*2.2. Assessment of Water and Irrigation Needs*

Reference evapotranspiration (ETo) was determined using Hargreaves' model [18] modified by Droogers and Allen [19] and expressed in Equation (2):

$$\text{ETo} = \text{HC} \times \text{Ra} \times (\text{Tmax} - \text{Tmin})^{\text{HE}} \times \left(\frac{\text{Tmax} + \text{Tmin}}{2} + \text{HT}\right), \tag{2}$$

where

HC = empirical Hargreaves coefficient = 0.0025,

Ra = extraterrestrial radiation (mm day<sup>−</sup>1),

Tmax = maximum air temperature (◦C),

Tmin = minimum air temperature (◦C),

HE = Hargreaves exponent = 0.5,

HT = Hargreaves temperature coefficient = 16.8.

Potencial evapotranspiration (ETp) was determined by the method of crop coefficients [20] using crop coefficients for asparagus as determined by Hargreaveas' model modified by Droogers and Allen [14]. To take into account the specificity of drip irrigation (limited wetted area of soil), Equation (3) was applied [21]:

$$\text{ETp} = \text{ETo} \times \text{kc} \times \text{kr}, \text{ (mm)}\tag{3}$$

where

ETo = reference evapotranspiration (mm),

kc = crop coefficients,

kr = reduction coefficients.

Reduction coefficients were determined according to Freeman and Garzoli's formula [22] based on the percentage of surface coverage of asparagus in the postharvest period [14]. The results are presented in Table 1.

**Table 1.** Values of the correction factor kr according to Freeman and Garzoli.


Drought index (D) according to Stenz [23] is expressed in Equation (4):

$$\mathbf{D} = \frac{\mathbf{E} \mathbf{\bar{\mathbf{\mathbf{\bar{D}}}}}{P}}{P} \,\tag{4}$$

where

ETo = reference evapotranspiration (mm),

P = precipitation in a given period (mm).

To describe the precipitation conditions during the experiment, the worldwide recommended Standardized Precipitation Index (SPI) was used [24]. Equation (5) was used to calculate SPI:

$$\text{SPI} = \frac{\text{f}(\text{P}) - \mu}{\sigma} \,\text{s} \tag{5}$$

where


## *2.3. Meteorological Conditions*

Precipitation conditions in individual decades of the growing and irrigation seasons for asparagus were described according to the season classification, based on the Standardized Precipitation Index, as presented in Table 2.

**Table 2.** Classification of the period as dependent on Standardized Precipitation Index values [24].


Numerical data from measurements taken in the meteorological station of the then Department (now the Laboratory) of Land Reclamation and Agrometeorology at the University of Science and Technology located in Mochełek near Bydgoszcz were used in the calculations.

Average air temperature values during the growing season (April–September) in the research years (2006–2007) were higher than in the long-term (1971–2000) by 0.8 ◦C and 0.5 ◦C, respectively (Table 3). In the first year of the study, high air temperature values were found in July (22.4 ◦C), with the value exceeding the long-term mean by as much as 4.2 ◦C. In the second year of the study, high air temperature values were found in June (18.2 ◦C, i.e., 1.9 ◦C above the long-term mean).


**Table 3.** Air temperature (◦C) in the 2006–2007 vegetation season.

Total precipitation of the growing season in the research years, in relation to the longterm mean, was higher by 54 mm (i.e., by 19%) (Table 4). Higher precipitation totals were recorded in the year 2007 (367 mm, i.e., 131% of the long-term mean). Particularly heavy rainfall occurred in June and July 2007, amounting to 103 mm and 112 mm, respectively, which constituted 172% or 167% of the long-term mean. In the first year of the study, the highest rainfall was recorded in August (115 mm, i.e., 225% of the long-term mean).


**Table 4.** Rainfalls (mm) in the 2006–2007 vegetation season.

Air temperature during the harvest of asparagus spears (from the third decade of April to the third decade of June) is a very important element of asparagus cultivation [1,14].

