemusing cores of peaches, an alternative to oil as a source of energy. Cores ofpeaches are abundant in Greece, as they are the side-product of peach canner-ies. One-year-old crowns of the three cultivars were field-planted in May 1997in a split plot design with four replications and four soil heating treatments.The experimental plot for each cultivar and replicate included 10 crowns pertreatment. The spacing of the crowns was 1.7 m between the rows and 0.2 mbetween crowns in the row. The crowns were planted at least 15 cm below soilsurface (Sanders et al., 1990) at a population of 29,000/ha. This density wasconsidered appropriate for the sandy loam soil and the cultivars used, as the

4 JOURNAL OF VEGETABLE CROP PRODUCTION

cost of the extra plants could be more than offset by the greater yields obtainedfrom the higher population (Loughton et al., 1996). Raised beds on the rowswere covered with black plastic mulch to blanch spears and control weeds.Fertilizing, irrigation and plant protection were the usual practices used byGreek farmers.

Spear harvest was planned to begin next year after planting. To stimulateearly spear emergence, heating began immediately after the bed shaping andmulching. For 1998, heating was started on 17 January 1998; for 1999, heatingwas begun on 17 December 1998; and for 2000, heating began on 3 January2000. The temperature of the hot water in the pipes was 35°C, which causedsoil temperatures to be about 16-18°C throughout the harvest season. Har-vesting started on 15 February, on 10 February and on 20 February for 1998,1999 and 2000, respectively. White spears were harvested three times perweek and classified as “A” or “B” grades. The “A” grade classification was ac-cording to standards of “fat white spears” (diameter > 12 mm) of “extra” and“I” category determined by E.U. committee decision (454/28-2-1992) whilethe “B” grade classification was according to standards of “II” category (diam-eter < 12 mm) determined by this decision. The yield until 20 March was des-ignated early yield, as after this date the prices of white asparagus generally godown rapidly in the markets of the E.U. countries. Harvesting lasted until lateApril. Yield variables were tested by ANOVA and where applicable meansseparation was with Duncan’s Multiple Range test. To investigate the effectsof cultivars and heating treatments on yield quality, the relation of early “B”grade spear yield as a percentage of “A” grade spear yield was estimated foreach plot. Before proceeding to ANOVA of the percentages, to increase the ac-curacy of comparisons, angular sine transformation was applied to them,which is defined by arcsin X1/2 (Steel and Torrie, 1981), where X is the per-centage value of “B” grade yield on “A.” For the test of percentages, meansseparation was with orthogonal comparisons.

RESULTS AND DISCUSSION

Cultivar and heating treatment, but not their interaction, affected compo-nent of yield (Table 1). Cultivar affected only early yield of “B” spears in 1998and early yield of “A” and “B” spears in 1999. In 1998 early yield of “B”spears was greatest for ‘Steline’ and least for ‘Dariana’. Whereas, in 1999early yield of “A” spears was greatest for ‘Dariana’ and early yield of “B”spears was least for ‘Larac’. ‘Dariana’ consistently had the highest total yield.In 1998 and 2000 the UC treatment improved the early yield of “A” gradespears. In 1998 early “B” grade spear yield was not affected by treatment. In1999, the AC treatment had the highest early “B” grade spear yield and in 2000

Paroussi, Koutsos, and Paroussis 5

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6

the UC treatment had the highest early “B” grade spear yield. Total yield wasaffected by heating treatment in 1998 where the UC treatment produced thehighest yield.

The early “B” grade spear yield as a percentage of early “A” grade spearyield appeared to be unaffected in 1998, highest due to AC treatment in 1999,and highest for controls in 2000 (Table 2). Furthermore, these quality percent-ages, “B” grade on “A” grade yield, increased year by year, which indicatesdeterioration of the quality. This might be due to the early first harvest appliedone year after planting. Benson and Motes (1982) reported that harvesting oneyear after planting reduced the number, diameter and total spear production insubsequent years, while Dufault (1994) reported that first harvest delay from18 to 30 months after transplanting did not affect later harvests. On the otherhand, it is unclear why the UC treatment inconsistently affected (in 1998 and2000 but not in 1999) early harvest more than the other heating treatments.

On balance, over the three years the results of the UC heating treatment wasmore effective than the AC and BC treatments or controls, in terms of earlyyield (Figure 1). For total yield there did not appear to be any difference due toplacement of heating pipes (Figure 2). The cv. ‘Dariana’ out-yielded ‘Steline’in terms of both early (Figure 3) and total yield (Figure 4).

