Capture of Sprinkler Irrigation by Container-Grown Nursery Crops

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Capture factor research sponsored by:

Background

Plants growing in containers can channel sprinkler irrigation water into the container that would otherwise fall un-intercepted between containers. This effect has significant implications for adjusting irrigation rates to accurately deliver required amounts of water to containers. The capture factor (CF) is used to describe the water-capturing ability of container-grown plants. In general terms:

CF is affected by several factors including plant species, plant size, leaf area, container size, and container spacing. For example, a plant with a CF = 1 (no effect) when small may exhibit a significantly larger CF when more fully grown (Fig. 1). Or, a plant may exhibit a high CF when spaced far apart but a low CF when spaced close together. The dynamic nature of CF presents challenges for CF monitoring/prediction in container nurseries that irrigate a wide range of plant species at various stages of production.

In this fact sheet we present results from research sponsored by the Southwest Florida Water Management District that describe how CF can be affected by plant species, plant size, and container size and spacing. We also provide an example calculation of how CF can be used to more efficiently irrigate container-grown nursery crops.

Research Methods

Irrigation test site

A 20 ft x 20 ft irrigation zone at the University of Florida, Gainesville, was used for CF testing (Fig. 2). Impact sprinkler heads (Rainbird® Maxijet 2045PJ-08, No. 8 nozzles - 3 gpm at a trajectory of 23 degrees) on 4-ft tall risers fitted with 30 psi pressure regulators were situated at each of the four corners of the irrigation zone. Spray patterns were adjusted to achieve a Christiansen’s Uniformity Coefficient (UC) 95% and a delivery rate of 0.56 inch/hour. See edis.ifas.ufl.edu/ae384 for more on irrigation uniformity.

Effect of plant species and container size on CF

Ornamental shrubs grown in trade #1 (6-7 inch diameter) and #3 (10-11 inch diameter) containers and representing seven different growth habits were surveyed for their capacity to capture sprinkler irrigation water. Plants were obtained from commercial nurseries and transported to Gainesville for testing. Plants chosen were of marketable or near-marketable size as larger-sized plants exhibit the greatest water-capturing effect. A list of the plant species tested is given in Table 1; photos of test plants are given in Fig. 3 (trade #1) and Fig. 4 (trade #3).

For CF tests, 10 plants of each species were placed on 2-ft centers in an equidistant, triangular pattern (Fig. 5). The wide spacing arrangement ensured that CF values would not be limited by competition from adjacent plants. Containers were placed in pails to collect leachate during irrigation tests. A plastic or aluminum foil skirt was used to form a barrier between container and pail to prevent irrigation water from directly entering leachate pail (Fig. 6).

For CF irrigation test runs, each container-pail assembly was weighed before and after irrigation. Three irrigation test runs were made for each plant species-container size giving a total of 30 measurements per plant species-container size. The difference in weight before and after irrigation was used to calculate the volume of water captured by the container with plant (V_p) based on the density of water (1 g= 1 cm³).

The amount of water that would be captured by containers without a plant was determined by placing empty collection pails in irrigation test area and applying the same amounts of irrigation water. The following equation was used to calculate the amount of water captured by containers without a plant (V_o):

Based on V_p and V_o, the capture factor CF was calculated for each species-container size:

Effect of plant size and container spacing on CF

For these tests, we selected three plant species from those surveyed in the previously described experiment:

1) Juniperus chinensis ‘Parsonii’ – Parsonii juniper

2) Raphiolepis indica – Indian hawthorn

3) Ligustrum japonicum – Waxleaf ligustrum.

Three sizes (small, medium, large) of each of the three plant species in both trade #1 and #3 containers were obtained from a commercial nursery. For each plant species-size, we measured CF at 3-4 container spacings ranging from close together to far apart. CF was measured using the methods described previously.

