Frost Cloth – What do the Numbers Mean?

Sitting in Tucson on a day when the temperature will reach 102^o,  I find it difficult to think about the need for frost protection on my plants. In reality, now is the perfect time to think about what the garden will be needing in a few short weeks. If I don’t plan now for frost protection, the cold will sneak up and plants will suffer or die as a consequence of my poor planning.

Frost cloth has many functions beyond protecting from frost, it offers wind protection, works as a pest barrier and it reduces light transmittance; but perhaps the second most popular reason people use frost cloth is to extend the growing season.  With Tucson as an example, using frost cloth in winter/early spring allows us to plant summer crops like tomatoes and pepper plants before our last date of frost (March 17). This lets us get an aggressive start on the growing season and harvest before temperatures climb to the point of shutting down the pollen. Applied in fall, frost cloth allows us to nurse those tomatoes straight through the winter. There is nothing like a freshly harvested tomato on a gloomy January morning.

In most areas of the country, one of the primary limiting factors for the types of plants that can grow there is how cold the temperatures become. Every plant has a temperature at which they begin to take on physical damage that will impact health and longevity. To determine the level of frost protection needed, it is critical to understand these two data points:

• The average lowest temperatures where you grow. 
• At what temperature do the plants you grow experience damage 

Your local County Cooperative Extension Office has a lot of information regarding minimum and maximum temperatures that is specific to your area. Intellicast.com also has minimum and maximum temperatures for every zip code in the United States.

Once we understand the minimum temperatures that may occur and what plants will have trouble tolerating them, it’s time to decide what type of frost protection to prepare for. Here are the options to consider.

Seedlings Protected by Frost Cloth

The Agribon line of frost protection is known for being high quality and their products are made of an ultra-light spun polypropylene that resists exposure to the extremes of elements. This fabric allows water, light and air to pass through, but not insects. The number in the product name tells you exactly what you can expect in terms of temperature protection.

AG-15 – The lightest weight (.45 oz. per square yard) and least frost protection available. This cloth allows 90% light transmission and is most widely used to protect plants from insects. It is only suitable for a couple of degrees of thermal protection.

AG-19 – Another light weight cover (.55 oz. per square yard) that provides frost protection up to 4^oF. It allows transmission of 85% of light and helps to reduce evaporation. This is an excellent choice for helping to prolong growing seasons in moderate to warm climates.

AG-30 – Medium weight cover (.9 oz. per square yard) that provides up to 6^oF frost protection. There is 70% light transmitted through this cloth.

AG-50 – A heavy weight (1.5 oz. per square yard) protection that works down to 24^oF – meaning it provides up to 8^oF of protection. Since the cloth is denser, light transmission is 50%.

AG-70 – The highest level and heaviest weight (2.0 oz. per square yard) protection. This cloth provides more than 8^oF protection while allowing 30% light transmittance.

When the cloth is doubled (or folded over), the protection doubles and the light transmittance is reduced. So, for example, the AG-30 numbers look like this:

• The temperature protection increases from 6^oF to 12^oF.
• Light transmittance decreases from 70% to 40%.

No matter the season, if you are growing, having some frost cloth on hand is a pretty good idea. The many helpful functions it provides will make your gardening easier and less troublesome.

Contributed by Deb N.