Nursery growers north of Florida frequently inquire about ideal plant fertilization recommendations in the fall. It’s a tough question, as excess nitrogen uptake late in the season can cause plants to go into winter too juiced up, leading to split bark and ultimately causing severe injury or even death.

As nitrogen/NPK is a must-have for the growing situation, Harrell’s POLYON® controlled-release fertilizer products are a smart, proven option to deliver steady nutrients all season long.

POLYON® in the Fall

As fall temperatures drop, POLYON® fertilizer slows its release, ultimately shutting down completely when the container media drops below 55 degrees. With the fertilizer slowing its release to a standstill, it prevents plants from gaining more nutrients than necessary in the dormant season and saves plants’ lives.

However, in recent years, many in the dormant season have used Harrell’s 0-0-22 K-Mag as a fall application and seen very promising results. Applied to container and nursery plantings in the six to eight weeks leading up to dormancy gives the plant time to take up the 0-0-22 before shutting down for the winter. Harrell’s 0-0-22 K-Mag fertilizer helps the plant store up nutrients in the fall before heading into dormancy. (Think bears feasting on big meals before hibernating for the winter.)

Potassium Benefits in the Fall

Harrell’s 0-0-22 K-Mag contains 22% Potassium. Among its many benefits, Potassium helps regulate the opening and closing of stomata. As fall temperatures fluctuate—warm in the daytime and cooler at night—healthy plants with adequate potassium can regulate properly, making them more cold-tolerant and resistant to stress.

Additionally, potassium promotes healthy root growth, and the healthier the root, the more the plants become cold & stress tolerant. Stored potassium also leads to a healthier and more vibrant initial flush in the spring and increased bloom/flower production.

Magnesium Benefits in the Fall

Harrell’s 0-0-22 K-Mag also contains 10.5% magnesium, which is yet another added benefit for the plant. Magnesium is the center of the chlorophyll molecule and helps plants produce more photosynthates (aka sugars) in the fall. More sugars help cold resistance by lowering the freezing point of the plant. This builds a stronger, healthier plant overall.

Magnesium gives plants the ammunition they need to go into dormancy and be completely ready for a strong start to the spring growing and selling season. That’s especially good news for growers selling to box stores, as many of those plants arrive at their destinations before flushing.

Application Rates

Harrell’s 0-0-22 K-Mag has been container and field-tested for several years, with more than impressive results. Rate-wise, based on the many unofficial trials we have conducted over the past few years, I recommend adding one tablespoon of Harrell’s 0-0-22 K-Mag per container size.

Thus, one tablespoon on a one-gallon plant, three tablespoons on three-gallon plants, and so on. For field-grown plants, we have found excellent results at a rate of 1/4 cup Harrell’s 0-0-22 K-Mag per inch of caliper—1/4 cup for a one-inch tree, 1/2 cup for a two-inch tree, etc.

Of course, it is important that we have a normal, cold winter. We need the plants to shut down after absorption, and if the winter months are unseasonably warm, the plants can utilize the benefits 0-0-22 before intended.

A Few Other Notes

Finally, it is worth noting that Harrell’s 0-0-22 K-Mag does contain 21% combined sulfur derived from sulfate of potash magnesia, but that the sulfate ion is NOT acidic. Therefore, this product will not affect the pH of the plants or media where applied.

Feel free to contact me directly with any questions you might have about this product, or reach out to your local Harrell's rep. Again, we have seen promising, horticulturally sound results with growers in TN, AL, GA, MS, TX, and IL and I highly recommend trialing this at other nurseries across the country.