It may be useful to supply time-release fertilizer in the pot to carry it through the growing season.
Photo courtesy of Sinclair Adam

Plant nutrition is a very important aspect of producing quality plants for the marketplace. The supply and absorption of chemical compound required for growth and plant metabolism is defined as nutrition, and chemical compounds required are referred to as nutrients.

Plant nutrients are: Carbon (C), Hydrogen (H), Oxygen (O), Phosphorus (P), Potassium (K), Nitrogen (N), Sulfur (S), Calcium (Ca), Iron (Fe), Magnesium (Mg), Molybdenum (Mo), Boron (B), Copper (Cu), Manganese (Mn), Zinc (Zn), and Chlorine (Cl). Lately, Silicon (Si) has been shown to provide some benefit in disease resistance and drought tolerance, and a number of potting media producers are putting silicon into their products. C, H, and O are generally derived from the atmosphere, while the others need to be provided in the potting medium or by supplemental fertilization.

Finding the right fertilizer

Once they get to the store, plants may not receive the consistent watering they did at your greenhouse.
Photo: Laura Watilo Blake

Many choices of fertility are available to a grower, with a good array of product formulas available. Both time-release products, such as Osmocote or Nutricote, as well as liquid soluble formulas, are options a grower can elect to use in a crop fertility program. Often Ca and Mg are incorporated in peat-based media as limestone in order to adjust the pH by the manufacturer. In many cases, this incorporated nutrition is adequate for the crop cycle, but with some plants, additional doses of Ca and/or Mg are required. Plants such as bacopa, calibrachoa and petunia are iron-inefficient uptake plants, and may require pH adjustment or additional applications of iron to finish well. Iron chlorosis can become a problem with these crops when pH or Fe availability are not adjusted properly.

Water condition considerations

By conducting a test of the water used for the crop irrigation, a grower can determine the alkalinity of the irrigation water and the pH of the water. Then, by using an alkalinity calculator such as the excellent online one from University of New Hampshire (, a grower can design the required pH adjustment for a good fertility program. Many operations inject acid into their water for irrigation to provide a proper pH for their crops in central Pennsylvania, with well water pH levels above 7.0.

Finishing phase fertilization

A number of flowering plant crops are grown with a finishing phase of production where nutrient levels are reduced to help tone the crop in its final phase. This practical approach to crop culture has been well-researched and documented over a number of years.

Plants like petunias, which are known to be fairly high-fertility requiring plants, are generally grown at 150 to 200 ppm N on a constant feed basis, or 1.0 to 1.5 mmhos/cm EC (based on a 2:1 extraction method). Recommendations for the finishing phase of petunia culture are to reduce the feed rate by half in the last two weeks of culture.

Perennials have also been successfully treated with this strategy, and plants grown at 136 ppm N were shown to be just as high quality when the rates were reduced to 75 to 80 ppm N in the final phase of crop cycling. A grower using this strategy would be aiming for an 85- to 95-percent level of fertility maximum for the peak growth phase, subsequently reducing the rate to half of the maximum for the finishing phase. Compared to a constant feed program, cycling can save money, reduce environmental impact, and growers can do this with a number of crops.

Ensuring success in big-box plants

In producing plants for the big-box stores, where crop fertility maintenance may not be as carefully tracked as with an independent operation, some important considerations need to be addressed. Usually, the shipment arrives from the producer company with a good fertility status. But if supplemental fertility and proper watering practices are not followed, the shelf life of that crop will be limited.

For example, two such big-box operations exhibited tomato plants at the beginning of June that were totally flagged and dehydrated after two days of considerably high winds. Many of these operations have the plants in exposed conditions, and natural rainfall also leaches nutrients. Obviously, watering was not carefully attended to at those locations, and supplemental fertility is not typically provided after receiving the plants from the grower either. This creates a situation for the big-box operation supplier, where the plants, although grown properly in production, will diminish in quality the longer they remain in the big-box store. The consumer, who is unlikely to provide additional fertility after purchasing the plants, will wind up considering the plants of inferior quality as they become chlorotic, and growth diminishes over time. While the producer was in no way at fault, the consumer’s tendency may be to blame the producer company, and not the big-box store maintenance program. The consumer frequently is not informed about the production system that makes it possible for plants to come to the marketplace, which could lead to further misinterpretations.

In an independent garden center, the consumer is usually informed of the need for supplemental fertility once the plant leaves the greenhouse or nursery. Many independent operations provide a good array of fertilizers for their consumers, and are quite likely to inform the purchaser of the proper fertility requirements for the plant they are purchasing.

Growers can feed at a higher rate before sending pots and baskets to big-box stores to prevent fertility problems for the consumer.
Photo: Laura Watilo Blake

Overcoming challenges

One strategy that may work to overcome this problem is to feed at a higher rate than typical prior to shipment to the big box. Plants that are sold in pots and hanging baskets can be provided with a time-release fertilizer product prior to shipment to the big box store. This fertility should be selected for minimum of four months duration, and a number of these time-release products can provide fertility for intervals longer than four months.

One such choice could be Osmocote tablets, which claim eight- to nine-month longevity, and have demonstrated this claim successfully in the Penn State Flower Trials where many of the plants are grown in 5-gallon containers. Proper fertility is provided through the summer and into early October with this strategy. Supplemental fertility is added as needed by using liquid soluble formulas, injected through the irrigation system. By monitoring the pour-through leachate, and taking potting media samples and tissue samples throughout the growing cycle, these levels can be recorded and adjusted as required. This is precisely what most growers are currently doing in our industry.

Such careful management of fertility is not applied at present in a big-box situation, as the leachates are usually not being monitored and levels adjusted by staff at the store. Certainly there are watering issues at times as well. These issues, coupled with the lack of information on plant care usually dispensed, result in an unsuccessful experience in many cases for the consumer.

To mitigate those problems, it can be very useful to supply time-release fertilizer in the pot to carry that plant through the growing season. If this is impractical with a product such as a pack or flat, consider getting custom blended feed incorporated in the potting medium. If custom blending of media is not practical, then at least finish the product with a rate 85 to 95 percent of maximum liquid feed rate. This fertility management program gives the product the best possible opportunity for success in a situation where crop maintenance may not be properly carried out.

Sinclair is an extension educator at Penn State University specializing in floriculture, propagation, breeding and nutrition.