Woody Ornamental Production


How can I maximize the amount of irrigation that actually gets used by plants?

Future Crops: The following factors should be considered prior to potting a new crop.

Substrate ● Polymer gels ●  Container Geometry Sub-IrrigationLarge Container Options Container Size

Existing Crops:  What about crops that are already growing at your nursery?

Shade ●  Antitranspirants ●  Water-collecting Basins ●  Soil Surfactants ●  Spacing

Future Crops

Substrates. Test your current substrate. Ideal physical property values initially are: 22 to 26% aeration, 45 to 50% waterholding capacity (OD) and 67 to 75% total porosity.

Polymer gels have been shown to have limited benefit within the range of plant water uptake, especially long term.

Containers with wider widths than heights (squat) require less overhead sprinkler irrigation than standard 1:1 (width:height) containers.

Containers with elevated drain holes (EFC ® ) also require less overhead sprinkler irrigation that standard containers.

Sub-irrigation incorporation of the overhead irrigation reduces the amount required down to a minimum of 0.18” daily, but has limitations based on substrate, and container and plant size.

Large container options for larger shrubs and trees, greater efficiency of irrigation (growth per volume irrigation) is achieved with porous bottom containers compared to common plastic containers.

Container size. Direct transplanting into market containers requires less irrigation generally than production in multiple transplanting steps.

>> Substrate. 

Substrates consist of solids (such as sand, pine bark, peat fibers, coir, perlite, etc.), and pore spaces, between or within solids, that are filled with either water or air. A typical landscape container substrate has about 55 to 75% pore space by volume, and the rest is solids. The best substrates have 67 to 75% pore space.

There are three main classes of pores found in substrates:

Air-filled macropores - these are pores which are filled with air after a pot is saturated with water and allowed to drain. The percentage of air-filled macropores is called the % Air Porosity.

Water-filled micropores - these are pores/holes so small that plant roots can’t extract water from them. A good example is the water held in moist-looking chunks of bark. The portion of these pores is called % Unavailable Water.

Mid-range pores hold water against gravity after the pot is saturated and drained, but this water is still available to be absorbed by plant roots. Mid-range pores account for the substrate’s % Plant Available Water.

The combined percentage of both mid-range pores and micropores in a substrate is referred to as the %Water Holding Capacity. An ideal outdoor container substrate consists of 67 to 75% pore space by volume, of which 22 to 25% is air-filled macropores, or % Air Porosity. The rest of the substrate’s pores consist of mid-range pores (% plant available water) and micropores (% Unavailable Water).

As the substrate settles over time, the % Air Porosity decreases a bit. As long as air porosity stays above 16 to 18%, plants will maintain good root development, even with normal summer rains. If your substrate has a % Air Porosity greater than 28%, you should strongly consider changing the ratio and/or size of solid components in your mix to increase the % Water Holding Capacity.

Determining substrate physical properties. (1.6mb PDF)

>> Polymer Gels.

Polymer gels have been studied since the late 1980s. In only one instance have they shown statistically significant positive effects in container substrates. This was a study by Fred Davies at Texas A&M University, which looked at desert roses grown on a very restricted irrigation regime.

There has been very little research looking at polymer gels recently. In the early 1990's we found that the polymer gel Aquasorb C gave up about half its water between saturation and the point of permanent plant wilt. In the normal range of container substrate water availability though, only 10 - 15% of the gel's water was available.

Later studies found that after dehydration of the gel-incorporated substrate, positive ions, especially calcium and iron, migrate into the gel and bind with the gel matrix, severely limiting re-hydration to as much as 1/100th its original water holding capacity. Based on this research, incorporating polymer gels is not a good strategy for maximizing water use, especially in Florida's calcium and iron laced water.

>> Container Geometry.  

The shape or design of the growing container has a big effect on the water holding capacity, regardless of the type of substrate used. In a series of experiments we conducted from 1997-2001, several commercially available container systems were evaluated for their ability to decrease the irrigation application rates needed for woody shrub production. Some of these systems would be useful during water shortages and to increase overhead irrigation efficiency anytime.

One shape used common 8" azalea pots, which are wider than they are tall, so the width: height ratio is greater than 1.0. In contrast, a standard pot is as wide as it is tall, with a width:height ratio equal to 1.0. Over the 3 years of the experiment, these "squat pots" consistently  produced Viburnum odoratissimum plants of the  same size and quality - or better - using only ½ (0.3") the daily irrigation volume of plants growing in traditional pots. During an average rainfall year, similar quality was achieved with 0.18” daily. The surface area of a squat pot is 60% larger than a standard container, so more of the overhead irrigation falls inside the pots. Squat pots are also able to hold more water in the same volume of substrate compared to a standard pot, due to its shorter height and wider diameter. Even greater water holding capacity would be achieved if there was no center drain hole in the pot bottom.    

