As we head into December, temperatures are dropping in many parts of the country, even in Florida. This blog will cover how to maximize the effectiveness of foliar nutrition around cold fronts. How this information can be used will depend on what part of the country you are in, but either way the information should help growers make better informed decisions on timing of nutritional sprays.

Soil Temperature

Container media temperatures vary far more than field soil temperatures, and more quickly. The air temperature can vary by as much as 50°, yet field soil will only very a few degrees, especially in the subsoil. Container media temperatures follow the diurnal (day and night) air temperatures fairly closely, generally being several hours behind.  In other words, when the sun comes up in the morning, the air temperatures will rise. Soilless media temperatures will also rise, but will be several hours behind the changes in air temperature. Potting soil surface temperatures will vary more quickly than temperatures in the center of the root ball. These observations are important for this discussion.

Media in large containers will respond more slowly to temperature fluctuations than media in small containers. Container type is also important.  Research has shown that media temperatures are about the same whether plants are grown in black or terra-cotta colored plastic  pots. However, media in a white plastic container can be up to 7° cooler. Realize however that evaporative  cooling is also part of the equation. Whether it is cold or hot out, slapping water on your face will reduce your skin temperature. In clay pots, media temperature can drop even lower than the coldest night temperature, by as much as 10°. This is because there is more evaporative cooling in clay pots.

Why does this matter? Think of a typical cold front. Cold, generally dry air blows in from the north or west. As it hits warmer, more humid air, the cold causes the moisture in the air to condense and precipitate, similar to the moisture that forms on a cold can of soda. Warm air can hold more moisture than cool air. This is why cold fronts are often preceded by precipitation, often rain, but sometimes snow or sleet if you are further north.

In a typical fall or spring cold front, it will often rain, which will leach major nutrients which have been released by the controlled release fertilizer  (CRF) prill.  After the rain, it is often clear, sunny and cold , often with wind and lower humidity. The wind and lower humidity will increase evaporative cooling, accelerating the temperature drop in the potting soil. Note that the temperature drop will often be greater at the soil surface which is where the fertilizer is for clients who top dress. Incorporated CRF will be only slightly less affected by dropping temperatures, though the differences may matter.

Think of a typical 3 gallon shrub growing outdoors the day after a cold front. It’s available nutrients are leached, and its media  is cold. If it’s sunny and breezy, the cold roots will have a hard time keeping the plant properly hydrated. The leaves may warm from the sun, but the roots are cold. Therefore, the stomates in the leaves will tend to be closed for a couple of days, which prevents carbon absorption from the atmosphere which is needed for photosynthesis. In short, the plant will not be growing for a couple of days. The media is leached, the soil is cold, the roots are cold, and the plant will basically be shutting down for a couple of days until warmer temperatures return.

Assuming you want your plants to keep growing, this is an ideal time to apply foliar fertilizer, especially N – P – K, as well as calcium and magnesium. You can bypass the roots and put nutrients directly into the leaves. The  N – P – K which was outside the prill is temporarily gone. Calcium absorption is temporarily reduced in wet, cold weather. Magnesium is of course essential for photosynthesis, in that magnesium is in the middle of the chlorophyll molecule.

For growers in the tropics and subtropics where continuous, year – round plant growth is desired, right after a cold front is a perfect time to apply foliar nutrition to keep the plants growing. In more northern climates, the strategy is somewhat different. You may not want to induce growth on dormant or cold – sensitive plants in the fall or winter. However, as spring comes, you may well want to break dormancy and induce new flushes of growth. You can help do this with foliar nutrition after spring temperatures temporarily drop from cold fronts.

Whether growers want to take advantage of this strategy will depend of course on location, time of year and plant variety, as well as market timing considerations. Know however that applying foliar nutrients after cold fronts can make a positive difference. Research in field crops shows that about 50% of foliar applied urea and potassium can be absorbed into the plant within eight hours of application. Urea is the most readily absorbed source of foliar nitrogen for most plants. Absorption of the secondary and micronutrients occurs significantly more slowly.

The take-home message is this: plant growth will almost cease for a few days after cold fronts. You can keep growth going when desired by bypassing the root system, putting nutrients directly into the leaves via foliar sprays. Foliar urea will tend to induce new vegetative growth, while foliar potassium can help increase resistance to cold and drought stresses in the plant.