Focus On Fertility: Coleus

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Coleus is a versatile plant that is favored in container and landscape settings due to its rapid growth and high tolerance to environmental conditions. To homeowners, it is known as the plant that almost anyone can grow well. Its varying and unique habits, from trailing to mounding, shade to sun and a rainbow of colors and leaf shapes allow flexibility for any landscape. So how does this affect fertility when producing these plants in the greenhouse?

At Propagation

At the youngest stage, either sown seed or vegetatively propagated, coleus can obtain nutrition in the media when kept in an evenly moist environment at a pH between 5.5 and 6.0. However, rather quickly after germination or transplant, it is important to focus on establishing a strong and healthy root system. Begin by applying fertilizer on a constant basis at a low rate of 50 to 75 ppm. You can achieve the most control at this stage by using a water-soluble fertilizer, feeding until root saturation and allowing for ample drying time for maximum drainage and disease prevention.

Early Growth

At the early stages of growth, coleus root development can be affected by high salts. A good target for EC in the root media is below 1.0 mS/cm. Keep ammonium nitrogen levels in the root zone less than 10 ppm by choosing high-nitrate-based fertilizers like, 13-2-13, 16-2-15 or 15-5-15. These formulas are designed with a balance of nitrogen that is easy for the plant to take up and also provide readily available phosphorus to encourage the growth and branching of a healthy root system.

Growing On

Growing on, coleus remain low to moderate feeders. The most efficient nutrient uptake will occur when the media stays in the targeted 5.5 to 6.0 range.  Understanding your water quality is key to choosing the best fit fertilizer for the remainder of crop cycle. In general, high-nitrate fertilizers are potentially basic and tend to produce sturdier, stockier plants, while ammoniacal and urea-based fertilizers are potentially acidic and produce softer, more green and lush growth. If your water source is higher in alkalinity (and usually higher in pH) plus it provides 50 to 70 ppm calcium at each feeding, select a fertilizer that is potentially acidic, like 21-5-20 or 20-8-20. If it is necessary to add more magnesium, you can add epsom salts directly to your stock tank.  

If your water source is lower alkalinity, you may need to increase or maintain your pH and add calcium at each feeding. Potentially basic formulas such as 13-2-13, 15-5-15 or 15-2-20 are very good options since they add both calcium and magnesium in one balanced formula. Neutral formulas such as 17-4-17 and 16-2-15 also add calcium and magnesium but are designed to keep the pH change in the root zone to a minimum. Compare these factors when selecting your fertilizer so that you can provide a balanced nutrient profile as well as keep within the pH target values. 

To Finish

Taking the plant to finish you will gradually increase your overall fertilizer rates to between 150 and 200 ppm. Monitor growing media to avoid a gradual rise in pH and EC, not to exceed a pH of 6.0 and an EC of 1.2 mS/cm. Over fertilization or build- up of salts in the media will lead to less intense color variation and decreased plant vigor.

When using coleus in combination containers, do not over plant. Coleus will have the tendency to overtake some of the lower and spreading plant material due to its rapid growth. Homeowners tend to use feed rates of typical garden formulas like 14-14-14 or 20-20-20. This can lead to plants that bloom and become leggy. A combination of pinching with a lower phosphorus fertilizer choice will keep container and landscape coleus looking beautiful all season long.

Dr. Cari Peters is vice president at J.R. Peters. You can e-mail her at caripeters@jrpeters.com.

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