If your blanket is covering something like a hydrangea or semi-hardy shrub, you put them on after there has been sustained cold and leave them on. Depending on your location that could be anywhere from now until the first of the year. I have some that cover a couple hydrangeas but I do not completely seal them up or fill with leaves until we have consistent temps in the 40s and below in the daytime, usually late December.

If you’re covering cool-season, late fall crops then it would be best to remove or at least open them during warmer, sunny days so you don’t run the risk of overheating the plants. They could be left on if the days are cold and blustery. I would treat them similarly to cold frames, just not letting interior temps get too high. The blankets are designed to keep the frost off the leaves but will only do so for a short period; once the weather turns really cold the ambient air under the blanket will be too cold and the blanket can no longer protect the plants.

According to the US Dept. of Energy, depending on how long the temps maintain freezing and below, this is not harmful. Why? Well, the water in a liquid state has to undergo a physical change to transform to a solid. When it freezes it gives off heat energy during that transformation. Once the freezing process stops, the ice will fall to ambient temperatures, so an extended period of freezing would require a continued spraying. The extended spraying, however, would create an increased amount of weight from the ice load, risking damage to the plant from breaking branches. But in addition to the energy released during freezing, two other reasons apply. Ice has a low thermal conductivity, especially if sprayed on in the form of a “snow” and the ice/snow helps to protect the plant from wind, which will wick moisture and heat away (that dreaded “wind-chill” factor). It’s why igloos work or why you burrow into the snow when caught in a snow storm. If you don’t want to water your plants in the freezing temps, make sure that the ground has sufficient moisture by watering the ground around the plants. If you have had a period of drought, your plants may already be stressed and less able to endure a sudden freeze, so in general keeping moisture consistent at the root level is also important.

Because of watering limitations in commercial use, new sprays are being developed to help plants endure sub-freezing temps. Covering
plants with cotton fabric or plastic to protect against cold and frost is a simple physical barrier against the elements, but be sure to remove in the mornings so sunlight can warm the soil and air can circulate around the plants.

The best way to avoid frost damage to your plants is to grow plants that can withstand the frost. The term ‘frost hardy’ is often misleading because of the degrees of frost (i.e., light frost vs. hard, killing frost). It is a good idea to ask a qualified local nurseryman what is suitable to grow in your area. Even better, look around your own neighborhood, and see what survives and thrives in other yards and gardens.

Choose varieties of plants that flower late, in areas where late spring frosts may occur. Some annual and perennial plants will survive frost on the foliage, but the same frost might kill any flower buds that have emerged.

Cold air is denser than warm air, so it sinks to the lowest point. Low-lying areas of the garden can be several degrees colder. Consequently, frost may occur in these areas when there is no frost evident anywhere else in the garden. Plant tender species on higher ground or on slopes where the cold air will flow past the plants as it moves to the low point. Any sloping area is less prone to frost, because the cold air won’t settle there as readily.

Precondition your plants to withstand cold temperatures by discontinuing fertilizing in early fall, so that no new foliage is on the plant when cold temperatures arrive. Older leaves are much tougher and more able to withstand frost.

When frost is predicted, there are several things you can do to protect your plants:

Water your plants thoroughly before nightfall. The soil will release moisture into the air around your plants during the night, keeping the air somewhat warmer.

Even a slight breeze will prevent cold air from settling near the ground during the night. You can help keep frost from forming by providing this breeze with an electric fan. Be sure to protect the fan and all electrical connections from moisture and the elements.

Cover up before dusk! By the time it gets dark much of the stored heat in the garden has already been lost. If you have time, build a simple frame around the plant, or row of plants. (Even a single stake can be used in many cases.) Then drape a cover of newspaper, cardboard, plastic tarps, bed sheeting or any other lightweight material over the frame to create a tent. If you don’t have time to create a frame, lay the protective cover directly onto the plant. This will help to slow the loss of heat rising from the foliage and the ground. Remove the covers in the morning, once the frost has disappeared, to let the light and fresh air back in, and to prevent overheating by the sun.

For smaller, individual plants you can use glass jars, milk jugs with the bottom removed, paper cups turned upside down, or flower pots as heat traps. Don’t forget to remove these covers in the morning.

You can collect heat during the day by painting plastic milk jugs black and filling them with water. Place them around your plants where they will collect heat during the day. Water loses heat more slowly than soil or air. This collected heat will radiate out throughout the night.

Potted plants are particularly susceptible to frosts because the roots are less insulated. If you are unable to move your container plants indoors or under cover, remember to also wrap the pot in burlap or bubble wrap, or simply bury the pot in the ground, in addition to protecting the foliage.

If your efforts were too late, or too little to protect your plants from a frost, resist the urge to cut off the damaged parts of the plants. To a certain extent, these dead leaves and stems will provide limited insulation from further frost damage. Your plants will still need to be pruned in spring.

USDA Zone Last Frost Date 
1 July 15
2 August 15 
3 September 15
4 September 15 
5 October 15
6 October 15
7 October 15
8 November 15 
9 December 15
10 December 15
11 No frost