FlowerTECH 2002, vol. 5/no. 1

Available land in the Netherlands isbecoming increasing scarce. Neverthelessbulbs are still propagated on over 20,000ha of land of which 50% is down totulips. It is not surprising therefore thattulips are still a Dutch national symbol.

Of the total number of bulbs grown inHolland some 76% - or 7 billion bulbs –are exported every year. Of this, just overhalf will be sold to home gardeners whilethe remainder are sold to professionalgrowers. The remainder stay in Hollandand nearly all will be cultivated by flowergrowers.

So how do they produce cut flowertulips in Holland? At the beginning ofthe 1990s the government began encour-aging a reduction in the amount ofchemical emissions into the environmentso more and more producers switched toclosed systems. However, problems withrooting media arose on grounds of avail-

ability, susceptibility to disease and dis-posal. Different substrates were investi-gated as well as hydroponic production.The advantage of this method is thatwater is cheap and readily available.

The only problem was how to keep thebulbs in place but this was solved by anumber of tray manufacturers. TheTriflor Company produced a tray fromTempex which held the bulbs in placewith tooth pick like projections. Thenthe Curtec company produced a similartray made of plastic. Kenn also intro-duced the Kenn tray which has a row ofpins at the bottom to anchor the bulbs.

A further problem arose due to bacterialgrowth on the roots which made themslimy. In fact it made many growers giveup on hydroponics but research, in par-ticular by the Dutch Bulb ResearchCouncil continued. The slime was shownto be due to lack of oxygen in the water

and it was found that the problem couldbe completely avoided by planting thebulbs three to one week before addingthe water. The success has meant thathydroponic production of tulip bulbs hasgrown from virtually nil in 1997 to over40% last year and the expectation is thatthis will rise to 90% within the next fiveyears.

ChallengeThe brothers Mark and Meindert-Jan deWit, of Andijk, previously grew bulbs onpeat and sand but switched, two yearsago, to forcing them in water. This wasmainly because they wanted a challengeand because it did not require any invest-ment. It also freed up some coolingspace. They now produce about 1.9 mil-lion tulips per year of seven cultivars ofwhich about 95% come from their ownnine hectares of fields.

Hydroponictulips succeed

at secondattempt

8 www.HortiWorld.nl

Lessons learnt from the past coupled tocontinuous research means that tulips cannow be cultivated successfully on water.

By Helen Armstrong

Tulip bulbs are secured on the pins of the Hydro Tray. One tray will

take about 100 bulbs.

The cool room. The

temperature is about 5°C.

FlowerTECH 2002, vol. 5/no. 1

Bulbs are secured by pins in Hydro-Trays, about 100 bulbs per tray of bulbssize 12+. Then, before any water isadded, the trays are put in the cold storeto break dormancy – the temperaturemust be under 9°C. The water is addedlater. The boxes are then returned to thecold store where the temperature is about5°C. It then takes three weeks, some-times only 10 days later in the season,before the bulbs are ready to transfer tothe greenhouse. No further water isadded during this period. Previously,with the peat and sand system, bulbsneeded to stay up to 18 weeks in the cold

store because the rooting system neededto be much more extensive in order toanchor the bulb.

“They only require about 4cm ofroots,” said Mark de Wit “because theroots are only necessary to take up water,not to anchor the bulb.”

As well as the time saving, the advan-tage of the water system is that is it notnecessary to buy in the sand and peat,the boxes with water are lighter (10 kgper box, compared wit 18kg), less roomis required in the cold store and less ener-gy is required to bring the temperature ofthe boxes down. Also harvesting is much

9www.HortiWorld.nl

The stacked trays are automatically filled with water.

When the tulip bulbs have about 4cm of root they can be transferred

to the greenhouse.

Tulips in the greenhouse, ready for harvesting.

BULBS

Tulip bulbs ready for

planting.

FlowerTECH 2002, vol. 5/no. 1

faster and the flowers are cleaner, saysMark. All together, it is one euro centcheaper per stem to produce tulips onwater than on soil.

The only problem is that some cultivarsare not suitable for hydroponic produc-tion.

And during dull weather the bulbs stilltake up water but they cannot transpireso readily so sometimes in winter it isnecessary to increase the EC of the waterin an attempt to slow down wateruptake. This they do by adding a solu-

tion of nitrogen, nitrate oxide and calci-um oxide.

GreenhouseAfter the cold store, bulbs are moved tothe greenhouse which in total covers1,200 m2. In the beginning the tempera-ture is set at 21°C which s graduallyreduced to 18°C and then 16°C. Thehouse is heated by hot water pipes whichare suspended above the crop – tulipslike to have “cold feet” – and watering isvia hoses which lie over the boxes.

Irrigating from above can lead to Botrytisand if the cultivar is susceptible to calci-um deficiency then overhead wateringcan cause the stems to fall over andbreak.

The boxes are sterilised between batchesby steam cleaning at 60°C for a couple ofhours. ❙

More information on hydroponic produc-tion of tulips is available from theInternational Flower Bulb Centre. Email:info@bulbsonline.org

10 www.HortiWorld.nl

Hyacinths are usually sold as pot plants,either in soil or with the bulb just float-ing on a vase of water. But the samebulbs, given a slightly different treatmentso that they produce longer stems, can beused to produce cut flowers. The majori-ty of these will be forced in soil but oneDutch grower has almost perfected thehydroponic production of cut hyacinths.

