Fertilization programs for perennials

Mineral nutrients are provided to plants using either water-soluble fertilizers (WSF) or controlled-release fertilizers (CRF). Just like comparing Pepsi and Coke, coffee and tea, or Dunkin’ and Starbucks, talking about WSF and CRF can elicit strong opinions. When we take our preferences and set them aside, there are distinct and different advantages to each of these fertilizers. But when it comes to containerized perennial production, who says you need to choose between them for perennial production?

Our approaches to producing containerized perennials has changed dramatically over the past 20 years. The ability to schedule, program, and grow crops with the predictability of potted flowering plants and containerized annuals is a testament to the research and development that have gone into these crops. In addition to refining containerized perennial crop culture, perennials also have a long marketing window. From the early spring season through fall, and into the winter in some parts of the U.S., perennials can be sold for use in outdoor landscapes. This wide window also allows for producers to use a variety of environments (greenhouses, outdoor spaces or a combination of the two) to grow perennials.

One of the first things to think about is simply the growth rate of perennials. Although perennials have been “annualized” to a degree, with respect to their predictable production, in general, they still do not have as fast a relative growth rate as annuals. Using WSF allows you to change the fertilizer concentration on an as-needed basis, increasing as perennials are established after planting, or after they come out of dormancy for forcing and begin actively growing. But CRF can also be useful, especially if care is taken to select CRFs with a release pattern that meets plant requirements.

While some CRFs simply start to release once they have been hydrated, some CRFs have specific release patterns. Some have a fast initial release, and this is meant for plants that start to establish and start growing in containers quickly, such as planting field-dug crowns into 1-gallon containers. Alternatively, some CRFs have a slow or delayed start to release, and these are well suited to crops slower to establish and start growing, such as planting smaller plugs (i.e. 288- and 128-cell) into a 1-gallon container.

In addition to the growth rate of perennials, fertilization needs to be matched to the crop schedule. Would you rather pay a premium for large propagules, such as field-dug crowns or extra-large “landscape” plugs that are pre-cooled, but the greenhouse minimizes the time to finish crops? Or perhaps you would rather start with more economical small plugs, and choose to bulk the crop up in their containers in a low-cost outdoor production space, before cooling them for dormancy and vernalization and subsequent forcing? Both strategies are fine, and WSF and CRF can be used in both instances.

Again, WSF is a straight forward and responsive approach to fertilizing containerized perennials. But if you select the right duration of release for CRFs, they also work. CRFs are available in a wide range of release rates, from as little as two months up to one year. It is important to know these release rates, especially the longer ones, are influenced by growing temperature. CRFs release more fertilizer as temperatures increase and decrease as temperatures decrease. When crops are dormant and being held at cold temperatures, CRFs do note release fertilizer. It is important to keep in mind that when comparing CRF rates (lbs/yd³) for products with different release durations, we are comparing apples and oranges … or petunias and poinsettias. If we compare 5 lbs/yd³ from CRFs with a three-month release and six-month release of the same N-P-K, half as many nutrients are released by the longer release period compared to the shorter. If CRFs with short release duration are used for crops with a production time that exceeds the release duration, additional CRF applications must be made by top-dressing containers, which can be a laborious task.

The production environment will also influence fertilization strategies. For containerized perennials with a summer bulking period, the temperature and precipitation will both influence fertilizer. When rains occur, unwanted nutrient leaching occurs. While some growers will follow a heavy rain with a WSF application to re-establish nutrition, CRFs are also useful for keeping nutrients in the container and available to plants. For outdoor production, the substrate temperature inside containers will affect the release rate differently throughout the year. As mentioned earlier, the release of nutrients from many CRFs is controlled by temperature, with more nutrients being released under warmer temperatures.

In the summer, excessively warm temperatures can cause an excessive release of nutrients. In the spring cool temperatures may cause insufficient nutrient release. This is especially true for cold-tolerant crops that can grow at cool temperatures.

Finally, keep the consumer in mind. What are the fertilization practices of your average consumer buying herbaceous perennials? Well, probably not great. First, sending out well-fertilized plant materials (WSF or CRF) will get plants started off after transplanting. While they don’t have the potential for a longer period of nutrient provision compared to CRFs, the fertilizer salts in WSF help provide nutrients to containerized perennials after planting. Using CRFs with release durations longer than the production time leaves for residual nutrients for the consumer. Remember, the more success consumers have with our products, the more customers we can look forward to in the future.

Much of this article is framed as comparing CRF and WSF. But, the best opportunity for perennial production may lie in using both. As discussed in this article, there are distinct advantages to both CRF and WSF. So why not take advantage of each? By using both CRF and WSF — both in low to moderate concentrations — provides a diversified approach to fertilizing perennials. Given the wide variety in production practices and schedules, try to take advantage of all the benefits WSF and CRF have for your crops. There is no need to pick a favorite. Have your Starbucks and your Dunkin’.