Weekly Express-News ArticleSaturday, November 26, 2005
By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist   “Cold Weather and Your Plants”  

            The cold weather over the last two weeks has put lawns in many neighborhoods on the road to dormancy.  You can recognize dormancy by browning.  The browning begins in the most open or lowest parts of the lawn, which are the coldest spots.  The browning means that it is too late to fertilize the grass with a “winterizer” fertilizer.  The lawn must have green fully-functioning grass blades to take up high levels of nutrients from fertilizer.  Dormancy also means that it is time to cut way back on watering if you have not already done so.  No level of irrigation will green-up a dormant lawn when the weather is cool.  What the watering may do is stimulate weed growth. 

St. Augustine grass is the last grass species to go dormant.  Some winters it will stay green long after zoysia, buffalo, and Bermuda grass have quit growing for the winter.  St. Augustine also is the most cold sensitive in terms of freeze damage.  A dormant lawn is less susceptible to freeze damage. 

There is another good reason to reduce lawn watering during the winter.  Water companies determine sewer service rates during the winter on the basis of average water use for 90 days during three consecutive billing periods beginning in November and ending in March.  The theory is that most of the water used during these months is used in the house, and all of that water goes down the sewer and must be treated.

Some gardeners received below-freezing temperatures during the cold spell.  It is not necessary to remove the leaves on tomatoes or tropicals that were tip-burned from the cold, but in situations where tomatoes and tender annuals were frozen back significantly it may be time to pull the plant.  Collect the full-size fruit to ripen in the house and compost the rest.

The freeze was probably not long or hard enough to kill stems on woody tropicals like bougainvillea, hibiscus or citrus.  Move them to freeze-proof shelter where it is possible and organize your protection for the next freeze.  A plastic tent works if it does not touch the foliage and for especially cold temperatures (under 29º F) a heat source is provided.  A mechanic’s light or even Christmas lights work for the heat source.  Better than plastic alone is a blanket or fabric inside the plastic.  Remember that a clear plastic tent warms up within minutes after the sun appears.  Within a short time the plants that escape the freeze may be fried!  Open up the tent for air circulation as soon as temperatures rise above freezing. 

For light freezes an agricultural fabric or old sheet draped over the plants may be enough and it is certainly easier than propping up a tent.  There are many plants that do not need any protection from San Antonio winter cold.  In your flower garden in the sun, the pansies, snapdragons, ornamental kale, calendula, dianthus, alyssum, and stocks will live through all, but the coldest weather.  In the shade the cyclamen and primrose will bloom every day through a normal San Antonio winter.  Vegetables such as broccoli, cauliflower, spinach, cabbage, carrots, rutabagas, turnips, Brussels sprouts, Swiss chard, and onions are also very cold tolerant.

A healthy well-watered plant survives cold better than a dried out plant, so watering when the plants are dry is essential, especially for one that is actively growing in the winter.  That does not mean, however, that watering at the last minute before a freeze will contribute to cold tolerance.  Cold tolerance is dependent on a plant’s genetics and its health at the time of a freeze.  For the watering to have an effect it must have been applied as needed to maintain the organization of salts and starches within the plant.  Happy Thanksgiving to San Antonio gardeners.