Fertilizing foliage

With the increasing popularity in foliage crops, have you reconsidered your fertilization approach? While there are certainly similarities to well-established practices in floriculture crop production, there are some key differences as well. Let’s take a look at how to adjust more traditional flowering crops’ fertilization plan to better suit containerized foliage plants.

One of the most fundamental questions is how much fertilizer, or what concentration, should be applied to foliage plants. Before we highlight how foliage crops differ from flowering ones, let’s at look how they are similar.

The range in nutrient requirements across foliage plant species is like the variation in floriculture species — crops range from low to moderate to high nutrient requirements. Now, setting species-specific requirements aside, foliage crops are generally provided with more fertilizer than flowering crops. But it can also be common to reduce fertilizer concentrations for floriculture crops to avoid unwanted or excessive vegetative growth before the crops are finished, as flowers are developing.

For foliage crops, they are not considered “finished” when flowering occurs; rather, foliage plants are considered “finished” when the plant size is appropriately balanced for the container. Since we do not need to promote both vegetative (foliage) and reproductive (flowers) growth, we do not need to use as much nutrient restriction as may be used for containerized flowering plants.

This is not to say that foliage plants should be provided with excess nutrients and finished plants have soft and stretched growth. Simply, the same restrictions don't need to be exercised for flowering plants to balance vegetative growth while flowers are developing.

Fertilizer concentrations should also be adjusted to the amount of light the plants are receiving, or daily light integral. Of course, light will vary over the course of the year in the greenhouse with the change in seasons. However, light may be manipulated for acclimating foliage plants for subsequent retail and consumer environments. In some instances, foliage plants are grown under high light intensities, and acclimation to less-intense light can improve their postharvest performance. When light is reduced, fertilizer should also be reduced. Conversely, if foliage plants are placed under higher light intensities, increase fertilizer concentrations.

In addition to the concentration of nutrients, we also want to consider the ratio of elements to one another. For flowering plants, it is common to have a nitrogen (N) to potassium (K) ratio of 1:1. For foliage plants, this can decrease. Using a higher proportion of nitrogen is desirable to promote the vegetative growth of foliage plants. Phosphorous can promote flowering (up to a point), as well as stem elongation. But with foliage plants, neither of those characteristics is desirable. For most foliage plants, there is no reason to promote flowering, much less enhance it with additional phosphorous. Additionally, while we want to promote vegetative growth for foliage plants to reduce the time it takes for the plants to be finished and reach a saleable size, we don’t want to achieve that with excessive stem elongation. While an N:P:K ratio of 2:1:2 or 3:1:3 is common for floriculture crops, an analysis like 3:1:2 can be good for foliage crops.

In addition to the N:P:K ratio, don’t forget about the ratio of micronutrients to macronutrients. For example, fertilizers with the name “Peat Lite” indicate that micronutrients are approximately double of those found in the original (non-Peat Lite) formulations. These formulations with a higher concentration of micronutrients per unit of nitrogen are useful for foliage crops that are inefficient at taking up micronutrients or simply have a higher requirement for them.

Finally, the type of nitrogen used for fertilizing foliage plants is essential, particularly in terms of their effect on substrate pH. Fertilizers with more nitrate (NO3-) raise substrate pH, whereas a greater proportion of ammonium (NH4-) or urea (which is quickly converted to NH4-) decreases substrate pH. The use of potentially basic or acidic fertilizers to steer substrate pH is used in both flowering and foliage crops. But other factors related to foliage crop production can influence fertilizer selection.

It can be common for air temperatures to be warmer for foliage plant production compared to floriculture crops. This is not necessarily because foliage plants are any more “tropical” or heat-tolerant (but in some cases they may be). Rather, the warmer temperatures increase developmental rates and reduce time to finishing. Although flower quality can decline with warmer temperatures, this is not a problem for foliage plants. Under cooler production temperatures, nitrate is useful when nitrifying bacteria are less active and ammonium toxicity can be a risk. With foliage crops, the warmer production temperatures make ammoniacal nitrogen sources, including NO4- and urea, very useful for production.

Designing fertilization programs for your foliage plants can draw on some fundamentals of flowering plant production. However, using some of the finer points of foliage plant production to further fine-tune your foliage fertilization programs can help reduce production time and increase the bottom line.