The other container shape that did surprisingly well was the EDH, or elevated drain hole container. These containers have drain holes that were elevated 1" above the bottom of the pot. They can be found commercially as "EFC" containers. These containers also produced viburnum that consistently matched the size and quality of the control plants receiving 0.6" daily irrigation, but at ½ the application rate. The reason the EDH pots work is because the bottom 1" of macropore space becomes filled with water; this is equal to 28% of the average volume of water used each day by a plant that is 3/4 marketable size during the summer, given a 23% air porosity of the substrate. The elevated drain hole also effectively lowers the substrate height, further increasing the water holding capacity.

>> Sub-irrigation.

Several container systems performed very well at increasing the application efficiency of overhead sprinkler irrigation. By increasing the efficiency, lower volumes of water were required to produce the same size plant produced on common porous ground cloth in either the same time period or a shorter time frame. All of these successful systems had two things in common, they captured water falling between containers and channeled it to container drain holes for uptake; and they survived intact for the duration of the season or beyond.

Water Collectors

The Water Collectors shown below are made from recycled 64 oz pop bottles. The collectors are white, and are produced in pairs. They are designed for 12" spacings in greenhouses.


Unfortunately, Water Collectors don't work outdoors. They reflect sunlight that severely stresses plants, resulting in stunted leaves and shoots. They deteriorate and become brittle in sunlight. After about 4 months, the collectors started to crack, becoming of limited water collecting value.

Water Saving Trays.

This product, produced by Landmark Plastics, was designed for greenhouse production of 6-inch containers. We evaluated this using common #1 nursery black plastic containers and a general substrate of 7 pine bark fines: 3 Florida sedge peat: 1 coarse sand in 1999. Three containers were place within each 6 cell tray and plants were growth in full sun. Four species were include in the experiment ( Viburnum odoratissimum (sweet viburnum), Ligustrum sinense `Variegatum' (variegated sinense), Rhododendron sp. `Mrs.G.G. Gerbing'(Mrs. G.G. Gerbing azalea) and Juniperus chinensis `Parsonii' (Parsonii juniper).

After 7 months, plants in the trays were significantly larger than the overhead irrigated controls on pervious ground cloth or plants irrigated using 1 microirrigation spray stake per container. With both the ligustrum and azaleas, the same volume of water was applied overhead as with spray stakes to obtain marketable size. Overall use of Water Saver Trays and 0.44” water daily accelerated growth of all plants, reaching marketable size sooner than control plants irrigated at 0.6” daily. This indicates irrigation depths could have been further reduced to maintain similar growth to control plants. For most of the production period, standing water was in the bottom of the trays through at least mid-afternoon. There was no incidence of disease.

These trays worked very well and helped the plant remain upright under windy conditions. By placed together, they formed a continuous surface so that all overhead irrigation applied was available to the plants that was not lost through overflow holes in the trays. At their spacing, the containers only occupied 33% of the irrigated area, but received water from 100% of their designated area. Unfortunately, these trays were not designed for nursery containers, which barely fit. Spacing was also not optimum for woody landscape plants. The benefit of capturing 67% more of overhead irrigated water for a plant is advantageous and may be economically feasible during drought conditions. In greenhouse production use of such tray should be considered.

Greenhouse Bench Trays. These 3 x 5 ft by 1 inch deep trays were manufactured for leachate containment in greenhouse production. For outdoor use, we add drain holes 3/8 inch up from the bottom in the side walls. Viburnum odoratissimum and Ligustrum japonica were grown during the spring to fall of 2001 and spaced on 10” centers. Plants irrigated at 0.15” daily outgrew control plants irrigated at 0.6” daily, reaching marketable size 4 to 6 weeks earlier than control plants. Containers stood in shallow water most of the time for the first ¾ of the production period, but developed no root diseases. Advantages of these trays are the same as the water saver trays above. In addition, their larger, non-structured size was advantageous for any container size or spacing and much easier to work with.