The following conclusion may be drawn from the data: (1) the best positionfor heating appears to be placement of the two plastic heating pipes undercrowns to increase earliness of outdoor asparagus production, and (2) the cv.‘Dariana’ responded to heating treatments similar to the cv. ‘Larac’ and betterthan the cv. ‘Steline’.

Paroussi, Koutsos, and Paroussis 7

TABLE 2. Comparisons of percentages of early “B” grade spear yield on “A”grade spear yield during 1998-2000.

Year

Cultivar 1998 1999 2000

Dariana 0.090bz 0.102b 0.258a

Larac 0.097b 0.116b 0.134b

Steline 0.152a 0.170a 0.148b

Heating

(C) Control 0.102a 0.119b 0.249a

(AC) Above Crowns 0.117a 0.167a 0.112c

(BC) Beside Crowns 0.133a 0.130ab 0.155abc

(UC) Under Crowns 0.100a 0.101b 0.204abz Values in a column followed by different letters are significantly different at p � 0.05.

8 JOURNAL OF VEGETABLE CROP PRODUCTION

700

600

500

400

300

200

100

0

Yie

ld(k

g/0.

1h)

Placement of heating pipes in relation to crowns

None Above Beside Under

FIGURE 1. Mean early yield of heating treatments over cvs. and productionyears.

1100

1000

900

800

700

600

Yie

ld(k

g/0.

1h)

Placement of heating pipes in relation to crowns

None Above Beside Under

FIGURE 2. Mean total yield of heating treatments over cvs. and productionyears.

LITERATURE CITED

Benson, B.L. and J.E. Motes. 1982. Influence of harvesting asparagus the year follow-ing planting on subsequent spear yield and quality. HortScience. 17(5): 744-745.

Dufault, R.J. 1994. Impact of forcing summer asparagus in coastal South Carolina onyield and quality, and recovery from harvest pressure. J. Amer. Soc. Hort. Sci. 119(3): 396-402.

Paroussi, Koutsos, and Paroussis 9

450

400

350

300

250

200

150

100

50

0

‘Darina’ ‘Larac’ ‘Steline’

Cultivars

Yie

ld(k

g/0.

1h)

FIGURE 3. Mean early yield of cvs. over heating treatments and productionyears.

1500

1300

1100

900

700

500

‘Darina’ ‘Larac’ ‘Steline’

Cultivars

Yie

ld(k

g/0.

1h)

FIGURE 4. Mean total yield of cvs. over heating treatments and productionyears.

Krug, H. 1996. Seasonal growth and development of asparagus (Asparagus officinalisL.) I. Temperature experiments in controlled environments. Gartenbauwiissnschaft.61 (1): 18-25.

Krug, H. 1998. Seasonal growth and development of asparagus (Asparagus officinalisL.) II. Influence of drought on grown activity. Gartenbauwiissnschaft. 63 (2):71-78.

Krug, H. 1999a. Seasonal growth and development of asparagus (Asparagus officinalisL.) IV. Crown activity as function of incubation temperature and temperature gradi-ent. Gartenbauwiissnschaft. 64 (2): 84-88.

Krug, H. 1999b. Seasonal growth and development of asparagus (Asparagus officinalisL.) V. Fern “ripening” and crown activity in open fields. Gartenbauwiissnschaft. 64(2): 165-172.

Loughton, A., R. Baker, and O.B. Allen. 1996. Yield and growth responses of aspara-gus to between-row spacing and planting depth. Can. J. Plant Sci. 76 (4): 841-847.

Makus, D.J. and A.R. Gonzalez. 1991. Production and quality of white asparagusgrown under opaque rowcovers. HortScience. 26 (4): 374-377.

Pill, W.G., T.A. Evans, and S.A. Garison. 1993. Forcing white asparagus in varioussubstrates under cool and warm regimes. HortScience. 28 (10): 996-998.

Sanders, D.C., C.A. Prince, P.P. David, and M.R. McMurtry. 1990. Effect of initial as-paragus populations on survival and yield. Acta Horticult. 271: 197-202.

Steel, R.G.D. and J.H. Torrie. 1981. Principles and procedures of statistics. A bio-metrical approach. Inter. Student Edn.

10 JOURNAL OF VEGETABLE CROP PRODUCTION