By: Fred Hulme, Ph.D., Published in Livescapes - Landscape Management, Jan/Feb, 2008

As everyday life becomes more hectic and complicated, many consumers are retreating from the time and worry associated with traditional gardening in soil beds. This has led to an explosion in container culture, especially promoting the use of mixed containers that offer instant gratification, variety, flexibility and aesthetic appeal. These items can offer an ever-changing season-long display that can be moved around the landscape area and have dramatic visual impact from day one. Unfortunately, many consumers overlook the extra care involved with proper watering and fertilization of these containers.

There are several basic types of fertilizers available for landscape use including water soluble fertilizer (WSF) and controlled release fertilizer (CRF). WSF may be in granular or powdered form. These fertilizers dissolve in water and instantly supply a veritable buffet of nutrients. However, the effect of each application is not long-lived, and WSF is best used when constantly supplied with each watering or at least as frequently as possible. CRF’s are generally coated to ensure a consistent and predictable supply of nutrients throughout the growing season, and because of this, they offer a high degree of plant safety. With CRF the entire fertilizer dose for a crop is applied only once at the beginning of crop production. This one application continuously feeds plants over an extended period of time.

Soilless System Needs
Plants growing in soil beds have the moderating influence of mineral soil to help them thrive throughout the growing season. Mineral soil will hold and supply many nutrients, remain fairly stable throughout the season and can hold significant moisture, allowing ornamental plants to survive periods of drought. As consumer preference switches from garden beds to mixed containers, the growing media generally used is “soilless,” comprised of peat moss, composted bark or perlite, for example. While these manufactured growing mixes are excellent for quickly, efficiently and consistently raising ornamental crops in greenhouses and nurseries, they do not hold nutrients or water for long periods very well, and can almost be considered hydroponics systems.

In greenhouse production, plants may receive a constant diet of a WSF that is fitted to the irrigation water and provides all the essential nutrients. However, when plants leave the protected environment of the nursery, water and nutrients often take a back seat to other considerations. Many garden centers can provide little care, especially if plants remain there for long periods and consumers may never feed these plants at all after purchasing them. In many cases, frequent feeding with WSF at the necessary concentrations is not a viable practice. Many mixed container plant species are heavy feeders and vigorous growers. Inadequate fertilization will quickly lead to nutrient deficiencies and a decline in appearance.

This presents an opportunity for landscapers. Consumers are buying bigger and showier mixed containers every year. Usually, these containers are quite expensive to purchase, and their quality will quickly decline if they are not fed properly.

Controlled Release Benefits
The use of CRFs can be an effective and economical solution to this problem. CRFs are very easy to apply. Fertilizer prills (or particles) can be blended into the growing media at planting or can be top-dressed on the growing media surface once the container is placed in the landscape. The product begins to activate very quickly, and within a week, the CRF will begin to dose nutrients to the root-zone.

After applying a CRF, the consumer only has to keep the container sufficiently watered. For the small cost of the fertilizer dose and the labor to initially apply it, landscapers can ensure mixed containers have adequate nutrition to reach their full potential throughout the growing season. CRFs also help maintain better foliage color and bloom production due to constant, uniform feeding.

Key considerations for CRF product selection and use include:

• Nutrient content, including micronutrients.
• Homogenous nature of the product – is it blended or are all particles the same?
• Stated longevity of the product – does it match your growing season?
• Appropriateness of the rate for intended crops and application.
• Other fertilizer sources – is a WSF also being used?
• Growing media make-up – tighter mixes should have lower rates.
• Plant types.

Combination Programs
So what is the best fertilizer method for maintaining mixed containers? Some researchers have reported that using combination fertilizer programs (a low-to-medium rate of CRF, along with supplemental applications of lower concentrate WSF as needed) can round out a potentially inefficient fertilizer program. Even if the current program contains only WSF, combination fertilizer programs with both WSF and CRF can be beneficial and certainly increase customization potential.

• For landscapes that contain many crops types, one simple WSF program can be used on all plants; then CRF can meet the additional needs of heavy feeders or plants with special fertilizer requirements.
• WSF can be used to spoon-feed specific nutrients (like iron) or acidify containers when needed.
• CRFs provide a base feed when one can’t use water solubles – during cool, cloudy weather (no need to irrigate), or when there is no time to irrigate or mix up fertilizer solutions.
• CRF will help maintain root-zone nutrient levels during periods of frequent and heavy rains.
• Since CRFs are coated, the use of them in the garden can help minimize nutrient run-off into the environment compared to liquid feeds only.

ICL-SF recommends landscapers choose a WSF program for mixed containers based on a complete water test. The CRF component should provide a steady and extended release of N-P-K, Mg and minor elements. Match CRF longevity with growing temperatures and desired delivery time. In most cases, a CRF with eight to nine months of longevity will perform well in the landscape. Refer to the label for specifics.

Recommended rates:

• Use the low CRF rate for salt sensitive, tender species such as fuchsia, begonia, fern and impatiens; bedding plants; high water retentive media; minimal leaching.
• Use the medium CRF rate for more vigorous species and heavy feeders such as trailing petunia, with low water-retentive media (containing coarse bark) or in situations with frequent leaching.
• WSF concentrations need to be lowered when used in combination with CRF. Reduce your constant feed to 50 to 75 ppm for bedding plants and 100 to 150 ppm N for flowering pot crops and baskets.

By following these suggestions and tips, you can provide your clients with mixed containers that provide consistent beauty to their landscapes throughout the growing season. Remember, when using any fertilizer, it’s best to study the product and consult with the manufacturer for optimum results.

This article was first published in the January/February, 2008 issue of Livescapes by Landscape Management.