By Marcy Britigan, MEI

Incorrect temperature at the store level is a major contributor to cut flower deterioration. The proper temperature helps keep respiration at a minimum, enables the flower to maintain its natural food source for longer periods, thus maximizing vase life. Lower temperature levels also help reduce the effects of ethylene gas, retard normal aging and can contribute to a more brilliant flower color.

Ideal temperatures and humidity

Unlike most produce, fresh flowers have a greater surface area to volume ratio. Cut flowers also have a much higher risk of water loss through transpiration. Cut flowers have a very low density, so response time to temperature, warm or cold, is much more rapid.

Relative humidity, the second component of the equation, is considered by some to be as, if not more, important than temperature for maintaining cut flower integrity.

A high relative humidity level helps flowers maintain moisture resulting in a fresher appearance and longer vase life. Low humidity coolers dehydrate flowers, accelerate the natural aging process and reduce flower life.

Good air circulation at relatively low velocity prevents hot spots and assures a constant temperature throughout a floral cooler. With the exception of pre-cooling and dry storage where product is typically protected and boxed, flowers should never be placed where they will be in direct contact with air exiting immediately off the evaporative coil.

Commercial vs. floral coolers.

A retailer should not simply take someone's word that a floral cooler being considered is truly engineered for cut flowers. Many reconditioned commercial food/beverage coolers are marketed for flowers but do not have the capability to meet cut flower refrigeration specifications.

Relative humidity levels
Optimum: 95 percent
Minimum acceptable: 80 percent

Two coolers similar in size, one properly engineered for flowers and the other designed for a common commercial application, will both deliver the desired temperature.

Because food or beverages are more dense than flowers, they must be cooled down faster and are typically humidity insensitive. To meet these requirements, common commercial units have a much higher air velocity (a factor that can be field corrected) and a considerable lower relative humidity factor of +/- 70 percent ( a factor that cannot easily be field modified).

he relative humidity in a properly designed floral cooler (minimum 80 percent; optimum 95 percent) is actually controlled by the refrigeration system itself.

It cannot be achieved by adding special humidity controls or recently-introduced misting systems. Either option will increase cost, misting will potentially create an environment for bacteria and botrytis to thrive and neither will rectify the inherent problem.

Effect of improper unit size

A refrigeration system's ability to control relative humidity levels is an engineered balance between the evaporative coil capacity at specific air velocities and the compressor/condenser capacity. Both selections are a function of box load or BTU requirement.

The compressor size (HP) is selected after the box load is determined. A unit sized too small will have insufficient capacity, will run constantly and will not have the ability to pull down and hold temperature.

An oversized unit will not cycle often enough to maintain optimum temperature and RH levels.

Evaporative coils are identified by their capacity or BTUs. In a typical commercial food or beverage cooler the coil capacity is 'matched' in BTUs to the compressor capacity to deliver a +/- 12 degree temperature differential (TD). This high TD factor results in low relative humidity levels.

A coil for cut flowers must be built as or modified to be a low velocity unit. Reducing air velocity (CFM) also reduces the unit's BTU capacity.

Also, a coil with increased fin surface will deliver a better balanced and a more gentle cooling system. The combination of lower velocity fans and 50 to 100 percent more fin surface will increase the coil size and cost.

Return to document index