December 2011

How Much Mist Are You Applying in Propagation?

Applying the right amount of mist in propagation is a constant challenge. Too little mist results in immediate damage or death, so most propagators tend to stay on the side of being too wet. However, excess mist wastes water and fertilizer while increasing nutritional and pathogen problems in propagation. So, we undertook a project to survey commercial propagators to determine how much water was being applied during the winter propagation of vegetative annuals. We visited 18 growers to measure the amount of water that they were applying in propagation. In this article, we will provide instructions on how to make your own in-house measurements to compare to the mist water use results that we recorded during our commercial propagators survey.

There are three steps to calculating the amount of water that is used in propagation. First, one must make measurements of how much mist is delivered to cuttings during a typical mist event. Second, one must decide upon a typical mist recipe in order estimate the number mist events that occur during a propagation cycle. Third, multiplying the number of mist events by the amount of water per mist event provides an estimate of the total amount of water delivered during cutting propagation.

Step 1: In-House Measurement of Mist Water Volume

Equipment:

• Kitchen scale (measures to 1 gram)
• Five trays for capturing the mist
• A towel for drying the trays

Method:

1. Number each tray and then measure the mass of each tray since these can be quite variable despite looking identical.

2. Measure the area of the trays (in square feet per tray).

3. Place trays in various positions within one misting section (Figure 1).

4. Let the mist system or boom engage for one mist event.

5. Gather trays immediately. Use your towel to remove any water that may be on the bottom or sides of the tray.

6. Weigh the trays. (Remember that 1 gram of water = 1 ml)

7. Repeat Steps 3 through 6 to observe the variability between mist events.

If you do not have a scale, one can roughly estimate the volume of each mist event simply by observing the size of the water droplets that settle on the trays. Figure 2 demonstrates the size of water droplets that occurred under mist equivalent to 3, 6 or 12 ml per square foot.

Propagators surveyed were located from Georgia to Michigan. As seen in Figure 3, a range of 1.75 to 11 ml per square foot per mist event was measured from greenhouses growing similar crops (vegetative annuals) at similar times during the year (February). These numbers are primarily affected by the size of the mist nozzle and either the duration of the mist event or the speed of the boom.

Step 2: Estimating the Number of Mist Events Per Day

After measuring the water volume per mist event, growers must decide on a general recipe for February propagation for vegetative annuals. While weather has a big impact on this recipe, we suggest that you describe a typical recipe for a typical weather day for your location. Figure 4 displays an example of a recipe for a time clock system. Given the recipe is for a winter crop, it is recognized that there are ten hours of day light. The number of mist events during the first 24 hours in propagation is 240. This numbers decreases throughout the 10-day propagation cycle; however, the total number of mist events to propagate this crop is 920.

Step 3: Calculate the Total Water Volume Provided During the Propagation Cycle

Multiply the mist volume per mist event by the total number of mist events per propagation cycle to get the total water volume used. Figure 5 displays results from the 18 growers that we surveyed. These results suggest a wide range of water use in propagation. Some of these differences can be explained by the greenhouse climate. For example, the lowest water user has a fog system to maximize humidity and thus to minimize the amount of mist applied. Some of the higher light environments were amongst the highest water users. However, in general, many of these propagators have quite similar temperature and humidity environments. The biggest differences are attributed to grower decision-making, not climate. Thus, these results suggest that 1 liter per square foot would be good target to shoot for. If you are providing more than that amount, you may consider the following options for reducing your mist volume.

Suggestions for reducing mist volume:

• Ratchet down your mist frequency.
• Increase the speed of your booms (1.3 feet/second is a typical boom speed) or reduce the duration of your individual mist events.
• Change mist nozzles to a lower volume nozzle. For example, TeeJet 8002 is a commonly used nozzle on irrigation booms. These output 0.2 gallons/minute. Simply switching to 8001 nozzles will cut the mist volume in half without changing your mist settings. ‘Fogger’ nozzles can also provide less mist than standard ‘mist’ nozzles in stationary systems.
• Remove or plug every other nozzle on your boom (during winter propagation).
• Reduce evaporation of water from leaf surfaces by increasing humidity in the greenhouse or decreasing the dehumidification cycles.

Summary

It can be difficult knowing how ‘wet’ or ‘dry’ you are in propagation. This project shows you how to determine the amount of water that you are applying in propagation and then to compare those results to other commercial propagators. It is clear that many propagators are applying much more water than is necessary to successfully propagate vegetative annuals during late winter. This excess water has many negative side effects, such as it wastes natural resources, costs you money in terms of water treatment, reduces your ability to control algae in propagation, increases nutritional problems resulting from leaching nutrients from the leaves and growing media, increases fertilizer costs, and increases disease pressure. So, there are many incentives to minimizing your water use in propagation. We challenge you to follow the steps outlined above to see how you compare to other propagators.

Acknowledgements: The authors would like to thank the USDA-ARS Floriculture and Nursery Research Initiative for their financial support of this project. Also, we thank the propagators that allowed us to survey their facilities.