Getting rosemary through a Colorado Springs winter

Rosemary is an herb that is popular to grow in Colorado. It has wide culinary use, anything from marinades to bread will benefit from a sprig or two. It is a very attractive, subshrub plant that looks great in the right spot in your garden. It is a tender perennial plant, which will not survive our winters without protection. 

This herb is from the Mediterranean, a region with long summers and gentle winters. Rosemary officinalis is described by the USDA as a zone seven plant, but other sites consider it zone eight. That means that it will probably not survive in temperatures below about 15 degrees Fahrenheit. There are some cultivars described as zone six hardy, meaning it might survive at zero degrees Fahrenheit.

You can overwinter rosemary indoors. My personal experience is that it always becomes infested with insects in winter, and I end up throwing the plant out. About a year ago I was told that it was possible to keep rosemary alive in the garden by putting a wall of water on the plant in late autumn. Honestly, I didn’t believe it would work. However, I had three rosemary plants in pots and thought why not give it a try?

So last year in late summer I took the first step in a trial that I was positive would end with dead plants. I moved the plants from pots, to a sunny (south-facing) space near a light-colored wall. My goal was to find the microclimate in my garden that would stay the warmest, and have good drainage. The plants adapted well to the transplant, and in late October I placed a season extender on each plant. The extender looks kind of like a plastic tepee, comprised of cylinders that are filled with water. They are widely used by gardeners to plant tomatoes a few weeks earlier than our climate allows. The water in the tubes heats up during the day, then warms the plant in the colder nighttime. Trade names for this type of season extender include Wall o’ Water, Tomato Teepee, Kozy Coats, etc.

The plants in this new warmer environment starting growing a bit, putting new foliage on that I was sure would turn black as soon as we had a hard freeze. The freeze came about two weeks into December. One night it went down to negative six degrees Fahrenheit, and I was sure the plants were gone. Dead. Imagine my surprise when I looked at the now-frozen solid water enclosure, and the plant still looked green. About two months later in late January one of the plants bloomed! The plants survived the winter. I was able to cut sprigs of rosemary for cooking whenever the structure wasn’t frozen.

One of the walls did blow down a couple of times. Refilling the walls in mid-winter was not fun, I will try to avoid repeating that. This year I will try to contrive some kind of external structure to prevent the structure from collapsing and emptying out mid-blizzard. Some kind of tomato cage will probably suffice.

This column generally tries to provide well-tested scientific information. This is not that. It is one trial, in one garden, during one winter. While we had some very cold weather last year, we also had lots of warm days in between. Except for the January windstorm I would categorize the winter of 2016-17 as mild. I encourage you to give this technique a try if you are okay if your plant doesn’t make it through next winter.

When you have questions, Colorado State University Extension has research-based answers. Get answers to your horticulture questions by visiting ask.extension.org any time day or night. Monday to Thursday from 9 a.m. – 12 p.m. you can call 719-520-7684 or email CSUmg2@elpasoco.com.

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