Results of CF testing

Effect of plant species and container size on CF

CF values for marketable-sized plants ranged from 1.2 to 4.2 depending upon container size and plant species (Table 1). CF values were greater for trade #3 than for trade #1 containers (2.9 vs. 2.1), which is attributed to the greater canopy volume per top area of container. The ratio of plant volume to container top area was 8 (1590 cm3 per 200 cm2 top area) for trade #1 and 11 (6390 cm3 per 560 cm2 top area) for trade #3 containers. Greater canopy volume per container top area implies relatively greater leaf structure for capturing irrigation water.

Plant species exhibited a wide range of CF values. Lowest CF values (1.2 and 1.6) were observed for the groundcover Agapanthus africanus which had low, arch-shaped leaves (Figs. 3 and 4). The downward shape of the leaves outside the container perimeter likely resulted in the relatively low CF values. The globose-shaped azalea (Rhododendron sp. ‘Red Ruffle’) also exhibited relatively low CFs (1.5 and 1.8). Three other small-leaved species, Buxus microphylla japonica, Ilex cornuta, and Ilex vomitoria, exhibited moderate CF values ranging from 1.7 to 2.9. Another two species exhibiting moderate CF values included the spreading vine Jasmine multiflorum and the broad-spreading Juniperus chinensis ‘Parsonii’. For J. multiflorum, branches tended to droop outside container perimeter so that even though plants were tall and wide, channeling was moderate. For J. chinensis, low plant height from broad-spreading habit appeared to limit ability of canopy to channel water into container.

Highest CF values (2.3-4.2) were observed for several woody shrubs with upright-spreading growth habit: Ligustrum japonicum, Podocarpus spp., and Viburnum odoratissiumum. A fourth species, Raphiolepis indica, also exhibited high CF (2.5 and 3.9). Although R. indica’s habit is spreading, branches within the canopy are stiff and upright resulting in high CF despite relatively low height to width ratio.

Based on the range of CF values observed, we can categorize plant species into three groups:

a) Low capturing ability (CF <2)

b) Moderate capturing ability (CF 2-3)

c) High capturing ability (CF >3).

Plants classified as ground cover or globose exhibited low CFs, semi-broad spreading, spreading vine, and broad spreading habits moderate CFs, and upright and spreading habits to have moderate to high CFs. Large-leafed species and species with greater height to width ratios tended to have higher CFs than plant species with small leaves and more spreading growth.

Effect of plant size and container spacing on CF

CF increased as plant size of each plant species increased (Table 2). It should be noted that CF results in Table 2 were observed at the widest spacing tested and, therefore, represent the maximum CF observed for each plant size.

For each plant species and size, measured CF decreased when containers were spaced closed together (Table 3 and 4). The reason for this is that when containers are spaced close together, there is a limited amount of water that can be captured outside each container. Table 5 and 6 shows how theoretical maximum CF values decrease as containers become more closely spaced. Maximum values in Table 4 are based on the principle that the irrigation water captured is limited to the area allotted each container divided by the container top area:

CF_max = A_t/A_c

where A_t = total area allotted each container (cm²) and A_c = top area of container (cm²). The calculation depends on the container placement pattern. For square pattern,

A_t = (container diameter + spacing between adjacent containers)²,

and for equidistant, triangular spacing,

A_t = (container diameter + spacing between adjacent containers)² * 0.866.

The calculation for top area of the container is:

A_c= π * r² = π * (container diameter/2)².

We can see from these results that CF is a function of plant species, plant and container size but may be limited by close container spacings. For example, large #1 Ligustrum japonicum had a CF of 2.9 when spaced 6 inches apart but only 1.8 when spaced 1.5 inches apart. In this case CF was limited at the 1.5 inch spacing where theoretical CF_max was 1.5. Similarly, large #3 L. japonicum had a CF of 4.0 when spaced 15 inch apart but only 2.9 when spaced 6 inches apart. In this case theoretical CF_max was 2.7 at the closer 6 inch spacing.

