Spring greening: Avoiding chlorosis on annuals

Fertilizers are a powerful tool for spring annual production. We aim to produce plants that are free of visible deficiencies that may reduce marketability. However, problems can occur and it is not uncommon to spot yellowing foliage among your crops. Plain and simple, chlorotic foliage, like any other visible deficiency or toxicity symptoms, can reduce crop marketability. This article will go over the three most common types of chlorosis you will see on spring bedding plants, including describing the different symptomatology, as well as the causes of and solutions for remedying the different deficiencies.

There are three different deficiency symptoms that cause leaves to turn chlorotic, and we will cover them here. First, the general yellowing or chlorosis resulting from nitrogen deficiency develops on the oldest leaves. Since nitrogen is a mobile nutrient, it will leave older growth for re-assimilation into the new growth. Magnesium is another mobile nutrient like nitrogen, so deficiency symptoms will also appear on older leaves. Unlike nitrogen, the chlorosis is interveinal; areas around veins in the leaf stay green with areas in between yellowing. Like magnesium, several micronutrient deficiencies including iron, zinc, copper and manganese appear as interveinal chlorosis, but on the newest growth as opposed to the old growth since these nutrients are immobile within the plant. Once you can determine which type of chlorosis you are seeing, it is time to try and determine what is causing it.

The first and most obvious cause of chlorosis from any of these nutrients is simply an insufficient supply to the plant. Using low concentrations of fertilizer during production is common to help curb excessive growth and keep plants more compact. It is also common to use low concentrations or withhold fertilizer altogether when holding plants at the end of production. While using lower nitrogen concentration obviously means less nitrogen is provided, all the other nutrients — including magnesium and micronutrients — are also provided in lower quantities. It can be common to use low fertilizer concentrations during spring crop production, for several reasons. First, less fertilizer provides some growth control, which most crops need. It can also be common to withhold fertilizers as a means of “holding” plants when sales are slow or weather isn’t cooperating with customers. However, there can be other causes of magnesium and micronutrient deficiencies. For magnesium, excess calcium from fertilizers or in the water from hardness can inhibit the uptake of magnesium, as these two nutrients have an antagonistic relationship. Magnesium is also taken up when water is taken up, so diminished transpiration can also cause deficiencies. For micronutrients, elevated substrate pH can make micronutrients less available to plants that require more of them (the “low pH plants” or “petunia group”).

There are approaches to managing crop fertilization to try and prevent or remedy the different deficiency symptoms. For nitrogen deficiencies, providing additional nitrogen is the answer. If plant size is below a target finished size, more nitrogen doesn’t pose a problem. However, if plants are already up to size and additional growth is unwanted, providing more nitrogen is not as simple. Unlike applying magnesium or micronutrients, nitrogen can promote vegetative growth, which is going to be unwanted. In these cases, consider applying a low-dose plant growth retardant (PGR) spray to keep plants compact with increased nitrogen. In addition to using a lower concentration for short-term growth control, consider using a chemical with a short residual life. This is especially important when holding annuals like packs and 4-inch pots, as we don’t want to apply excessive PGR right at the end of production to inhibit growth of these crops when they are transplanted.

If a magnesium deficiency is visible, apply a drench of magnesium sulfate, or Epsom salts, at a rate of 1 pound per 100 gallons to green the crop up. However, depending on how often this problem occurs, it may warrant looking more at water sources and fertilizer selection. If excess calcium is inhibiting uptake, look at if it is hardness in the water or fertilizers causing the imbalance. Consider lowering the calcium or increasing the magnesium in your formulation to get to 3 to 5 ppm calcium per 1 ppm magnesium. If the magnesium deficiency is a result in a lack of uptake, make sure horizontal airflow fans are circulating air sufficiently and try reducing the humidity to promote transpiration. If supplemental light is available, it can also be useful.

Micronutrient deficiencies can be corrected quickly. Applying a one-time drench using a micronutrient blend like Mix of Soluble Traces (MOST) or Soluble Trace Element Mixture (STEM) is useful when you are not sure of the specific element causing the deficiency. If you know what the element is, there may be an opportunity for a more targeted correction. For instance, if the deficient micronutrient is iron, drench using a chelated iron as opposed to the whole suite of micronutrients. Be sure to rinse foliage off after application to avoid any damage.

Again, if micronutrient induced interveinal chlorosis is common, consider longer-term solutions. Perhaps better pH management for your root zone can keep micronutrients more available for uptake. Alternatively, consider changing fertilizer formulations that offer higher micronutrient concentrations at lower nitrogen concentrations. There are specific formulas out there with elevated micronutrient packages for the low nitrogen concentrations common during spring bedding plant production.

Just because you are trying to control growth or hold plants by reducing fertilizer concentrations doesn’t mean you need to tolerate chlorotic foliage. Try to maintain magnesium and micronutrient fertility throughout production to keep foliage green and healthy for customers in retail.