Selecting the Right CRF for Your Greenhouse: Q & A with Dr. Fred Hulme

February 20, 2014
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Q: Fred, I’d like to start using controlled release fertilizer in my greenhouse operation, but I’m not sure how to select the right one for my crops. What are the factors that I need to consider?

--Dan, Grand Rapids, Michigan

A: Dan,

Using controlled release fertilizers (CRF) products is a great opportunity for today’s greenhouse growers to save time and money, produce high quality crops and reduce potential nutrient run-off. CRFs can be used alone or in combination with water soluble fertilizers to efficiently fertilize greenhouse crops. Many growers in the past few years have begun to work CRFs into their production systems, but there is still a lot of confusion with all the CRF products on the market today, each with a different combination of nutrients, components and technologies.

Since greenhouse crops are relatively salt-sensitive and grown in smaller containers in tighter, peat-based growing media, many CRF products that work very well in the nursery may not be suited for the greenhouse. When selecting fertilizers, first consider other nutrients that may be present in your irrigation water, growing media and any accompanying water-soluble fertilizers you may be using.

Once you identify the gaps, read the guaranteed analysis of CRF candidates to determine their nutrient content, including nitrogen types and secondary nutrients like magnesium and micronutrients. For your application, here are some general guidelines to follow. First of all, choose a product that:

  • Is 100% coated (for safety)
  • Is homogeneous in nature so that different types of granules are not unevenly distributed throughout pots, a situation which can cause inconsistent feeding. Remember that blended products can segregate and lead to uneven fertilization, especially in small containers.
  • Offers a controlled release of N-P-K and magnesium
  • Includes micronutrients for complete feeding
  • Has the longevity to match cropping systems and any need for sustained post-production feeding

Product rates are an essential part of success. Label rates on a fertilizer bag are generic and suggested for a wide range of situations, and these suggestions should be considered as starting points for growers. To determine which rate works best for any situation, conduct a trial. In the greenhouse, I would advise growers to be conservative and start with lower rates to ensure a high degree of safety while assessing the benefits of the CRF. This would especially be true if water soluble fertilizer is being used. The CRF can provide a base feed, and you can use water soluble fertilizer to tone plants or treat them with prescriptive additives like chelated iron.

It’s important to note that different crops or cropping cycles have different nutrient requirements during certain points in their growth cycle. Some plants, like annuals and perennials, are heavy feeders early on, they need extra nutrition at that time or plant quality may suffer. That’s why it’s essential to match the CRF product to the nutrient demands of the plants being fed—not just what nutrients they need, but also when they need them. For example, the Osmocote® Plus family of coated fertilizers from ICL Specialty Fertilzer incorporates  Patterned Nutrient Release Technology™ that feeds plants earlier or later in the growing season depending on their specific feeding habits and formulation you select. An Osmocote® Plus Hi-Start® product can supply nutrients early in the crop cycle when they are needed. An Osmocote® Plus Lo-Start® product can be used to provide post production feeding for the garden center and consumer long after the plant has left the greenhouse. The bottom line is that choosing a CRF that releases the right nutrients at the right time is key to the crop’s overall success.  Osmocote® Bloom is a mini-prill option that is perfect for smaller containers, it is available in 2-3 month and 5-6 month formulations.  Osmocote® Plus 7 gram tablets are also very easy to insert into mixed hanging baskets near the high nutrient demand plants like petunias.

If you select a coated fertilizer that’s not suitable for your climate or crop type, or apply the fertilizer at an improper rate, nutrients may be released when your crops don’t need them. Always carefully consider crop type, feeding habit, length of growing season, timing of application, container size in relation to plant size and irrigation/rainfall.

If you need additional help, keep in mind that companies that produce CRF, like ICL Specialty Fertilizer, usually have field representatives who can help greenhouse growers develop successful fertilizer programs on-site. Good luck with your fertilizer program!

Have a question for Dr. Hulme? Simply e-mail it to [email protected].