#### *2.4. Irrigation Needs*

Irrigation was carried out after the harvest period, during the summer months, as recommended [25]. In 2006, irrigation began on July 4 and ended on August 31. The irrigation season lasted 59 days. During the time, 10 single doses were used, amounting to 92 mm. In 2007, the irrigation season was shorter and lasted 37 days (from 15 July to 20 August). A total of 54 mm water was used in 6 doses.

The values of Stenz index, presented in Table 5, indicate the irrigation needs of asparagus cultivars in July and August of the years 2006 and 2007.



The values of the Standardized Precipitation Index, summarized in Table 6, reflect the actual precipitation conditions occurring in individual decades of the irrigation season. The first decade of July in 2006 was extremely dry, while the second decade of July in 2007 was moderately dry.

**Table 6.** Values of the Stenz index for the 2006–2007 irrigation period.


The smallest daily water needs under drip irrigation (below 1 mm) were found in the third decade of June when summer stalks were just beginning to grow (Figure 2). However, irrigation needs were much higher in the next 2 months of the irrigation season (from 3.2 mm to 4.4 mm). Depending on weather conditions (particularly temperature), asparagus irrigation needs variation in individual years. For comparison, in 9 years of trials with European cultivars (Gijnlim, Ramos, Vulkan) conducted in central Poland (region of Bydgoszcz), the average daily value of field water consumption at drip irrigated plots in June, July, and August, was 1.7 mm, 3.5 mm, and 4.4 mm, respectively [14]. In the lysimeter studies by Paschold, et al. [12] daily water consumption under drip irrigation was, on average, 1.6 mm in June, 2 mm in July, and 3 mm in August.

**Figure 2.** Daily values of the potential evapotranspiration (mm) of the asparagus plants under drip irrigation conditions in particular months of the irrigation period.

The total water needs of asparagus in the postharvest period were higher in the first year of the study (258 mm) than in the second year of the study (241 mm) (Figure 3). For comparison, in the Rolbiecki study [14], the water needs of asparagus plants under drip irrigation ranged from 205 mm to 272 mm in individual years. The results of tests carried out by Paschold and Kunzelmann [26] in Germany's climatic conditions indicated that the water needs of asparagus plants from June 20 to September 1 ranged between 150–241 mm. Moreover, in prior studies carried out in Germany, Hartmann [8] estimated that, depending on precipitation conditions, the water needs of asparagus plants ranged between 179 mm and 240 mm. In turn, Pardo et al. [27] recorded the seasonal water consumption of asparagus under lysimeter conditions ranging from 274 mm to 294 mm. In the latest research on asparagus water needs in various regions of Poland reported by Rolbiecki et al. [28], the irrigation needs of asparagus plants were found to be, on average, 228 mm in the period from July 1 to August 31 in the central-north-west region of Poland, which also covers the area considered in the present study.

**Figure 3.** Cumulative potential evapotranspiration (mm) of the asparagus plants under drip irrigation conditions in particular months of the irrigation period.

### **3. Results and Discussion**

#### *3.1. Height, Number, and Diameter of Asparagus Summer Stalks*

Drip irrigation, used after the green spears harvest in the growing season preceding the next asparagus harvest, significantly increased the height of asparagus summer stalks for the studied cultivars and years, on average, from 152 cm to 172 cm (Table 7). Stalk height due to drip irrigation increased, on average, by 20 cm (13%). The highest increase in stalk height (above 180 cm) under drip irrigation, on average, for 2 years of research, was obtained for the cultivars Apollo (196 cm), Grande (185 cm), Atlas (182 cm), Jersey King, and Purple Passion (181 cm). The best response to drip irrigation, with reference to this characteristic, was noted in Atlas cultivar. Drip irrigation increased the height of its summer stalks by 40 cm (i.e., by 28%). Some researchers have also shown a significant impact of irrigation on the height of summer stalks in the year preceding the harvest. For example, in the third and fourth year of cultivation, Drost and Wilcox-Lee [29], Hartmann [8,9], Pardo et al. [27], Paschold et al. [12], and Sterrett et al. [30] obtained an average increase in summer stalk height of 14 cm in comparison to non-irrigated control variants.