Outdoor capillary mats. Also in 2001 we evaluated two prototypes of a bi-layer capillary mat. The bottom layer held and relocated the water as it was taken up by a container, while the top layer acted as insulation, greatly reducing evaporation between containers while aiding conduction of water into the center drain hole under a container. We found the model of 1 gal container was not critical as long as there was at least a center drain hole. The more Canadian peat incorporated into the substrate, the better it was able to wick water into a container and slightly increase the growth rate. However a common 7:3:1 (pine bark fines: Fla. Sedge peat: sand) substrate worked well also. Plants were grown faster with 0.25” daily than similar control plants on ground cloth at 0.6” daily. Because the mat is covered with a ground cloth type material, it can be walked on with deterioration of water holding capacity. The trick to successful use of this system was to supply about 0.15” of irrigation initially to re-establish any broken water columns with the soil substrate in a container, then apply a second, lower volume later to re-fill the lower layer as a water buffer. This was more important as plants approached ¾ marketable size.

Since this experiment, the better of the prototypes was commercialize in 2004 and is being sold as Aquamat. It has been used at one commercial landscape nursery in the Tampa area since the summer of 2006 with excellent accolades from the owner. Installation of this capillary mat should be considered as a long term investment with a life span greater than 5 years. Similar benefits could be achieved in greenhouse production.

A critical factor in the success of any subirrigation system, with or without overhead irrigation, is the ability of a substrate to pull water up into the container relative to the depth of the root ball. Substrates with good physical properties would generally be successful in 1 gal containers with liners several weeks in the pot with root growth into the substrate. However these same substrates in 3 gal containers would not likely be able to lift water high enough to be the sole water source for liners for about 3 to 4 months, depending on how rapidly roots growth towards the bottom of the container. In contrast transplanting a quart or 1 gal into a 3 gal container would put the roots within reach of capillary water very soon if not immediately after transplanting.

With any subirrigation system, substrate and roots in the bottom of a container will be near 100% water saturated most or all the time. The Aquamat would be the least saturated. In several years of working with these systems, and the experiences of Ticknor and his similar sand beds at the North Willamette Research Station in Aurora , Oregon , root diseases have not been a problem. Woody species associated with acceptability to root rots, such as junipers and azaleas have grown exceptionally well with subirrigation. The key is to use a substrate with good aeration.

>> Large Container Options.

Research conducted in 2002 -2004 with a live oak clone found greater growth and more efficient use of irrigation with systems that incorporated porous material underneath the cylinder containing the substrate and tree. The most efficient systems were those with mimimum porosity of the cylinder wall to limit circling roots. The advantage of porous bottoms is roots are able to either penetrate porous fabric or grow over it into the ground, allowing the plant to absorb additional water and nutrients. An added benefit is the plant is better anchored and in many cases does not require cabling. However, it is difficult to lay such plants down in preparation for a hurricane and more difficult to return upright unless well rooted into the substrate.

>> Container Geometry.  

The size of container used can also help growers make more efficient use of the water that is applied. In the early 1990's, many nurseries were potting liners into 1 gallon pots, and later upcanning to 3 gallons for market. Ideally, most liners should be potted directly into the size container they are to be marketed in, or at least into the largest size possible. Using a larger pot provides the plants with a larger soil volume, and therefore, a larger water reserve. Under overhead sprinkler irrigation, the larger surface area collects more irrigation, thus required application depths are lowered. Little irrigation is shielded from the surface of a container until a plant's canopy extends over the container lip.

An experiment MREC conducted at a commercial tree farm in the mid-1990's revealed that potting live oak seedlings directly into 15 gallon containers shaved 8 months off production time compared to upcanning from 1 or 3 gal containers into 7 gal, then the 15 gal containers. Starting seedlings in 15 gallon containers isn’t practical or recommended, however. For seedlings, pot them into 1 gallon containers initially, and upcan the best trees within 6 months, or when the root ball holds together enough to transplant. Discard any runts at this time. For cuttings, start with the largest pot feasible - at least a 3 gal unless your market is 1 gal plants.

Existing Crops

Shade. Put up temporary shade or move plants into the shade of existing larger plants or trees.

Antitranspirants offer only temporary relieve and are more suited for landscape installation than production.

Soil wetting agents. Soil surfactants work and restore the water holding capacity of hydrophobic pockets of in substrates. Their use is strongly recommended where root balls have shrunk away from container walls.

Handwater. Spot irrigate drier areas by hand.

Irrigation. Alternate day irrigation is ok during a substantial portion of the production period.

Sub-irrigation. Temporary subirrigation systems would be labor intense to install, but offer the greatest benefit for landscape plants 2/3 grown or larger.

