Whether you are working with container ornamentals, field grown trees, landscape plants or turfgrass, understanding fall and winter fertilization strategies is important. Despite this being a steamy August in much of the country, believe it or not your plants are already preparing for winter. Even though it is warm, the days are getting shorter, and most plants sense that. Pretty soon, many of them will start responding to lower temperatures, especially at night. I will briefly discuss here the relationship between plant cold tolerance and nutrition.
I once read a 700 page book on the relationship between plant nutrition and cold tolerance. You can basically boil the entire 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”. Why are some plants more tolerant to cold weather than others? On a cellular level, it basically comes down to how permeable the cell membranes are. Think ofan ice cube tray. When you fill the ice cube tray with water and then freeze it, the ice cubes are a little bigger than the volume of the water that was in the tray originally. The same thing happens when cells freeze. They expand, often beyond the ability of the cell wall to contain them. When the plants thaw, the liquid in the cells melts, and it often leaks out through the broken cell walls. That is why recently frozen tissue often looks water- soaked.
So what can plants do about this, and how can we help? As plants sense the approach of fall and winter, they increase the content of sugar alcohols in their cells. The sugar alcohols help the sap resist freezing. Think of it as “plantifreeze”. The ethylene glycol antifreeze that you put in your radiator for winter is a sugar alcohol. Your plants or turf will be starting to do the same thing pretty soon, putting antifreeze in the form of sugar alcohols in their cells to help them resist freezing later.
To better understand fall fertilization strategies going into winter, it helps to understand what nutrients are important in sugar alcohol production. That partly comes down to stimulating chlorophyll. We don’t want to apply excess nitrogen in the fall, as young tender growth will likely be freeze damaged. However, nitrogen deficient plant tissue is less tolerant to cold then tissue with normal nitrogen levels. Nitrogen and magnesium are both part of chlorophyll, so make sure your plants are not low in these nutrients. Potassium is very important in helping the plants close the stomates (pores in the leaves), and in maintaining root health. It often helps to shift the nitrogen – potassium ratio toward potassium as winter approaches. This way the plant can “close the window” when cold, dry, windy winter weather approaches.
The micronutrients are also important in cold tolerance. The reason is that all of them are directly or indirectly involved in chlorophyll manufacture and ultimately, sugar alcohol production. As days shorten and soils cool, micronutrient absorption can be reduced. This is a good time to apply foliar micronutrient sprays, before the tissue begins to shut down for winter. Calcium and silica are also important in cell wall structure and function . Having good strong cell walls will help a plant resistant the potentially damaging effects of freezing weather.
Spraying foliar micronutrients while you still have functioning foliage will make sure your plants have adequate minors going into the winter for cold resistance. Adding potassium nitrate to the micronutrient spray will help shift the plant balance toward stress – fighting potassium. It will also aid in foliar absorption of the micronutrients. I like to do this two or three times before fall really sets in. Next, spraying a Cal – Mag chelate will help in two ways : the calcium will help strengthen cell walls, and the magnesium will help stimulate chlorophyll so the plants can make enough sugar alcohols. Finally, sprays with potassium silicate or other silica sources will help strengthen plant cell walls going into winter.
While you still have time, think about whether your plants might be low or deficient in anything going into winter. Making these adjustments and corrections can have a significant impact on how well your plants will survive and recover from the effects of cold weather. Footballs are starting to fly around the country. Make sure your plants have a strong defense going into winter.