Dutch Rhododendron grower Frans Kortenhorst was looking for a way to get rid of weeds and moss in his pots for good. He cut squares out of a roll of cling film and a bin liner and secured them around the top of the pot with a rubber band. In the middle he made a hole for the plant. Six years later, this idea has a name and it has the support of his horticultural supplier.

With his engineering background, rhododendron grower Frans Kortenhorst from Heeten in the east of the Netherlands is always looking for ways to innovate and automate his business processes. But his best ideas come during quiet periods at the nursery. He was fed up with having to remove weeds and moss by hand to get the plants ready for delivery. “Having 300,000 pots pass through your hands is a massive job that has to be done every year.” Laughing, he says that preferring an easy life makes a healthy starting point for innovation and encourages him to optimise and get creative.

Weed and moss problem

According to René Janssen, Substrate Product Manager at horticultural suppliers Horticoop, the Cleanpot System is an enormous improvement. Unsurprisingly, their enthusiasm has resulted in collaboration with Frans Kortenhorst.
The weed and moss problem in pot plant cultivation is tackled by covering the surface of the pots. Bark chips go quite a way to reducing weeds and moss, but not enough for Kortenhorst’s liking. “The retail channel is not very happy with the stuff either,” the grower says. “Bark makes a lot of mess: it lies loose on the surface of the pot, it falls on the shop floor, on the conveyor belt in the greenhouse and in the customer’s car – those kinds of complaints.”

Step by step

The idea of covering the pots with film was something Kortenhorst thought worth trying. He ran a trial with both transparent and dark film on five pots in the first year, followed by thirty pots in the second year. It immediately became clear that the film didn’t harm the plants. The transparent film was dropped straight away, but the dark film suppressed weed and moss growth completely. All the additional benefits were an added bonus. “We cut our watering by 25% and I got rid of Sciarid fly in one fell swoop,” he says.
Kortenhorst designed a machine to fit the film to the pots and built the prototype in his own workshop. “We introduced it at our nursery in stages – 10,000 pots in the first year, 100,000 in the second year, and our total production of 300,000 after that.”

Longer shelf life

Retailers have welcomed the system. Kortenhorst made sure of that before introducing it throughout his operation. Most of his dwarf rhododendrons are sold in Lidl Europe stores. The most positive aspects for the retailers were the longer shelf life and the fact that the pots were cleaner to work with. The shelf life of the plants is a good four days longer because the potting soil stays moist. What is more, most consumers don’t even notice the film to begin with when they buy the plant. They have to remove it when they get home, of course, along with the handle.
However, from the point of view of sustainability, it is still plastic, and that needs work. Kortenhorst: “We would really like to switch to a biodegradable material. But while the pot, the label and the cover are still all made of plastic, it is better if the film is plastic too. Then they can all go into the recycling together.”

Wide range of uses

A substrate specialist from Lentse Potgrond (a division of Horticoop) spotted the film on the pots during a routine customer visit around six years ago, and it immediately caught his attention. Janssen envisages a wide range of uses for cover film in the trade channel. “We were very impressed with the idea,” he says. “After all, the use of film also has a bearing on the growing medium. It doesn’t physically touch it, as there has to be a 2 cm air buffer between the film and the potting compost for oxygen exchange and to stop mildew from forming.”
Horticoop is keen to raise awareness of the Cleanpot System because it could have potential for many growers and groups of crops. The company has therefore worked with Kortenhorst, the inventor, and machine builders Linthorst Snijtechniek to develop a machine that combines potting up, applying the film and planting the plants in one fluid movement.
Apart from the great advantage of reducing moss and weed growth, the initiators report a whole host of other beneficial effects that occur in the pot. First off, Janssen mentions the homogeneous moisture distribution in the pot. That is because there is no transpiration taking place; the water condenses against the film and drops back into the growing medium. It also reduces the amount of crop protection products, water and fertiliser needed and extends the shelf life of the plant.

Specific reasons

According to Janssen, this is an interesting concept for many ornamentals. From the point of view of sustainability, the water efficiency aspect is something that is bound to appeal to all growers, he believes. “It is also a sustainable way of dealing with soil pests. There are some specific reasons as well. In cyclamen, for example, the film can prevent a botrytis attack on the bottom leaves. And in orchids the system can stop roots crossing into other pots. Sometimes the roots from one pot will grow into other pots nearby, and when you pick up the pot, you drag the other plant along with it.”
The more experience growers gain with this system, the longer the list of applications and benefits gets. Bunnik Plants from Bleiswijk in the west of the Netherlands are also testing the system. Improving shelf life in the supply chain is an important issue for them. Kortenhorst again: “If it was only about moss and weeds, it would have died a slow death. But when it is a question of sales or no sales, then it’s a no-brainer.”

Running own business

Kortenhorst runs the 2.5 hectare rhododendron nursery with his sister and brother-in-law. Plastic film greenhouses are not what you’d necessarily expect to see in the middle of verdant countryside resplendent with farms, villas, cows and sheep. But the oldest greenhouses have been here for almost 30 years.
His sister Gonny set up the nursery after completing her horticultural training, on the spot where their parents’ cows used to graze. Her husband Jos joined her five years later. And five years after that Frans decided to give up his job as a mechanical engineer at a machinery factory and join them. “What attracted me to running your own business is that you can shape it the way you want it.”

Time for innovation

Today’s nursery looks nothing like the one that started out all those years ago. Back then they grew a wide range of plants which they sold locally, focusing mainly on the retail market. Twenty years ago they decided to sell their products via Flora Holland and gradually reduced the size of their product range. These days Kortenhorst is well known as a specialist grower of dwarf rhododendrons. The plants go to retail and all sales take place by auction. The bulk of the plants are exported.
With the exception of the two busy periods – when the cuttings are planted and in the selling season – the three entrepreneurs do all the work themselves. “We are very happy with our present size; we wouldn’t want to get any bigger. We make time to innovate, which we think is very important and fits well with who we are,” Kortenhorst concludes.


It started out as a solution for suppressing weeds and moss in pots, but the system created by grower and inventor Frans Kortenhorst is much more than that. With a simple piece of plastic film on the pot, the benefits are there for the asking. The system has nothing but beneficial effects during production, for retailers and for the consumer at home, he says. The concept also offers scope for a wider range of applications in horticulture.

Text: Suzan Crooijmans. Images: Rikkert Harink.