Woody Ornamental Production

 

How much water should be applied at each irrigation event?

This is a question that UF researchers have been trying to answer since the late 1980's.

 

The data shown here was gathered by Gary Knox at the North Florida Research and Education Center in Monticello. These values represent the ounces of water used per day by liners grown under shade.

From June to Oct in 1994, daily water use ranged from a low of 1.7 ounces to a high of 5.4 ounces. For plants in 1 gal containers, these volumes would equal 0.03 to 0.1 inches per day. From spring to summer, water use increased considerably, from a low of 2 ounces per day to a high of 11.9 ounces per day. Translated for 1 gal containers, the range is 0.04 to 0.24 inches per day.

Remember, these plants were grown under shade, so these values shouldn't be used to set application rates for crops grown in full sun. Still, this table illustrates some important points:

? First, irrigation application rates should be adjusted to reflect seasonal differences in water consumption.

? Secondly, by knowing the appropriate application rate and placing plants with similar water needs together in the same irrigation zone, growers can apply only the volume of water that is needed for each crop - further conserving water.

This table contains similar data to that of Knox, but collected at the former Central Florida Research and Education Center , in Sanford . The values represent the mean daily water use of 1 and 3 gal plants, which averaged being of marketable size, continuously, for the entire years of 1995 and 1996.

For marketable-size plants, average daily water use ranged from 9 to 15 oz for 1 gal plants, and 24 to 40 oz for 3 gal plants. These are equivalent to 0.18" per day for a 1 gal Rhaphiolepis indica ( Indian hawthorn), up to 0.3" per day for a 1 gal Viburnum odoratissimum (sweet viburnum). In a 3 gal container, a marketable-size Indian hawthorn averaged 0.21" per day, and a sweet viburnum averaged 0.35" per day. More importantly though, these volumes are equal to 24 to 41% of the Plant Available Water in a 1 gal pot, and 21 to 34% of Plant Available Water in a 3 gal pot.

Note that these values are the average over 2 calendar years. Maximum and minimum daily values vary from these averages by as much as 50%. Thus, on a hot, clear day in May, a market size 1-gal sweet viburnum could use as much as 55% of the plant available water in the container.

This is critical in that depending on species, when between 75 to 85% of plant available water has been transpired, the plant becomes permanently wilted, and growth comes to a halt. This is true whether you're talking about a liner in a 3 gal pot or a marketable plant in a 3 gal pot. The volume of water available to a plant increases as the root ball increases, up to the point where roots fill the container. Thus, another advantage of starting plants in larger containers is that it will take longer for a plant to exploit the full volume of available water. In studies since the turn of the century where we have tracked root volume growth concurrently with shoot growth of plants in 3 gal containers, the trend among Ligustrum japonica (green ligustrum), sweet viburnum and Indian hawthorn has been for roots to fill out a container when the shoots are around 2/3 marketable size, as defined by the Florida Grades and Standards for Nursery Crops. At the research station in Apopka, this has been around 6 or 8 months after potting. However the rate will vary depending on substrate physical properties. Low aeration will decelerate root growth into the substrate, while excessive aeration will accelerate it.

This figure depicts water use of marketable-size 3 and 7 gal sweet viburnum plants, in terms of the percent of available water used over time, measured in December 1995. Containers were saturated at day zero, and no irrigation was applied thereafter .

Interestingly, there was little difference between 3 and 7 gal plants. Permanent wilt occurred after 4 days without irrigation, at which time the plants had used more than 70% of the available water. Keep in mind that this was in December; in late spring to early fall, a sweet viburnum ¾ marketable size or larger could reach permanent wilt during the second day without additional irrigation.

For Indian hawthorn, a more drought-tolerant plant, it would probably take 3 to 4 days for permanent wilt to occur in the summer. Remember, permanent wilt means the end of plant growth.

In 1994, an experiment was conducted at the Sanford Research Station to look at the concept of Managed Allowable Deficits, or MADs. The goal was to conserve water by withholding irrigation until plants had used a certain percentage of the available water. MAD refers to the percentage of available water used between irrigation events. A 20% MAD means plants would be irrigated when at least 20% of available water had been loss. An 80% MAD means the plants had to loss at least 80% of the available water before being irrigated again (plants would be permanently wilted at this level of MAD).

For the 3 shrub species tested, the less frequently the plants were irrigated, the smaller they were after six months. The plants in this picture received 0, 20%, 40%, 60%, and 80% MAD, from left to right. A MAD level of 0 means it served as the control treatment, reciving 0.75" daily. For Indian hawthorn, allowing a 40% deficit still produced commercially acceptable plants after 6 months. A 60% deficit resulted in plants that were thinner and less branched, but still not too bad.

