August 2002

A Primer on Hydroponic Cut Tulips By Bill Miller

Tulips are a good example of a flower bulb crop that can beadapted to hydroponic culture. In Holland, approximately 30-35 percent of thecut tulip crop is forced hydroponically, and we have been evaluating thisproduction method at Cornell over the past two seasons. While our experienceshave been very positive, there are several important details to understand andact upon before a high-quality crop can be produced.

Hydroponic bulb basics

The basic procedure with hydroponic tulip forcing is to giveapproximately 75-80 percent of the cold requirement to dry, unplanted bulbs(see Figure 1, below). Depending on the cultivar and time of year, this mightbe 12-14 weeks. Bulbs are then “planted” into the system, and adilute calcium nitrate solution is added for rooting (about 1.0-1.2 mmhos/cm2).Rooting proceeds at 40° F for 3-4 weeks for early crops or 2-3 weeks forlater crops. After the entire cold requirement has been provided, bulbs aremoved into the greenhouse for forcing. The plants are then fed with calciumnitrate, with the goal of maintaining an EC of 1.2-1.5 mmhos/cm2.

It is important to realize that the longer the rootingperiod (above 2-4 weeks), the lower the eventual quality of the flower. This isbecause longer roots cause more rapid oxygen depletion of the solution andbecome more susceptible to disease. Also, the longer and more entangled theroots are, the more difficult harvest is (harvesting one stem pulls up manymore with entangled roots). Realizing the relatively small root system neededto produce a good-quality plant is the key to successful hydroponic tulipproduction; the small root size is probably much less than is necessary for cuttulips in soil- or peat-based forcing.

Compared to traditional “soil” culture in”boxes” (where bulbs are planted in crates, cooled, then forced),hydroponic forcing has the following advantages:

• itis 3-5 days faster than soil culture;

• muchless cooler volume is required for chilling bulbs (because most of the coldperiod is given to densely packed, unplanted tulips in their shipping crates);and

• harvestingis easier and cleaner — there is no wasted soil at the end, greatlyreducing material handling problems.

Why do hydroponic plants force faster than plants grown intraditional soil culture? It is not due to any inherent superiority ofhydroponics; it is simply due to the prevailing temperature (ca. 40° F) ofthe plants during the 2- to 4-week rooting period. This is 6-8 degrees warmerthan is typical during the last few weeks of cooling, where, normally,temperatures of 32-33° F might prevail to reduce stem growth in the cooler.These few degrees over a 2- to 4-week period can easily account for the reducedcrop time in the greenhouse. Plus, in traditional cut flower forcing in boxes,the mass of the bulbs and soil is substantial, probably taking 1-2 days to warmto prevailing greenhouse temperatures.

The disadvantages of hydroponic forcing are:

• Whengrown at the same temperature, the ultimate quality of the stem is not quite asgood as when the same cultivar is grown in soil (hydro stems tend to be 1-2inches shorter and 6-8 percent lighter compared to substrate-grown stems);

• notall cultivars are suited to this system;

• veryhigh-quality and disease-free bulbs are required, especially for laterplantings (careful attention must be placed on proper bulb storage, includingtemperature, humidity and ventilation);

• especiallyfor individual trays, a level bench or tray support system is critical tomaintain a level nutrient solution (old, uneven benches won’t cut it);and

• theneed for exceptional cleanliness. The trays and components are sometimesdifficult to wash and sanitize (although, on a large scale, a machine could beused for this).

Options and solutions

The weight and length issues of hydroponic tulips aresolvable problems, and ongoing work in The Netherlands indicates thatadjustments in rooting period and aeration can compensate for most of theweight and length reduction.

After considering the biological merits of hydroponicforcing, thought must be given to the hydro system itself. A tray designed bythe Bulbfust Company (24 x 16 inches, approximately four inches tall, designedto fit inside a black plastic bulb crate) is still the major one used in TheNetherlands. It is characterized by a grid of plastic “pins” thatthe bulb is pressed onto for upright support. The tray has two drainage holesto maintain the proper solution depth when the tray is level. A number of othersystems are available from manufacturers in The Netherlands. These systems arevery similar in appearance to large plug trays and come in a variety of sizesto match the size of the bulb being forced (4-5 inches, etc).

As expected, each system has its own plusses and minuses.The Bulbfust “pin tray” has proven to be popular because it isdurable, and the pins, while causing some injury to the bulbs, are usable fornearly all sizes of bulbs and provide an infinite number of spacing andarrangement options. The plug-like trays are designed for specific bulb sizes,and multiple trays are needed if a company forces different sizes of bulbs. Ineither case, there are two components to handle: the crate and the water trayitself (though this is being solved by new, larger-scale systems designed toutilize larger ebb-and-flow greenhouse benches).

Our research at Cornell has indicated that static nutrientsolution (as is characteristic in individual trays) is often difficult tomaintain at optimal EC, aeration and pH levels. The volume of solution in eachtray is only about 10 liters; this is not a lot of solution for 60-80 tulips.We adapted our irrigation and fertilization practices such that new calcium nitratesolution (at an EC of 1.2) was used during the week, and only clear water wasapplied on weekends. In this way, we were able to maintain the EC at anacceptable level and grow excellent-quality tulips.

And even though the solution in hydroponic trays is onlyabout 1.5-2 inches deep, it is possible for the dissolved oxygen level in thesolution to drop sufficiently low that root growth is reduced. Research in TheNetherlands at the Zwaagdijk experiment station confirms this and hasdemonstrated the expected advantages of larger nutrient reservoirs withsolution that flows constantly over the roots. In this way, there is a greaterbuffering of nutrients, a slower rate of change of pH and EC, and betteraeration of the solution. Thus, it should be easy to adapt existing Áebb-and-flow benches to hydroponic tulip production. An indication of the plantand root response to simple aeration of the solution is shown above.Interestingly, the dissolved oxygen level of our non-aerated treatment wasstill above the minimum needed for good growth of hydroponic lettuce crops;perhaps this indicates tulips have an especially high dissolved oxygenrequirement for growth. Currently, the answer to this question is unknown.

The Future for Hydroponic Tulips

It is easy to see hydroponic tulip production continuing toincrease worldwide. In the United States and Canada, one can envision its usefor large-scale production with all the advantages noted in this article. It isalso easy to see it as an interesting component for smaller retail greenhouseoperations, where a few trays could be forced weekly to provide veryhigh-quality, locally produced products. Because a smaller cooler volume isneeded, capital costs are lower, making it easier to get into cut tulipproduction. Eliminating the direct cost of the substrate and the associatedhandling costs probably allows for payment of the hydroponic trays in two years(although costs and savings would vary tremendously between companies). Our ownexperience with hydroponic tulips at Cornell has been very positive, and inmost cases, the advantages more than compensate for the negative aspects ofthis way of forcing. One thing is clear: An ultra-fresh cut flower tulip is abeautiful thing and is rarely seen by most consumers in North America. Therewould seem to be many opportunities to incorporate hydroponics into the productmix of many smaller growers.

Thanks are expressed to the Dutch Exporters Association forflowerbulbs and nursery stock, and SePRO, Uniroyal and Valent USA for financialand material assistance with aspects of the research reported herein. Alsothanks to Cornell’s Barbara Stewart and Jeffrey Wagemaker and PieterHeemskerk, two Dutch student interns involved with this work.