Irrigation By Bodie V. Pennisi and Marc van Iersel

Reduction of pesticide and fertilizer runoff from greenhouses has been a repeated theme in the argument for a sound, protected environment. Many North American growers have taken the cue from their European counterparts by implementing subirrigation systems that can decrease or virtually eliminate chemical runoff from their operations. It would seem like a logical move in the face of future increased environmental protection laws. But the answer is not quite that simple.

As a grower, you must deal not only with environmental pressures but pressure from your customers for the best quality plant material.

Take for example, poinsettia production. It's no secret that a poinsettia crop requires an intensive fertilization program. Growing poinsettias with a subirrigation system can give your crop the nutrients it needs while reducing chemical runoff, which threatens the environment.

However, some previous research has suggested that subirrigated poinsettias may drop more leaves after leaving the greenhouse than plants irrigated by other means. Leaching is often touted as the answer to this problem. Leaching, however, adds an extra step to the production cycle, which means more time in the greenhouse, more labor and higher production costs.

So just how do you protect the environment, keep your costs down and produce a high-quality, long-lasting poinsettia crop for the holiday season?


Research at the University of Georgia has shown that subirrigation is a viable method for producing high-quality poinsettias. Here's how subirrigation works: benches (trays, really) or whole greenhouse sections are flooded with a fertilizer solution pumped from a holding tank. The fertilizer solution is picked up by the potting medium through a wicking action. After approximately 15 or 20 minutes, the fertilizer solution remaining on the bench or the flooded floor is drained back into the holding tank, where it is stored until the next fertigation (usually the next day). Runoff is not allowed with this irrigation system. As an added benefit, these subirrigation systems can be easily automated, saving on labor costs and requirements. And with the current job market, any irrigation system that provides a labor savings offers an advantage over other systems.

Subirrigation versus


After a plant has been subirrigated, the soil and plant begin to lose water. As this takes place, fertilizer salts from the bottom layer of the growing medium are brought up to the top and middle layers where they accumulate. In fact, soluble salt levels (measured as electrical conductivity, EC) in the top soil layer of subirrigated plants can be up to 10 times higher than those in the middle and bottom layers.

If salts reach prohibitively high concentrations, root growth stops and roots may become damaged in the affected soil layer. This may make the plants more susceptible to disease. Research with New Guinea impatiens, peace lily and poinsettia has shown that most root growth in subirrigated plants is concentrated in the bottom and middle layers of the growing medium. Therefore, high soluble salt levels in the top soil layer won't have much effect on these roots.

So it would seem that subirrigation works well for poinsettias during production. But, what happens once plants reach the garden center or florist shop, or eventually the consumer's home?

For starters, the method of irrigation will most likely change from subirrigation to top-watering. The water applied from above will pick up the fertilizer salts concentrated in the top layer. As the water flows down toward the drainage holes, it will carry those salts to the middle and bottom layers, exactly where the roots are. This change in irrigation method can cause salt stress in an otherwise healthy poinsettia plant. In fact, research has shown that in the first three weeks after production, a plant that has been watered through ebb-and-flow irrigation may drop more of its leaves than a top-watered plant.

However, accumulated salts can be removed from the media before plants reach the garden center or consumer tabletop. By top-watering the medium at the end of the production cycle, you can leach the fertilizer salts from the pots. But the efficiency of this method depends greatly on the volume of water applied. A small volume of water may simply wash the salts from the top into the bottom layer, where they are most likely to cause damage. Thus, sufficient water must be applied to leach the salts from the bottom of the pots. The question is, how much water should be applied?


To answer this question, researchers at the University of Georgia grew poinsettia plants in 6-inch pots with a soilless medium on ebb-and-flow benches. Plants were fertigated daily with 210 ppm nitrogen (the recommended rate, EC = 1.5 mS/cm). To simulate a worst case scenario, plants were fertilized until the end of the production cycle. The poinsettias were then top-watered with various amounts of water: half (0.5x), equal to (1x) and double (2x) the volume of the growing medium.

Leaching reduced the EC in the top and middle soil layers with each water volume. (Figure 1). However, applications of half the volume (0.5x) and the full volume (1x) increased the EC in the bottom of the pots. Using 0.5x or 1x the volume of the potting soil simply washed the salts from the top into the bottom layer of the pot, closer to the plant's roots. But even at these rates, the EC of the bottom layer remained well within the recommended range (2 to 3 mS/cm, measured with the SME method).

Applying a larger volume of water (2x) decreased EC in the bottom soil layer, indicating that the water volume applied in top-watering must be larger than the volume of the growing medium to reduce the EC of the bottom layer.

Top-Watering Effects

on Photosynthesis

To determine the effects of light levels, fertilizer levels or, in this case, salt accumulation on floriculture crops, researchers often measure photosynthesis. Photosynthesis is known to be extremely sensitive to salt stress. So how did post-production top-watering affect photosynthesis in poinsettias? Surprisingly, leaching with different volumes of water had no effect at all on photosynthesis. Even though leaching with 0.5x and 1x the volume of media did increase EC in the root-containing bottom layer, it had no detrimental effect on the poinsettias. Watering with 2x the medium volume resulted in a decrease in the EC of all soil layers. Since increased EC in the bottom soil layer did not negatively impact photosynthesis, the performance of subirrigated poinsettias is not likely to be impacted by top-watering the plants in the post-production environment.

An Unnecessary Step

What does all this mean to the grower? A high-quality poinsettia crop can be produced with subirrigation without the need for leaching at the end of the production cycle to remove accumulated salts from the top soil layer. Top-watering in the post-production environment is not likely to cause any damage to the plants, because the level of fertilizer salts remains well within the recommended range. Leaching salts from subirrigated poinsettias does not appear to be necessary to maintain high plant quality, so it is not recommended. The only exception to this may be for plants that were over-fertilized during the production period. In that case, soluble salt levels may become too high and adversely affect the keeping-quality of the plants. Leaching may provide some benefits, but keep in mind that over-fertilizing will almost always result in poor-quality plants; a problem that leaching cannot fix.

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Bodie V. Pennisi and Marc van Iersel

Bodie V. Pennisi is an extension floriculture specialist at the Tifton Campus and Marc van Iersel is an associate professor of floriculture at the Griffin Campus of the University of Georgia.

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