 

Ligustrum japonica showed the most clear-cut reductions in size with decreasing frequency. Plants grown with a 60% MAD had about half the canopy volume of control plants after 6 months. A 30% MAD was estimated to be the maximum deficit for ligustrum that still resulted in commercially acceptable plants after 6 months. While no plants were lost during this experiment, there was some dieback among plants receiving the 80% MAD treatment between irrigations in July. An alternate day irrigation schedule would likely correspond to about a 60 to 80% MAD for medium to high irrigation-requiring species as they near marketable size.

 

At the end, we calculated a canopy volume (growth index) for each plant by multiplying the greatest width times the width perpendicular to that times the average canopy height, essentially assuming a rectangular box shape. When these canopy volumes were plotted against the cumulative water use of the plants, we found a direct linear relationship between the two. An even tighter relationship was found between shoot mass and cumulative water use. In simple terms, the more a plant transpired, the bigger the canopy. Thus anything that reduces transpiration, given all other conditions are similar, will directly and linearly decrease plant growth and size.

Following the same logic, for a plant to reach a certain size, there is a certain volume of water it must transpire. This occurs because of the tight linkage between transpiration and photosynthesis. If transpiration is restricted due to closure of stomata or plugging of stomata from antitranspirant films, photosynthesis will be restricted accordingly. Thus, reducing transpiration extends the time required for a plant to obtain marketable size.

 

Actual water use

Average water use over seasons and values for market size plants are good for planning purposes and maintaining market-size plants during water restrictions. However the majority of plants at a given time in a nursery or greenhouse operation are in the production stage. During this period, as liners grow from a few leaves to market size, producers have a tremendous opportunity to manipulate application rates, conserving water in the early stages that can then be applied in the latter stages as plants are being finished off for market or held until sold. For this manipulation to be successful requires advance planning and placing crops in irrigation zones by delineated by plant size and relative irrigation demand.

In the mid-1990's we surveyed FNGA members as to their opinions on the relative irrigation requirement of the majority of ornamental landscape plants grown in Florida . These opinions were compiled and published in an extension publication . While local experience may contradict some of the ranking in this publication, it is offered as a starting point for those species you may not have experience with.

Detailed information is available for daily water use of 2 species of woody landscape plants. Work is in progress to add a third species and several species of greenhouse grown foliage plants in 6 in containers. These will be added as the data becomes available. This research has been conducted at MREC since 2001, the first major drought of this century to impact nursery irrigation. Since similar droughts occurred in 1998 and 1992, the industry should recognize that droughts have been a near-regular occurrence the past couple of decades and anticipate them.

The detailed information for the woody landscape plants provided here is on a calendar day basis, beginning at transplanting of liners into containers. Actual daily amounts given could be different if the start of your production is much sooner or later than the beginning of each experiment. Actual amounts also would vary if climatic conditions are much different than those that occurred when the data was collected. However differences in amounts relative to plant size are substantial enough that the data could be used for reference for shifting irrigation among plant sizes during irrigation restrictions, and as a rough guideline under normal production.

To account for both climatic and year-to-year variation, we have developed mathematical models to predict plant water use based on daily reference evapotranspiration (ETo), plant size and container spacing. Two models evaluated over the 2005-2006 production season adequately predicted irrigation requirements and produced marketable crops of Ligustrum japonica within a reasonable time with less irrigation than conservative manually controlled rates. These and additional models are under evaluation and are planned to be made available through FAWN in 2009.

In lieu of models for predict to precision irrigation rates, the graphs presented below should allow growers adjust their irrigation rates to limit plant losses during severe irrigation restrictions. Data collection for both species began in March and was completed by early April each year. Plants were irrigated with overhead sprinklers after midnight as required. Irrigation volume varied night to night and was based on replacing water loss the previous day plus 0 to 15% extra to account for less than 100% irrigation uniformity, canopy shedding and to offset increases in plant mass that occurred with growth. All liners were transplanted in 3 gal containers using a 7:3:1 blend of pine bark fines: Florida sedge peat: course sand.

Ligustrum japonica

Ligustrum is considered to be a medium irrigation requiring species according to the survey ranking. When setting up the experiment, we also included 2 containers with plastic foliage to mimic the shading effect of a plant on the substrate surface. The amount of plastic foliage was increased proportional to growth of the ligustrum. In the figure, the mean evaporation from these 2 containers is given as the red circles. Plants were initially set at half a container diameter apart, 15 in on center, and remained so throughout data collection.

