Hello All,

I hope everyone had a wonderful summer. School started last week and people are just getting use to their new schedule. The normally sedate summer Miami traffic has degenerated back to the maniacal form of movement where a red light means speed-up so you can pass through cross street traffic like fast thrown pencil moves through the spokes of a spinning bicycle wheel. Anyway, I hope you are readjusting to the fall schedule. Since it is going to start cooling off, I thought we might look at how we protect plants from the cold. Of course, this is Miami so by cold we mean temperatures below 40 °F (that is about the temperature inside your refrigerator).

For plants that don’t have enough sense to come in out of the cold, we can send them to their room. This is the banana room (picture below). Tropical plants, ones who like it hot all year round, will start to get cold 

damage when temperature’s drop into the low forties (40 °F). That is because tropical plant enzymes (proteins that will grab a specific compound and hold it still, then grab a different specific compound and hold it still so that the two different compounds can combine to form something a plant really needs),    . . . tropical plant enzymes don’t like cold. For example if a plant needs to make starch, it has to take two smaller sugar molecules called glucose and bond them together. The enzymes (it takes more than one) are shaped so that they can only grab a glucose molecule. If it gets too cold the enzymes loose their shape and can’t grab glucose, so no starch is made. In Florida, temperature’s can get into the low forties during winter nights. To keep it nice and warm for bananas, we grow them in these open-air rooms. In the day, the sun heats up the walls. At night, the walls release that heat and banana can stay warm. This method works if the temperature does not get too far below 40 and only lasts for a few days.

Another trick for plants that can’t come in from the cold, is to spray water on them. When water turns to ice it releases heat known as the latent heat of fusion. As the air temperature drops to around 34 °F, you spray a fine mist of water above plants. If the temperature drops to freezing (32 °F) water turns to ice and releases enough heat to keep the temperature near the plants from dropping any lower. This trick works at temperatures a few degrees on either side of the freezing. By-the-way, freezing does not necessarily kill the plant. Freezing damage usually occurs because plant cells thaw unevenly, cracking cells open and allowing all the good stuff inside to leaks out.

Some outdoor plants can be moved indoors when the weather gets cold. We put them in our greenhouse or in a room that gets the right amount of sunlight. Plants that grow in the open are put on a windowsill that gets a lot of light; plants that grow in the shade are put on a counter that does not get direct sunlight. A greenhouse usually has clear glass walls and ceiling. Light comes through the glass and heats up the greenhouse. Whereas, the light can penetrate the glass, heat cannot. Once sunlight heats a greenhouse the glass walls trap heat inside and keep it warm despite cold temperatures outside.

This is our cacao greenhouse. Cacao is a mid-story forest plant (larger trees grow around it blocking some of the sunlight). Because cacao likes partial shade we painted the glass panes white so the right amount of sunlight enters.

When it gets a little too chilly we give plants a blankie.

Or when they go outside we wrap them in a nice warm sweater.

When it gets too cold in the greenhouse, we turn on a heater.

The grass in trays at the bottom is sugarcane.

The fan helps circulate the warm air.

Small plants get a little jacket. It is clear, like the greenhouse walls, to trap heat. This plant only needs protection for the sensitive aboveground growing parts.

These plants need protection down to the pot surface. During a south Florida winter the ground tends to stay warmer than the night air. Plants that have a growing point below the surface can have their top parts dieback. Underground parts survive freezing temperatures in the warmer soil. In spring they can regrow.

Some of our Madagascar palms like this stylist red-striped cover.

Some go with a conservative white stripe.



But regardless of what color of protection they like, tropically evolved creatures like avocado, cacao, and people need to keep warm when temperature gets near freezing.