**Table 7.** Height of the asparagus summer stalks (cm) as dependent on cultivar and irrigation.


2006: LSD0.05 for: Irrigation = 6.343; Cultivars = 16.591; Interaction: Cultivars/Irrigation = 25.831; Irrigation/Cultivars = 13.599. 2007: LSD0.05 for: Irrigation = 7.225; Cultivars = 14.739; Interaction: Cultivars/Irrigation = 22.341; Irrigation/Cultivars = 12.664. 2006–2007: LSD0.05 for: Irrigation = 2.663; Cultivars = 11.936; Interaction: Cultivars/Irrigation = 16.879; Irrigation/Cultivars = 9.603.

Postharvest drip irrigation, applied in the growing season preceding the next asparagus harvest, significantly increased the number of asparagus summer stalks per plant for the studied cultivars and years, on average, from 7.9 to 11.1 (Table 8). Stalk number per plant due to drip irrigation increased, on average, by 3.2 pcs (40%). The highest increase in stalk number per plant (12 and more) under drip irrigation, on average, for 2 years of research, was observed for Jersey King (13.3), Apollo (12.6), Jersey Supreme, Atlas, and UC 157 (12) cultivars. The best response to drip irrigation, with reference to this characteristic, was noted in UC 157 and Grande. Drip irrigation increased the number of stalks per plant of these cultivars by 5.0 and 5.1 pcs (i.e., by 72%), respectively. The positive impact of drip irrigation on the increase in the number of summer stalks was also presented for Germany's weather conditions, which are similar to Polish weather conditions, by Hartmann [9], Hartmann et al. [10], and Paschold et al. [12]. Drost [31] reported that under dry climatic conditions, lowering sandy soil moisture below −50 kPa reduced the number and height of stalks in comparison to drip-irrigated variants. Sterrett et al. [30] obtained an increase in stalk number due to irrigation, on average, of four stalks per plant, and Battilani [32] obtained an increase of one stalk per plant. The diverse response of asparagus cultivars to drip irrigation has been confirmed, i.a., by the results of Dutch [33] and Polish [14] studies.

**Table 8.** Number of the asparagus summer stalks (pcs) as dependent on cultivar and irrigation.


2006: LSD0.05 for: Irrigation = 0.870; Cultivars = 2.190; Interaction: Cultivars/Irrigation = 3.655; Irrigation/Cultivars = 2.413. 2007: LSD0.05 for: Irrigation = 0.987; Cultivars = 1.839; Interaction: Cultivars/Irrigation = 3.196; Irrigation/Cultivars = 2.417. 2006–2007: LSD0.05 for: Irrigation = 1.160; Cultivars = 0.719; Interaction: Cultivars/Irrigation = 1.017; Irrigation/Cultivars = 0.578.

> Drip irrigation, used after the harvest in the growing season preceding the next asparagus harvest, significantly increased the diameter of asparagus summer stalks for the studied cultivars and years, on average, from 11.2 mm to 12.6 mm (Table 9). Stalk diameter due to drip irrigation increased, on average, by 1.3 mm (12%). The highest increase in stalk diameter under drip irrigation, on average, for 2 years of research, was observed for Apollo (15.3 mm) and Grande (14.6 mm) cultivars. The best response to drip irrigation, with reference to this characteristic, was also noted in Apollo and Grande. Drip irrigation increased the diameter of their summer stalks by 3.6 mm and 3.2 mm, respectively (i.e., by 31% and 29%). Researchers such as, i.a., Sterrett et al. [30], Contador [34], Rubio [35], and Battilani [32], also observed a significant impact of drip irrigation on stalk diameter. As numerous authors have stated, irrigation significantly affects the growth of summer stalks and, consequently, the yield of asparagus spears in the next year [4,36–41]. Based on the results of a cultivar trial in 2003 (carried out as part of the third IACT) in which 31 asparagus cultivars were compared, Rolbiecki and Rolbiecki [2] found a significant impact of drip irrigation on the formation of summer stalks and marketable yield in the first harvest season. The impact of irrigation was manifested by the increased accumulation of carbohydrates in

asparagus rootstocks of the plants growing in optimal humidity conditions, used in spring of the next harvest year [30,31,42–48].


