Buyers' guide - controlled-release fertilisers for ornamental plants

Growers can use these products to increase efficiency and improve profitability, Sally Drury explains.

Container trees: long-term nursery stock - image: HW
Container trees: long-term nursery stock - image: HW

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Q: What can controlled-release fertiliser (CRF) do for growers such as me?

A: The bottom line is that CRFs provide growers with an opportunity to increase efficiency and boost their profitability. At the same time they can also help to promote sustainability - and basically it is down to the fact that CRFs last a long time.

Q: So what are the actual benefits to the crop, to me the grower and to my customers?

A: Once a CRF is put into a growing medium it will last, approximately, for as long as it states on the bag. Repeated applications are not required. That saves you time and money, as well as reducing storage and the disposal of packaging.

Because of their longevity, using CRFs means that plants leaving your control are sent off with a larder stocked to see them into the future. Potted items and planted hanging baskets can leave the nursery with a high concentration of fertiliser that will be delivered safely over a long time. It will last, without being strong at any one time.

That helps when the next feed is uncertain. Hortifeeds technical manager Bill Riley explains: "Liquid feeding in a nursery situation means that the plant is probably getting a perfect feed. But when the plants leave through the sales chain, through storage and planting, they will probably be given little feed. The CRF, however, goes on working and gives shelf life - benefiting the garden centre and, ultimately, the end user."

There is an additional advantage for those growers with outdoor production. Last winter and even this spring have certainly shown us how much it can rain in the UK. When it is wet like that for weeks on end, there is little chance to put a liquid feed on outdoor crops. But with CRFs you are separating the nutrient from the water gift. You still get the controlled release.

Also, where liquid feeding is applied, the method of fertigation is crucial. A lot of commercial nursery stock nurseries have irrigation that is applied overhead. It is applied per bed (and path) rather than per pot and in such situations anything that falls to the ground is ultimately wasted - and that can be as much as 50 per cent. Using CRFs can cut down on that waste and reduce the environmental hazard.

Q: Are there any downsides to the use of CRFs?

A: Once you have put the CRF in it is difficult to take it out again. This means it is essential to get the product right and to get the rate right. Too much can result in over growth and/or damage.

Q: How do these fertilisers actually work and what gives them their longevity?

A: Talking about CRFs rather than slow-release fertilisers, the products are coated. The coating and technology may vary from brand to brand and even within brands, and some may have more than one coating. Inside is a granule containing the nutrients. These can be any combination but usually they consist of NPK plus trace elements, often coated on top of the NPK granule, and then the whole is coated in a semi-permeable membrane through which water passes by the process of osmosis.

As water is absorbed by the granule, so the nutrient content - usually a dry powder in granular format - is dissolved. As more water enters, pressure builds up until the dissolved nutrients are forced out.

The longevity of a product is controlled by the permeability of the coating and by temperature. Like all reactions, there is more activity at higher rather than lower temperatures. So when the outdoor temperature is low, molecules are less energetic and they respond slowly. As the temperature increases, so molecule activity increases and nutrient release accelerates accordingly. Effective release temperature is around 5 degsC upwards and you may expect a five per cent increase in response per degree increase - this is a very rough guide and all products will vary.

Q: Why do manufacturer's state release at 21 degsC?

A: This is the standard or test temperature used to give a level playing field and enable product comparison. It is not meant to reflect growing conditions.

Q: How do I choose the best CRF for my crops?

A: Riley suggests you first choose the longevity to suit the crop. "If you are growing a short-term herbaceous perennial, you might only need a fiveor six-month product. But if you are growing long-term nursery stock subjects, you might be looking at a 12- or 18-month product to suit the cultivation period," he says.

The next thing is to look at the analysis. Riley has more examples: "For woody plants you would need a high N - a typical one would be 16-8-12, for instance. For flowering plants you are likely to want something with more potassium and less nitrogen and a typical analysis would be 15-6-16 or 14-3-19."

Finally, choose a reliable product with proven performance to ensure a timely delivery of nutrients. Then it is a matter of ensuring that you apply the fertiliser chosen at the correct rates for your crops.

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