Using CF to improve irrigation efficiency

CF is important for adjusting irrigation rates to deliver a prescribed amount of water to containers. For example, if the container water deficit was determined to be 0.5 inch, a grower NOT considering CF would apply 0.5 inches of irrigation water. However, if it was determined that CF=2, the grower would know that 0.5 inches of irrigation would supply 1.0 inch or twice the desired irrigation amount to containers. Knowing CF, the grower could irrigate with 0.25 inches to deliver 0.5 inches of water to the containers. The general equation for adjusting irrigation is:

Irrigation (inch) = Container water deficit (inch) ÷ CF

Example calculation

If container water deficit = 0.6 inches and CF = 1.7, then

Irrigation = 0.6 inch ÷ 1.7 = 0.35 inch

The above example demonstrates the potential value of using CF. Using CF to adjust irrigation rates in this example saved 0.25 inch of irrigation water (0.6 vs. 0.35 inch).

Even if the container water deficit is not known, CF can be used to group plants with similar irrigation requirements. For example, all factors being equal, i.e., same plant size, container size and spacing, growers could group plant species based on CF (Table 1). A working knowledge of CF, including the ability to measure CF in the field, is paramount if sprinkler irrigation water is to be applied efficiently in container nurseries.

Fig. 1. Capture factor (CF) values >1 indicate that the plant canopy is channeling water into the container that would otherwise fall between containers.

Fig. 2. Site used for CF testing at the University of Florida. Side curtains were used to protect irrigation area from winds during tests.

Agapanthus africanus

Rhododendron x ‘Red Ruffle’

Juniperus chinensis ‘Parsonii’

Jasminum multiflorum

Raphiolepis indica

Ilex vomitoria ‘Schellings Dwarf’

Ligustrum japonicum

Buxus microphylla japonica

Ilex cornuta Burfordii ‘Nana’

Viburnum odoratissimum

Fig. 3. Photos of plant species grown in trade #1 containers used in CF survey testing.

Agapanthus africanus

Rhododendron x ‘Red Ruffle’

Juniperus chinensis ‘Parsonii’

Jasminum multiflorum

Raphiolepis indica

Ilex vomitoria ‘Schellings Dwarf’

Ligustrum japonicum

Buxus microphylla japonica

Ilex cornuta Burfordii ‘Nana’

Viburnum odoratissimum

Fig. 4. Photos of plant species grown in trade #3 containers used in CF survey testing.

Fig. 5. For plant species testing, containers were placed on 2-ft centers in a triangular, equidistant pattern.

Fig. 6. Plastic or aluminum foil skirts were placed between collection pail and container to prevent irrigation water from directly entering collection pail.

Table 1. Plant species tested for their capacity to capture overhead irrigation water. The capture factor (CF) is the relative volume of water intercepted by the container with a plant in relation to what would be intercepted without a plant. Capture factor values were measured at wide container spacings and therefore represent maximum values.

¹GC=ground cover, US=upright spreading, SBS=semi broad spreading, SV=spreading vine, BS=broad spreading, S=spreading, GL=globose

²Container diameter was 6 inches for trade #1 and 10.5 inches for trade #3 containers

Table 2. Effect of plant size on measured capture factor (CF) for three woody ornamental species in either trade #1 or #3 containers. The capture factor is the volume of irrigation water intercepted by the container with a plant in it relative to what would be intercepted without the plant. Capture factor may be less if limited by container spacing (see Tables 3 and 4).

¹Trade #1 container diameter = 6.4 inches; trade #3 container diameter = 10 inches

²Widest of four spacing tested; spacing is distant between adjacent containers in equidistant, triangular pattern

Table 3. Measured CF values for trade #1 (6.4-inches diameter) containers at various container spacings. CF values are the average of 30 measurements.

¹Equidistant, triangular pattern

²Numbers in parentheses are average plant height and plant width in inches

Table 4. Measured CF values for trade #3 (10-inches diameter) containers at various container spacings. CF values are the average of 30 measurements.

¹Equidistant, triangular pattern

²Numbers in parentheses are average plant height and plant width in inches

Table 5. Theoretical maximum CF values for various spacing arrangements for trade #1 (6-inch diameter) containers.

Table 6. Theoretical maximum CF values for various spacing arrangements for trade #3 (10.5-inch diameter) containers.

The information contained herein has not been subjected to scientific peer review and is not a recommendation of UF/IFAS.