**Table 9.** Diameter of the asparagus summer stalks (mm) as dependent on cultivar and irrigation.

2006: LSD0.05 for: Irrigation = ns; Cultivars = 3.293; Interaction: Cultivars/Irrigation = 4.657; Irrigation/Cultivars = 2.677. 2007: LSD0.05 for: Irrigation = 0.143; Cultivars = 1.532; Interaction: Cultivars/Irrigation = 3.221; Irrigation/Cultivars = 2.774. 2006–2007: LSD0.05 for: Irrigation = 0.499; Cultivars = 2.234; Interaction: Cultivars/Irrigation = 3.159; Irrigation/Cultivars = 1.797.

### *3.2. Marketable Yield, Weight and Number of Asparagus Green Spears*

Postharvest drip irrigation treatment of asparagus plants used in the growing season preceding the asparagus harvest significantly increased green spear yields from 4.21 t·ha−<sup>1</sup> to 6.23 t·ha−<sup>1</sup> in the next growing season (Table 10). The marketable yield increase in green spears due to drip irrigation for the studied cultivars and 2 years of research was, on average, 2.02 t·ha−<sup>1</sup> (48%). The highest marketable yield increase under drip irrigation—above 7 t·ha−1, on average, for 2 years of research—was obtained for Apollo, NJ 953, Jersey Deluxe, and Grande cultivars. High marketable yields above 6 t·ha−<sup>1</sup> were obtained for UC 157, Jersey King, UC 115, Jersey Supreme, and Jersey Giant cultivars. Marketable yields above 5 t·ha−<sup>1</sup> were obtained for Purple Passion and Jersey Knight. The lowest yields under drip irrigation (above 4 t·ha<sup>−</sup>1) were collected from JWC 1 and Atlas. The best response to drip irrigation was noted in Jersey Deluxe and Jersey Giant. Drip irrigation increased their marketable yields by 4.37 t·ha−<sup>1</sup> (157%) and 3.91 t·ha−<sup>1</sup> (142%), respectively. No significant response to drip irrigation was found in Purple Passion, Jersey King, and Atlas. The response to drip irrigation in other tested cultivars was positive, and the observed marketable yield increases ranged from 1.71 t·ha−<sup>1</sup> to 2.78 t·ha−<sup>1</sup> (35–79%). The results obtained are consistent with the results of studies conducted by other scientists. Hartmann [8,9], in weather conditions characteristic for Germany, obtained yield increases of 50% in his trials on sandy soil and 25% on loamy sand due to drip irrigation. Moreover, Paschold et al. [4,11,13], carrying out a cultivar trial using drip irrigation, recorded the highest marketable yields for Gijnlim cultivar. In Poland, Rolbiecki [14] also obtained the highest marketable yields for both control (no irrigation) (6.5 t·ha<sup>−</sup>1) and drip irrigated plots (10.9 t·ha−1) for Gijnlim. Pinkau and Grutz [49] obtained significantly lower marketable yield increases in spears, on average, of 12.4%, observing the highest yield increases in dry years. Mulder and Lavrijsen [33], in an asparagus cultivar trial under sprinkler irrigation conducted in the Netherlands, obtained the highest yields for Gijnlim, amounting to 4.5 t·ha−<sup>1</sup> in the first harvest year and 12.8 t·ha−<sup>1</sup> in the second year. Numerous other authors have also presented a significant impact of irrigation on yield increases in asparagus spears grown in different than Polish climate zones [15,16,29,30,47].