Peter Knijnenburg and his wife Lenie,of Noordwijk, first started forcing cutflower hyacinths in 1974, firstly on soiland since 1994, on water. It took themabout three years to master water cultiva-tion and still they are the only people inthe Netherlands using this technique.Currently they produce 400,000 cut

flower hyacinths between December andMarch, half on water, half on soil fromseven cultivars: White Giant; Blue Giant,Mulberry Rose, Delft’s Blue; PrinsesMarie Christina, Prins Hendrik andNereus.

Many growers buy bulbs from specialistgrowers or via a bulb auction while otherspropagate their own lifting them at theend of May, early June. The bulbs arethen stored at 25°C, followed by a periodat 23°C and then 13°C. And then toforce them, the temperature in the cold

store will be reduced to 9°C.In order to produce cut flowers from

late November Mr. Knijnenburg plantsthe bulbs in wooden boxes, previouslyused to grow chicory, at the end ofAugust. The difference between potplants and cut flowers is that cut flowersrequire longer cold treatment in order toproduce longer stems.

There is quite a difference betweenthose bulbs being grown on peat andsand and those being forced on water.Bulbs in peat require 13 weeks at 9°C

Forcing hyacinths for winter cut flowers

Given the right treatment, hyacinthsmake very fragrant cut flowers in winter and can bring a splash of blueto bouquets, a difficult colour to findduring this period of the year.

by Helen Armstrong

Hyacinths make a

fragrant, short cut flower

in the winter.

FlowerTECH 2002, vol. 5/no. 1

BULBS

while those grown on water require justnine weeks of cold treatment. This isbecause the peat takes longer to reach9°C and, because the bulbs are respiringand producing heat, it is possible that thepeat around the bulbs never reaches 9°C.On the other hand, the bulbs growinghydroponically are in direct contact withthe water and it is easier for this to bemaintained at a constant 9°C.

Despite the saving of four weeks,Knijnenburg says that he will continue togrow some hyacinths in soil because notall varieties are suitable for water culture.

“Not all varieties are possible, because,the roots, although thicker become toobrittle and break easily,” he says. This is

the main reason why it is also not possi-ble to transfer water grown plants to soiland vice versa.

Bulbs for cut flowers required after 1January are planted at regular intervalsfrom the end of September onwards.Again they will be forced at 9°C andrequire 8-12 weeks dormancy breakdepending on the variety. If during thisperiod the stems become too long tooquickly, the temperature will be droppedto 2°C.

Knijnenburg tops up the water in theboxes during the forcing period but heno longer feels it necessary to change it.He used to renew it for fear of the fun-gus Penicillium building up, but even ifthis does occur it is not harmful to thebulbs, he says.

The bulbs grown on peat have a layerof sand poured over them which is neces-sary to weigh them down and preventthem being forced upward by the grow-ing roots.

GreenhouseAfter the forcing period Knijnenburgtransfers the boxes to his greenhouse,which covers an area of 110m2, andwhere the temperature is 18°C. Thebulbs grown on water are held in placeby two layers of plastic-covered mesh (seephoto). He used to use metal wire tosecure the bulbs but this lead to zinc tox-icity. Unfortunately, the plastic wire alsopresents problems because when they

touch it, rather than growing betweenthe holes, they stop growing. Therefore,he is currently experimenting with a hon-eycomb type plastic which is usually lainunder gravel paths to hold the stones inplace. The bulbs are placed on the plastic

11www.HortiWorld.nl

Bulbs being forced in

chicory boxes.

Peter Knijnenburg.

Plastic covered mesh hold the bulbs in place while the roots are

growing

Mr. Knijenburg is experimenting by putting the bulbs on a

honeycomb type of plastic mat.

FlowerTECH 2002, vol. 5/no. 112 www.HortiWorld.nl

which also has the advantage that itremains much flatter than the plasticwire.

Flowers from the bulbs raised on waterare ready for cutting after one week inthe greenhouse while those grown on soiltake about ten days. And as the seasonprogresses it becomes possible to harvesttwo consecutive water crops for everypeat crop. Another benefit of the hydro-ponic hyacinth is that harvesting is threetimes faster and the flower is cleaner.Those on water are grabbed at the base

of the stem while those in the peat haveto be worked loose by putting a finger ininto the soil and under the bulb.

Post harvestAfter harvesting, the bulbs are removedfrom the stems by a machine invented byKnijnenburg himself. It first cuts theroots off the bulb and then pushes thestem out of the bulb so that some of thebulb remains on the stem. This allows theleaves to remain attached to the stem. Ifthe stem is cut above the bulb the leaveshave no point of contact and fall off.

The stems are bunched in fives and areput in water for about five minutesbefore being packed ready for transportto the nearby auction. At the momentthere is no difference in price between ahyacinth grown in peat or on water but ifsupplies of hydroponic hyacinths were to

increase they might, in future, receive apremium because they are cleaner, hopesKnijnenburg. ❙

Hyacinths in the greenhouse where the temperature is maintained at

18°C.

The machine (pictured open) removes the roots and

pushed the stems out of the bulb.

BULBS