Chrysanthemum grower Maurice van Os, like many Dutch colleagues, has a problem with thrips in the nursery. He solves it mainly by using Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold season. He steams to control primarily Fusarium, nematodes and Rhizoctonia. Good hygiene practice during steaming and the stimulation of soil life after steaming are also necessary, he says.

Maurice van Os has a nursery of 2.5 ha in the southwest of the Netherlands and grows the varieties ‘Feeling Green Dark’ and ‘Alana’. The grower comes from a true chrysanthemum family. He started in 1997 as the third generation. His grandfather, father and uncle also grew chrysanthemums at a different location. When the family built a new greenhouse in 1998 his uncle left the company and his father followed later.
The location of the company was deliberately chosen for its ‘good’ ground: a sufficiently airy soil, with a sandy subsoil containing humus. The good soil structure makes watering easy.
He steams the nursery annually just like his father, uncle and grandfather. They used to hire a steam boiler but in 1998 Van Os purchased a high-pressure steam boiler. In those days heating and steaming with one boiler was the trend but now he wouldn’t purchase such a high-pressure boiler. “The mandatory annual inspection is an extra cost. The advantage is that the soil becomes less wet and the sheet cover over the soil gains a sphere shape faster.”

Steam after the summer

Opinions about the best time to steam vary. The chrysanthemum grower steams annually between week 30 and 39. “Some of my colleagues steam before the holiday. I chose afterwards. After steaming the plants grow better.”
Combined with the greenhouse temperature, which during the summer can rise to more than 32ºC, it can lead to too many buds in the bunch; ten instead of six. “We call that a ‘wild’ stem. In addition, the varieties that I grow are already quite heavy. If we steamed before the summer they would become too heavy. Then you also have more chance of yellow leaves. If the autumn is disappointing then you are very pleased with the heavier stems.”

Steam via the drainage

The chrysanthemum grower laid a steam drainage system 55 cm deep. Before steaming, Van Os loosens the ground to a depth of 50 cm using pens. Then the ground is nice and airy and the steam can spread optimally through the ground. He rolls out the steam sheet with an automatic roller and anchors it at the front and rear with steam chains and the sides with heat proof water pipes. Then the mesh is lowered down and the steam boiler and fan are turned on. The small pipes belonging to the steam drainage system stick out above the cover. The fan sucks the steam through the pipes, pulling it from under the cover deep into the ground.
“I steam for about 6.5 hours. Usually I start steaming as soon as the staff go home at the end of the day. I leave the cover in place and keep the fan running until the next morning.” He uses 3.5 m3 gas per m2 for steaming. Contrary to advice he does not use an extra energy cloth over the steam sheet. “As far as I am concerned, the mesh and supports can also get hot. The steam scorches everything it touches.”


Only steaming is not enough according to the grower. Steaming is carried out in one area while the rest of the nursery is in full production. Therefore it is important to ensure that the clean ground is not immediately contaminated again. “Before steaming we thoroughly sweep the path and we ensure that the tilling machine and tractor are clean.”
He purposefully doesn’t partition the greenhouse by lowering the internal walls around the steamed area because he doesn’t believe this helps combat the spread of thrips.

Thrips the biggest problem

Thrips is the biggest problem at the moment. “No one really has the answer.” Every week since the spring he has released Amblyseius swirskii: 200 units per m2. “Every two weeks I take samples to see if there are still plenty of predatory mites in the crop. Many colleagues release Amblyseius cucumeris. I only do that in the colder period from week 48 to week 3. The idea behind the early release of swirskii is that you create a biological balance early in the crop. That tends to work better with swirskii than with cucumeris. So far it doesn’t help enough but I don’t need to correct too much in between.”
Before harvesting he sprays the crop clean with Vertimec and Actara.

Quickly bring soil in balance

According to the grower healthy soil leads to more resistant plants, which are essential for reducing the risk of pests and diseases. “After steaming the soil is sterile and the plants, which grow faster have a softer leaf that is extra attractive to thrips. That is not an ideal situation,” says the grower. Therefore once or twice in the winter, between week 40 and week 10, he scatters poultry fertiliser and lime granules to redress the soil balance as quickly as possible. “The fertiliser also helps create an airy soil at the bottom. That’s good because as autumn approaches the ground is wetter.”


Chrysanthemum grower Maurice van Os uses a high-pressure steam boiler and special steam drainage at a depth of 50 cm to combat Fusarium. He uses Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold months, to combat thrips. In addition he maintains a healthy soil by ensuring an airy soil structure and applying poultry fertiliser and lime granules after steaming.

Points of attention for steaming

René Corsten of the Delphy chrysanthemum team has some points to remember when steaming the greenhouse:
– Make sure that the ground to be steamed is as dry as possible. Trying to steam wet ground is a waste of energy so stop the watering early enough.
– Use insulation material over the steam sheet. Insulation saves up to 0.5 m3/m2 and there is less condensation under the cover so the ground shuts itself off less quickly especially at the outer edges.
– With a little effort the steam transportation pipes can also be insulated.
– Preferably use rainwater.
– Flushing on time and good water treatment are important for effective steaming.
– Measure the temperature to check what you are achieving. These days good thermometers are available.
– If there is a problem with nematodes or soil fungi aim for 60-60-60: 60 centimetre deep, 60ºC and 60 minutes. The aim is not to use as little gas as possible but to get the best result. Vacuum (suction) steaming is a must. After steaming allow the fan to run for around 12 hours. Then there should be a continuous heating effect to a greater depth.

Text and images: Marleen Arkesteijn