**Table 10.** Marketable yield of the asparagus green spears (t·ha<sup>−</sup>1) as dependent on cultivar and irrigation.

2007: LSD0.05 for: Irrigation = 0.473; Cultivars = 1.572; Interaction: Cultivars/Irrigation = 2.223; Irrigation/Cultivars = 1.274. 2008: LSD0.05 for: Irrigation = 0.581; Cultivars = 1.822; Interaction: Cultivars/Irrigation = 2.451; Irrigation/Cultivars = 1.422. 2007–2008: LSD0.05 for: Irrigation = 0.211; Cultivars = 0.946; Interaction: Cultivars/Irrigation = 1.337; Irrigation/Cultivars = 0.761.

> Drip irrigation, used after the harvest in the next growing season preceding the asparagus harvest, significantly increased the mean spear weight from 35.33 g to 40.35 g (Table 11). The mean spear weight due to drip irrigation increased for the studied cultivars and 2 years of research, on average, by 5.02 g (14%). The greatest mean spear weight under drip irrigation—above 40 g, on average, for 2 years of research—was obtained for the cultivars Grande (48.79 g), Jersey Knight (43.32 g), Apollo (43.03 g), Purple Passion (42.99 g), Atlas (42.71 g), and UC 115 (41.24 g). The best response to drip irrigation, with reference to this characteristic, was noted in Atlas and UC 115 cultivars. Drip irrigation increased the mean spear weight by 24% and 32%, respectively. For comparison, the spear weight for Gijnlim cultivar in asparagus trials under irrigation conducted by Mulder and Lavrijsen [33] was approximately 40 g. Paschold et al. [13] obtained values at the level of 35 g, and Sterret et al. [30] obtained values at the level of 25 g.


**Table 11.** Weight of the asparagus green spears (g) as dependent on cultivar and irrigation.

2007: LSD0.05 for: Irrigation = ns; Cultivars = 8.822; Interaction: Cultivars/Irrigation = 11.316; Irrigation/Cultivars = 7.066. 2008: LSD0.05 for: Irrigation = 6.343; Cultivars = 6.022; Interaction: Cultivars/Irrigation = 9.416; Irrigation/Cultivars = 6.116. 2007–2008: LSD0.05 for: Irrigation = 1.031; Cultivars = 4.622; Interaction: Cultivars/Irrigation = 6.536; Irrigation/Cultivars = 3.718.

> Drip irrigation, applied after the harvest in the growing season preceding the next asparagus harvest, significantly increased the number of green spears per plant from 6.50

to 9.49 (Table 12). Green spears number per plant increased due to drip irrigation for the studied cultivars and 2 years of research, on average, by 2.98 pcs (46%). The highest increase in green spears number per plant (10 or more) under drip irrigation conditions, on average, for 2 years of research, was observed for the cultivars Jersey Deluxe (11.10), Jersey King (10.90), Jersey Supreme (10.78), Jersey Giant (10.72), NJ 953 (10.46), and Apollo (10.00). The best response to drip irrigation, with reference to this characteristic, was noted in Jersey Deluxe cultivar. Drip irrigation increased the number of green spears per plant in this cultivar by more than six spears (i.e., by 123%). Hartmann [9], Paschold et al. [11], Rolbiecki and Rolbiecki [2], and Rolbiecki [14] also obtained an increase in the number of asparagus spears due to drip irrigation.

**Table 12.** Number of the asparagus green spears (pcs) as dependent on cultivar and irrigation.


2007: LSD0.05 for: Irrigation = 0.747; Cultivars = 2.116; Interaction: Cultivars/Irrigation = 2.993; Irrigation/Cultivars = 1.732. 2008: LSD0.05 for: Irrigation = 0.824; Cultivars = 1.811; Interaction: Cultivars/Irrigation = 2.459; Irrigation/Cultivars = 1.562. 2007–2008: LSD0.05 for: Irrigation = 0.372; Cultivars = 1.666; Interaction: Cultivars/Irrigation = 2.356; Irrigation/Cultivars = 1.340.
