Tulips are a good example of a flower bulb crop that can be
adapted to hydroponic culture. In Holland, approximately 30-35 percent of the
cut tulip crop is forced hydroponically, and we have been evaluating this
production method at Cornell over the past two seasons. While our experiences
have been very positive, there are several important details to understand and
act upon before a high-quality crop can be produced.
Hydroponic bulb basics
The basic procedure with hydroponic tulip forcing is to give
approximately 75-80 percent of the cold requirement to dry, unplanted bulbs
(see Figure 1, below). Depending on the cultivar and time of year, this might
be 12-14 weeks. Bulbs are then “planted” into the system, and a
dilute 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 for
later crops. After the entire cold requirement has been provided, bulbs are
moved into the greenhouse for forcing. The plants are then fed with calcium
nitrate, with the goal of maintaining an EC of 1.2-1.5 mmhos/cm2.
It is important to realize that the longer the rooting
period (above 2-4 weeks), the lower the eventual quality of the flower. This is
because longer roots cause more rapid oxygen depletion of the solution and
become more susceptible to disease. Also, the longer and more entangled the
roots are, the more difficult harvest is (harvesting one stem pulls up many
more with entangled roots). Realizing the relatively small root system needed
to produce a good-quality plant is the key to successful hydroponic tulip
production; the small root size is probably much less than is necessary for cut
tulips 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:
• it
is 3-5 days faster than soil culture;
• much
less cooler volume is required for chilling bulbs (because most of the cold
period is given to densely packed, unplanted tulips in their shipping crates);
and
• harvesting
is easier and cleaner — there is no wasted soil at the end, greatly
reducing material handling problems.
Why do hydroponic plants force faster than plants grown in
traditional soil culture? It is not due to any inherent superiority of
hydroponics; it is simply due to the prevailing temperature (ca. 40° F) of
the plants during the 2- to 4-week rooting period. This is 6-8 degrees warmer
than 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 reduced
crop 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 warm
to prevailing greenhouse temperatures.
The disadvantages of hydroponic forcing are:
• When
grown at the same temperature, the ultimate quality of the stem is not quite as
good as when the same cultivar is grown in soil (hydro stems tend to be 1-2
inches shorter and 6-8 percent lighter compared to substrate-grown stems);
• not
all cultivars are suited to this system;
• very
high-quality and disease-free bulbs are required, especially for later
plantings (careful attention must be placed on proper bulb storage, including
temperature, humidity and ventilation);
• especially
for individual trays, a level bench or tray support system is critical to
maintain a level nutrient solution (old, uneven benches won’t cut it);
and
• the
need for exceptional cleanliness. The trays and components are sometimes
difficult to wash and sanitize (although, on a large scale, a machine could be
used for this).
Options and solutions
The weight and length issues of hydroponic tulips are
solvable problems, and ongoing work in The Netherlands indicates that
adjustments in rooting period and aeration can compensate for most of the
weight and length reduction.
After considering the biological merits of hydroponic
forcing, thought must be given to the hydro system itself. A tray designed by
the Bulbfust Company (24 x 16 inches, approximately four inches tall, designed
to fit inside a black plastic bulb crate) is still the major one used in The
Netherlands. It is characterized by a grid of plastic “pins” that
the bulb is pressed onto for upright support. The tray has two drainage holes
to maintain the proper solution depth when the tray is level. A number of other
systems are available from manufacturers in The Netherlands. These systems are
very similar in appearance to large plug trays and come in a variety of sizes
to 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 is
durable, and the pins, while causing some injury to the bulbs, are usable for
nearly all sizes of bulbs and provide an infinite number of spacing and
arrangement options. The plug-like trays are designed for specific bulb sizes,
and multiple trays are needed if a company forces different sizes of bulbs. In
either case, there are two components to handle: the crate and the water tray
itself (though this is being solved by new, larger-scale systems designed to
utilize larger ebb-and-flow greenhouse benches).
Our research at Cornell has indicated that static nutrient
solution (as is characteristic in individual trays) is often difficult to
maintain at optimal EC, aeration and pH levels. The volume of solution in each
tray 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 nitrate
solution (at an EC of 1.2) was used during the week, and only clear water was
applied on weekends. In this way, we were able to maintain the EC at an
acceptable level and grow excellent-quality tulips.
And even though the solution in hydroponic trays is only
about 1.5-2 inches deep, it is possible for the dissolved oxygen level in the
solution to drop sufficiently low that root growth is reduced. Research in The
Netherlands at the Zwaagdijk experiment station confirms this and has
demonstrated the expected advantages of larger nutrient reservoirs with
solution that flows constantly over the roots. In this way, there is a greater
buffering of nutrients, a slower rate of change of pH and EC, and better
aeration of the solution. Thus, it should be easy to adapt existing Á
ebb-and-flow benches to hydroponic tulip production. An indication of the plant
and root response to simple aeration of the solution is shown above.
Interestingly, the dissolved oxygen level of our non-aerated treatment was
still above the minimum needed for good growth of hydroponic lettuce crops;
perhaps this indicates tulips have an especially high dissolved oxygen
requirement for growth. Currently, the answer to this question is unknown.
The Future for Hydroponic Tulips
It is easy to see hydroponic tulip production continuing to
increase worldwide. In the United States and Canada, one can envision its use
for large-scale production with all the advantages noted in this article. It is
also easy to see it as an interesting component for smaller retail greenhouse
operations, where a few trays could be forced weekly to provide very
high-quality, locally produced products. Because a smaller cooler volume is
needed, capital costs are lower, making it easier to get into cut tulip
production. Eliminating the direct cost of the substrate and the associated
handling costs probably allows for payment of the hydroponic trays in two years
(although costs and savings would vary tremendously between companies). Our own
experience with hydroponic tulips at Cornell has been very positive, and in
most cases, the advantages more than compensate for the negative aspects of
this way of forcing. One thing is clear: An ultra-fresh cut flower tulip is a
beautiful thing and is rarely seen by most consumers in North America. There
would seem to be many opportunities to incorporate hydroponics into the product
mix of many smaller growers.
Thanks are expressed to the Dutch Exporters Association for
flowerbulbs and nursery stock, and SePRO, Uniroyal and Valent USA for financial
and material assistance with aspects of the research reported herein. Also
thanks to Cornell’s Barbara Stewart and Jeffrey Wagemaker and Pieter
Heemskerk, two Dutch student interns involved with this work.
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