Garden retailers are reminded that annual flowering plants waiting for sale may need to be fertilized. Rainfall and watering with plain water will leach out nutrients and plants will exhibit deficiency symptoms such as yellow or reddish foliage. If no additional fertilizers are applied, plant quality will quickly deteriorate, especially plants that will not be transplanted, such as hanging baskets. Plants in packs, 4-inch and other small containers are also susceptible to nutrient deficiency. The little soil volume in the smaller containers allows the nutrients to quickly leach from the container.
Depending on the plants and container size, options include using a water soluble fertilizer or topdressing with a controlled-release fertilizer according to directions (if it was not applied by the grower prior to shipping). Retailers should communicate with their wholesale suppliers about controlled-release fertilizer or look for fertilizer prills in planters.
Before fertilizing, check hanging baskets and mixed planters for controlled-release fertilizer (CRF) prills. Squeeze a few to see if the prills are depleted. If you find mostly empty prills, or if prills were washed out, another application of CRF may be needed. A good option for mixed planters and hanging baskets is to topdress with controlled-release fertilizer according to the label when the plants are sold.
If water soluble fertilizers are used, ideally plants should be fertilized with every irrigation using a fertilizer injector. For retailers who don't have a fertilizer injector, an inexpensive "Hozon" proportioner can be used. A Hozon is a much better option than lawn and garden injectors which are designed for home gardeners. A Hozon has a ratio of about 1:16; 1 gallon of fertilizer stock will make 16 gallons of fertilizer solution. The goal is to maintain plant quality, not to promote growth. Therefore, the optimum fertilization regime should include a neutral fertilizer (e.g. 17-4-17) at a low rate (between 50 to 100 ppm N) with low phosphorus (P). The purpose is to maintain the pH and electrical conductivity (EC) of the growing media and feed the plants with enough nutrients to prevent nutrient deficiencies. Fertilizing with every watering may not be an option if staff and/or injectors are limited. In that case, a higher fertilization rate (150-200 ppm N) may be used once a week. Maintaining fertility levels in the sales area will extend the life of plants in small containers and the period of time the flowering basket looks good for the consumer.
Tina Smith, UMass Extension Greenhouse Crops and Floriculture Specialist