Spacing. Maintain the minimum spacing necessary for commercial quality during production and especially when plants are pulled for sale. Ideally canopies should be at least touching, while 25 to 50% overlap is better.

>> Shade.

Placing plants under 30 to 45% shade will extend the longevity of water held in a container. Shade reduces transpiration rates of plants and evaporation from the substrate. Both transpiration and evaporation are driven by energy input from the environment and down gradients. Solar radiation accounts for around 90% of the water loss by transpiration. Gradient differences in relative humidity inside a leaf (usually >99.7%) and the air surrounding the leaf (or water in a substrate and air above), and air movement (wind speed) account for the remaining portion. Thus reducing the amount of solar radiation hitting a plant will have a larger impact on reducing transpiration than raising the humidity of the air around a plant, such as by short durations of overhead irrigation. Shading will also reduce whole plant photosynthesis, but not proportionally as much because most sun leaves reach light saturation at around 75% of full sunlight. While reduced photosynthesis could reduce growth, the higher plant water status of shaded plants usually compensates with larger cells, so growth is minimally affected. Simply put, plants grown under light shade will generally be as big or bigger than plants grown in full sun, however shade-grown plants require less water.

>> Antitranspirants.

Another potential tool for reducing water requirements for existing crops is the use of antitranspirants. Antitranspirants have be available since the early 1970's, and like polymer gels, they created a flurry of research early on. The allure of antitranspirants is that they can significantly reduce plant transpiration. By reducing transpiration, they reduce the need for plant uptake of water from the soil. Hence, they have been touted as an excellent tool for transplanting or other conditions where soil water availability is limited.

However, there are some important drawbacks to antitranspirants. First, the film-forming antitranspirants tend to crack and become discontinuous within a couple of weeks or less after application. They are effective only on the foliage initially sprayed. So, they would need to be re-applied at least monthly, if not more often.

Secondly, because CO2 molecules are 1.6 x larger than water molecules, antitranspirant films actually block photosynthesis more than they block transpiration. While antitranspirants become discontinuous relatively quick, they tend to clog stomata for much longer, prolonging reduced photosynthesis.

From a transplant perspective, where the plant has enough stored carbohydrates for root growth, reduced transpiration may be good and reduced photosynthesis may be tolerated. But when growing plants for market, reduced photosynthesis equals reduced growth. Unlike the cyclic high & low photosynthetic rates which could occur with alternate day irrigation, photosynthesis would remain low throughout production with regular antitranspirant application. So, applying antitranspirants is not a good strategy for dealing with the severe water restrictions.

Since 2005, Valent has been evaluating an abscisic acid ( ABA ) formulation that has shown positive results in reducing transpiration on a wide array of annual and woody crops. ABA is a natural plant growth regulator that induces stomata closure in response to water stress. As with film forming antitranspirants, inducing stomata closure reduces photosynthesis more than transpiration. However in personal communications, the effect of this ABA formation is relatively short term. As of May 2007, the product can be obtained on an experimental basis by nursery and greenhouse growers. Continual use would likely significantly reduce growth. However occasional use could be beneficial.

>> Soil Wetting Agents.

Soil surfactants are products that reduce the surface tension of water in the substrate - just as surfactants or wetting agents due in foliar sprays. Without the surfactant, water beads up on leaves. With the surfactant, water spreads out and covers the leaf.

In substrates, especially older plants, the substrate will dry out in pockets, and become very resistant to re-wetting. Dry pockets are especially likely during late spring/early summer, when rainfall is scarce and plant water demand is high. These dry pockets lower the water holding capacity of the container; the problem is compounded when irrigation is limited due to water restrictions, and plants use the majority of the available water in the pot.

Other than a good soaking with frequent thunderstorms, the only other solution is to treat the containers with a soil surfactant. A surfactant can be either added to the substrate prior to potting, or applied after the fact. In either case, it will enhance water absorption by the substrate, even if the root ball has pulled away from the pot.

>> Hand Watering.

Hand watering is an excellent way to conserve irrigation if used correctly. It is most appropriate around the edges of beds or wherever a substantial portion of a plant is exposed to light and air movement. The extra exposure of the border plants causes them to dry quicker than plants surrounded by, and mutually shaded by other plants. Hand watering these plants can often gain hours or even another day before the entire block requires irrigation. To be efficient, the water should be placed directly in the container, not over the top of the plant. It should also be done in short pulses, not filling the space between the top of the substrate and container lip, then moving on. The amount applied should be based on knowledge of the water holding capacity of the substrate and the size of the container. General recommendations would be no more than a pint for 1 gal or 1 to 1.5 quarts for 3 gal.

