So do many other growers. That’s why we’re starting a new series called FAQs--frequently asked questions. The first installment (and the source of the idea) is from flood floor expert Ratus Fischer, who says that growers have so many questions about flood floor technology that he wrote them all down to help his staff. Here they are, to help you decide if flood floors are appropriate for your business.

Q: How do you prevent the spread of
disease through the water?

A: By preventing pathogens like phytophthora and pythium from getting into the water in the first place. With short watering times of less than 10 minutes, the water on the floor begins draining away while the last bit of water is still entering the container. This one-way movement practically prevents pathogens from leaving the container.

Spores in the water are likely to settle in the water storage tanks, where we try to let them rest undisturbed until clean-out time.

Floor heat will provide optimal temperatures to keep the roots happy and make them less susceptible to pathogens. Subirrigation keeps the foliage dry, which keeps foliar diseases at bay. Growers with flood floors commonly report their disease problems have decreased rather than increased.

Q: Can we use fungicide drenches on flood floors?

A: No fungicides have been labeled yet for application through subirrigation water. They can be applied from above as usual.

Q: Isn’t salt accumulation a problem with
subirrigation?

A: With any subirrigation, some salt accumulation in the top layer of the soil occurs. Yet very few crops seem to respond negatively, not even Easter lilies with their crown roots. Of course, low salt fertilizers should be used.

Q: Do we have to change our fertilizer
concentrations?

A: Yes. Start out with 50 to 75% of the concentration recommended for overhead watering. Since the water is recirculated, all fertilizer will end up in the plants and not in your neighbor’s well.

Q: Are special containers needed to grow on the floor?

A: No, most pots and bedding trays on the market will work. On rare occasions, trays with unusually small drain openings have been seen floating.

Q: How deep should the water be on the floors?

A: As soon as the water makes contact with the soil, the capillary action begins. For all but very large containers, 1 in. of water is sufficient. The systems are normally designed for water depths up to 2 1/2 in.

Q: Which crops will grow best on flood floors? Can we grow bedding plants?

A: Flood floors will work great with all but very few potted crops, from miniatures to baskets. Most growers use the floors for bedding as well. There are some limitations, though. With flood floors, each watering is a full watering, and the small cell packs tend to stay saturated longer than larger containers. Bedding crops that need withholding of water should be watered overhead during those phases of growth. Since the watering with flood floors is very even, crops can be dried out more between waterings before the first plants get damaged.

Q: Can different crops and container sizes be grown on the same floor section?

A: Yes, as long as the crops have similar watering needs. The size of container doesn’t matter much, since any size of container with an appropriate soil mix will soak up water to its capacity within the normal flooding time.

Q: How do I provide different fertilizer solutions to the floors?

A:Most flood floor systems have two or three tanks with different fertilizer solutions or plain water. Each floor section can be watered with the appropriate fertilizer mix or concentration.

Q: What kind of a fertilizer mixing system do I need for flood floors?

A: Most likely a simpler one than your equipment supplier thinks. If you’re willing to switch some suction tubes, a basic proportioner will do. Fully automated systems require a bit more hardware. Since the water that comes back from the floor is practically unchanged from what goes to the floor, we only need to supply a preset mix to each flood floor tank.

Q: Can we retrofit existing greenhouses with flood floors?

A: Yes, the systems can be adapted to just about any greenhouse layout, as long each flood section in itself can be kept or made level. The watering sections usually correspond with the bays or houses--typically 1,000 to 6,000 sq. ft. in size. The same is true for floors in newly constructed houses.

Q: For which sizes of greenhouses are flood floors suitable?

A: There’s no limit how large. One flood floor tank system can supply six acres or more of growing area. The limit is only how long a watering time for the whole system is acceptable.

There’s also no real limit how small. Even a system of just a few hundred square feet can be feasible, if the added complexity of full automation is not required.

Q: Can flood floors be used outside or in retractable roof greenhouses?

A: Yes. Rainwater will drain through the same system as the floodwater, but is directed to the outside instead of into the flood system tanks.

Q: Is floor heat necessary flood floors?

A: Yes, unless the greenhouse is located in a subtropical climate. Floor heat maintains the ideal root temperatures, prevents excess humidity in the crop zone and dries the floors quickly, preventing the growth of algae. And you still need overhead heat. The floors would have to be too hot for the plants if they were to provide all the heat in the greenhouse.

Floor heat maintains the ideal root temperatures, prevents excess humidity in the crop zone and dries the floors fast, preventing the growth of algae.

Q: How much pitch does a flood floor need?

A: The less pitch there is, the less water is needed to cover the whole floor, but the risk of puddles increases accordingly. A good concrete crew can pour a largely puddle-free floor with 1/2 in. of fall over 6 to 10 ft.

Q: How can you keep the floor from cracking?

A: Pouring good concrete floors is as much an art as a science. Small cracks are usually only cosmetic and don’t cause leakage. If large cracks occur, probably some of the following advice has been ignored: Keep the moisture in the concrete while curing, cover the floor with plastic (or water) for 7 to 14 days, and create sufficient control joints.

Q: How can we avoid puddles, and when are they a problem?

A: Use a good vibrating screed, adjusted to a perfect V or W shape; fight with your concrete crew to maintain a stiff concrete mix with a slump of no more than four; place the concrete carefully, without unmixing it by excessive raking and pushing around.

A puddle is a problem if it doesn’t dry out within a couple of hours after watering: algae will grow, and plants sitting in the puddles can get watered beyond their comfort. A rule of thumb: Throw a dime into any standing water--if the water covers it, it’s a puddle.

Q: Do you need a special contractor to pour a flood floor?

A: Yes and no. Most quality-minded contractors can pour a good flood floor. Since pouring precisely pitched floors is quite different from pouring flat floors, it helps considerably if the contractor is willing to listen to others who have done it before. If you want to eliminate the guinea pig factor completely, try to work with someone who has done flood floors before.

Most quality-minded contractors can pour a good flood floors.  Since pouring precisely pitched floors is quite different from pouring float floors, it helps considerably if the contractor is willing to listen to others who have done it before.

Q: Should I get professional help in designing a flood floor system?

A: If you wish the system to perform as desired, that’s an excellent idea. Once the concrete is poured, changes tend to be on the expensive side.

Q: What’s the cost of a flood floor system?

A: A complete system will cost in the range of $4.50 to $5.50 per square foot. Costs will be lower if much of the work can be done with in-house labor. These figures include everything from design to operating system: piping, all components, concrete, floor heat (without boiler) and labor. The concrete work accounts for around 75% of the cost.

With a flood floor system, you not only pay for a watering system, but you also have a concrete floor for maximum flexibility and sanitation and floor heat for optimum control of growth. Savings in labor, improved plant quality and low maintenance costs over many years of use offset the relatively high initial investment.

While there's still not much equipment available for putting pots and flats down on the flood floor, Visser's forklift-based Space-o-Mat works well for larger batches of same-size pots.

Q: Is there equipment available to move pots and trays?

A: Not as much as we’d like to see. Conveyor belts are quite common. Visser’s forklift-based Space-o-Mat works well for larger batches of same-size pots. The latest development for
larger operations is growing flats and pots on floodable pallets, which are moved by a gantry system.