 

Evaporation remained relative constant from potting in late March until mid-Aug (day 225), when it began a gradual decline into early November (day 305). For the first 4 ½ months, simple evaporation from the substrate surface accounted for around 0.15 in daily. Evaporation declined as canopies began to interact/overlap and shade lower areas in early fall. Evaporation became relative constant for the rest of the production period at around 3 oz or 0.08 in per day. Maintaining plants with a closed canopy , recommended for existing plants achieves the same minimum evaporative loss shown here.

The variation in evaporation and in evapotranspiration (ET A , plant transpiration + substrate evaporation) seen in this graph is directly related to daily variations in sunlight, temperature, humidity and wind (ET o ). Strong dips were due to rainfall, with dips to the zero line indicating all day rain.

During first 30 days after transplanting, evaporation from substrate surface accounted for most ET A . The projected canopy areas of the liner leaves at this point covered 7 to 16% of a container's substrate surface. Through these first 30 days, 0.2" in daily would have been sufficient to maintain the liner. This is the about 7 oz per 3 gal container per day.

It is important that liners be irrigated nightly during the first 30 days. Early on liners have no roots into the container substrate and therefore are almost totally dependent on re-wetting of the liner root ball. We have found over the years that 30 days generally allows for initial (1 to 2") root growth into the container substrate. However we have also observed that tightly root-bound liners have required 7 to 10 weeks to develop roots in the substrate. Under severe water restrictions, only high quality liners should be used to obtain rapid growth into the container substrate.

After this initial 30 days, irrigation of 0.25" nightly or preferably 0.5" on alternate days would keep the liners moist & promoted growth for the next 75 days. Alternate day irrigation would be preferred at this point for 2 reasons. First evaporation from the substrate the day after irrigation is less than the day of irrigation . Drying of the substrate surface the day of irrigation results in a longer, more torturous path for water evaporation the second day. Hence there is a small net gain of water available to a plant with alternate day compared to daily irrigation. Second, though less important for small plants, canopies retain a certain amount of overhead irrigated water before it trickles/ drips down to the container substrate surface . The effect is the same as standing under a tree at the beginning of rain shower. After irrigation stops, the canopy still retains that volume of water acquired initially to wet the canopy. This then evaporates without reaching the root ball. Alternate day irrigation also results in one volume of water equal to canopy retention penetrating to the substrate.

With root and shoot growth, ET A increased slowly until a substantial shoot flush began the first of August (day 210). Up until this point, plant canopies were still generally within the projected cylinder of the container and 0.5" on alternate days would have been sufficient irrigation. As the flush grew and matured, daily water use increased to around 0.5" per day through to mid-December. Around mid-October roots had full encompassed the substrate volume. With cold weather from mid-December through mid-January, plant growth stopped and plant water use dropped in half. With warm (mid-70's) winter temperatures, water use returned to around 0.4" daily until early March 2002, when the first shoot flush of spring push plant water use to 0.5 to 0.6" daily and the plants to marketable at the end of the month.

Viburnum odoratissimum

Viburnum is consider to be a high irrigation requiring species, and is generally grown throughout Florida. The experimental setup was similar to the ligustrum except containers were spaced one container diameter apart, 20" on center. Additionally irrigation problems severely limited plant water use for about 2 weeks shortly after potting. Like the ligustrum measured 4 year earlier, most water loss from a container the first 45 days was from evaporation from the substrate. As before, evaporation was generally around 0.15" the first 7 months before declining to around 0.1" daily around the first of December. The decline in evaporation was slower for the viburnum because of the wider spacing. It took longer for the plants obtain canopy closure, thus longer to reduce substrate evaporation.

After an initial 30 days of nightly irrigation of 0.25", irrigation for the next 75 days or so could have been applied at 0.5" on alternate nights like the ligustrum. Despite irrigation problems at the beginning, ET A increased rapidly with a substantial shoot flush at the beginning of August (day 210), similar to the ligustrum. Through these first 5 months, plant canopies were still generally within the projected cylinder of the container. As the flush grew and matured, daily water use increased to around 0.35" per day until the next flush around late September (day 270). Unlike the ligustrum, roots did not fully encompassed the substrate volume until December (9 months) In mid-November, daily water use was up to 0.5" per day. As 4 years earlier, with cold weather and the decline in ETo from mid-December through mid-January, plant growth stopped and plant water use dropped to about 2/3. During these cool periods, irrigation of 0.6" on alternate days would have been sufficient. Increases in ETo in late January with warming temperature and increases sunlight, and the spring flush in late February, drove plant water use from about 0.5" to nearly 1.0" per day of a 30 day period.

While plants would hopefully have been shipped before they reached such high daily ET A , it would not be uncommon to have such market size plants remaining for at least a short time. Given the high percentage of canopy shedding of marketable size plants and their high daily water loss, implementation of temporary subirrigation.

Trees

For tree water use, please access the following web page .

 

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