>> Alternate Day Irrigation.

A substrate with close to ideal physical properties can support a plant with alternate day irrigation throughout much of the production period. This window spans the time beginning after liners have initiated good root growth into the substrate until the roots have explored the entire root volume. For Ligustrum japonica and Viburnum odoratissimum , we have found this to begin 3 to 5 weeks after potting and to extend to around when plant canopies are 2/3 grown, roughly about 7 to 9 months after potting in 3 gal containers at the Apopka REC. During this period, roots are constantly growing deeper, expanding into virgin substrate and untapped sources of water. Daily evapotranspiration during this time is less than the 30% plant available water, especially in early in the period. Thus over a two day period, plants to do exceed 60% cumulative plant available water.

>> Subirrigation Basins.

Subirrigation basins can include anything that captures and holds water that falls between containers or drains out of containers. As plants grow and roots descend toward the bottom of a container, subirrigation is the best way to supply sufficient water for optimum growth. In the Future Crops section, there was discussion of incorporating Water Saver Trays ® , large greenhouse trays, and the Aquamat ® capillary mat into production areas before placement of a new crop. The trays could also be used with exiting crops, but only the large greenhouse trays could be used for containers larger than 1 gal. Their incorporation would be labor intensive and the availability of these trays on a large scale is questionable.

Any other systems with the same characteristics would work just as well, provided that:

- water that falls between the containers is collected and made available to the plants.

- the water level cannot exceed about ½ inch above the bottom of the container.

Though labor intensive, transferring large plants to shallow basins, as simple as very shallow depressions in the ground covered with thick plastic; or ½” pvc pipe laid as a border, covered with plastic on level ground, would be an excellent way to maintain and grow market size plant during drought. This would require the site to be reasonably level, although basin size and shape could be adjusted to accommodate some slope. Incorporation of the pvc pipe would be a temporary solution which could be easily dis-assembled if not glued and removed when rainfall was again available. Only one end would need to be removed to prevent impoundment of rain water.


>> Spacing 

Of all the factors listed, spacing is one of the most important factors in plant water use management. Therefore, spacing is one of the more critical aspects to keep in mind when growing crops with alternate day irrigation.

This photo is an example of what not to do if you’re looking to save water. Ideally, new potted plants should be set pot-to-pot in an alternating, triangular configuration, re-spaced at least twice between potting and marketing to minimize the space between the canopies, and re-packed when more than 20 to 25% of the plants in a bed have been removed.

Realistically, most growers probably can set the plants pot-to-pot after potting and spread them out once before marketing. However under severe water restrictions, “desperate times call for desperate measures”. As plants are removed for sale, beds should be re-spaced as available and unnecessary sprinklers shut down. This will require more labor; however, the point may be reached where water becomes more critical than the labor cost.

The effects of spacing and canopy interactions were examined in 1994 using 3 gal. Full-size 1 gal plants were transplanted into 3 gal containers, which were then set in a square arrangement on 15-inch centers, so that plant canopies were initially 7 to 9 inches apart. Water use was recorded with suspension lysimeters daily.

This is a graph of the water use normalized by horizontal canopy area of these 3 gal ligustrum over time. For the first 70 days, water use per unit of horizontal canopy area decreased as the plants grew closer together. After 70 days, plant water use per unit of canopy was essential a flat line. This occurred about the point when the canopies grew together and formed a closed canopy. Plant water use continued to increase as the plants became larger and canopies intermingled. However, water use per unit canopy surface area remained the same. If we had randomly removed 25 to 30% of the plants from this area, plant water use would have almost doubled!

This near doubling would have resulted from changes in the portion of the canopy that is transpiring. When plants are close enough together that the canopies touch, termed 100% canopy closure , nearly all the measurable transpiration occurs from leaves in the uppermost 40% of the canopy, while the sides of the plants are shaded. In contrast, isolated plants (0% canopy closure) are exposed on all sides to solar radiation and relative humidity gradients, thus transpiration occurs over most of the entire leaf surface area of the plant. In a study with 7 gal Viburnum odoratissimum in 1997, transpiration was similar between isolated plants and plants with up to 67% canopy closure (horizontal projected canopy areas totaled 67% around the measured plants). When repeated in 2003 with 1, 3 and 7 gal viburnum, the same response occurred, independent of plant size or height. Thus once the canopy area of a block of plants declines to a maximum of around 70%, transpiration of the remaining plants will nearly be double that of plants when the bed was at 100% canopy closure.

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