1/17/2022 - By Lynn Griffith

Fertilization strategies are generally different for plant production and turf maintenance in winter. The differences between winter and summer conditions of course vary depending on what part of the world you are in. However, the laws of chemistry, physics and plant physiology are constant. Because of reduced sunlight and thus reduced photosynthesis in winter, those nutrients that are important for sugar production and carbohydrate metabolism are critical.

In turfgrass, fall fertilization is important, often with emphasis on potassium, depending on rainfall and soil conditions in your area. Helping turf establish a deep root system will help turf make it through severe winter conditions. Irrigation strategies and timing can also be important. Irrigate less often but for longer duration to encourage deep root growth.  As always, the best way to control weeds is to grow good grass.

Plants and cold tolerance

I once read a 700-page book on the relationship between plant nutrition and plant cold tolerance. While the book was educational and informative, I will save you the trouble of taking it on. You can basically boil this book down into one sentence, “A plant will tolerate cold better if it has everything it needs nutritionally than if it is deficient in something.”

Some experts, even those in the tropics, say that you should not fertilize outdoor plants at all in the winter. The common thinking more or less, is that if you induce tender new growth with nitrogen fertilizer, for example, that growth is susceptible to being burned off when a cold snap hits. Okay, fair enough. If I were growing plants outdoors, say in the Midwest or New England, I wouldn’t want to be putting out nitrogen fertilizer right now. Even if you get a nice stretch of weather, inducing new growth this week can lead to disaster next week.

Nitrogen

However, plants do need some nitrogen for enzymatic processes. Enzymes are proteins which have metabolic functions. Proteins are made of amino acids. The amino group is NH2, so clearly, some nitrogen is required. How much depends on your situation. Outdoor plants in the north probably don’t need any nitrogen fertilization in January, nor should they receive it.

What if you are producing Easter lilies or hydrangeas for a spring crop in the greenhouse? Nitrogen fertilizer would certainly be required in order to finish the crop on time. How much? Well, it depends in part on what region of the country you are in and what greenhouse temperatures you’re able to maintain. Many greenhouse growers apply roughly half the amount of fertilizer in winter as they do in summer.

Potassium

Potassium has over 200 functions in plants. Three of them are most important for cold tolerance.

• One, potassium helps regulate the opening and closing of the stomates. This is very important if cold winter winds are blowing and humidity is low.
• Two, potassium is critical for root growth, especially in monocots such as turf. Many fall and some winter turf fertilizer programs emphasize potassium.
• Three, it plays a large role in carbohydrate metabolism.

Carbs

Let’s talk carbs for a minute. In the winter months, the temperatures of course tend to be cooler both day and night. Not only are the days shorter, but the angle of the sun is lower and the total amount of light plants receive is reduced. This lowers the rate of photosynthesis and thus the rate of carbohydrates within the plant. This brings the role of micronutrients into play.

Micronutrients

Here is micronutrient function oversimplified:  Iron is involved in chlorophyll synthesis. Manganese is involved in break down of carbohydrates into sugars. Zinc is involved in transformation of carbohydrates and regulates sugar consumption in the plant. Boron helps plants make sugars and move them around. What do all these have in common?

Sugar

Sugar! With shorter days and lower light intensity, the plant has less light to catch, threfore plants are making less sugar. So, if one of these sugar producing micronutrients is deficient, the sugar problem in the plant is compounded. I like to build up micronutrient levels in the fall using foliar sprays such as Harrell’s Minors. In greenhouse production and during good stretches of weather outdoors, you can also apply foliar micronutrients to help improve cold tolerance.  This activity can help increase cold tolerance without inducing tender new growth.

Plant nutrition is important in both winter and summer, though the strategies are significantly different. In short, I do not believe in starving plants during the winter for most situations. However, growers have to be very judicious in how and what they apply to their crops. They also need to be ready to put the pedal to the metal fertility wise when spring comes.

For more information on any of these products or strategies on how you can best serve your plants, reach out to your local Harrell's Representative.