Duration of cold — not temp itself — key to plants
If we use a cold frame, what is the lowest temperature that we could place our over-wintering geraniums, lantana, etc. outside?
Lantana will generally be more cold hardy than geraniums, and within lantana, some varieties will be hardier than others. But in general, lantana will take short exposures to the upper teens. Our problem here isn’t the temperature reading itself; it’s the duration and frequency of the cold. This is a common question and misunderstanding from people who have moved here from warmer regions of the country.
I’ve talked to people from Albuquerque and Tucson who want to grow non-hardy plants here who say “Well, it gets that cold back there so it should grow here.” The truth is that we actually do get colder than these places, maybe by not that much, but the real difference is how often we get down to those temperatures and how long we stay down there.
Albuquerque may get down to 10 or 15 degrees on an especially cold night, but they only do it a handful of times a year and they’ll drop down to that temperature for only a short period of time and quickly pop up to warmer temperatures. We, on the other hand, will see the low to mid-teens for a low for two or three months typically and each night we’ll spend several hours down at that temperature.
This makes a world of difference in whether a marginal plant survives or not. I would not let your cold frame go below 25 degrees for much more than a couple of hours. For the geranium, I’d try to keep it above 28 degrees.
I want to have a lavender bed in our yard. The spot I am thinking about is in front of a deck on the south side of our house. Will the lavender tolerate the direct sun and heat in the summer? It gets great protection from the cold during the winter months.
Lavender should do great where you’re thinking of putting it. They thrive in hot, sunny locations. In fact, I see more problems with lavender where they get too much shade. They tend to get thin and leggy and just don’t thrive. The most important thing to remember is that all lavender like well-drained soil, so do a great job amending your soil before you plant. Most plant loss here in the valley isn’t due to cold but to water-logged soil that causes rots in the plant.
There are quite a few varieties of lavender you could consider. The standard around here for years has been a compact variety called munstead. This variety grows 12-18 inches tall in a neat, compact form that doesn’t need support. It has attractive gray foliage with striking spikes of dark violet purple flowers in early summer. It’s hardy to 15 below zero.
Another popular variety is Provence lavender. This variety grows a bit taller, getting up to 30 inches tall. This is one of the varieties grown in France for its oil and flowers. It has light purple flowers that are very fragrant. This variety is more moisture tolerant than other varieties and is hardy to 5 below zero.
Hidcote is similar to munstead but gets just a bit bigger. Growing to 24 inches with a uniform compact habit, it bears deeply colored violet-blue flowers and is hardy to 15 below zero. Goodwin creek grey is not as cold hardy as other varieties, but its wider, toothed powder gray foliage provides a striking accent to any garden. It grows to 2 to 2 1/2 feet tall and has soft, lavender-blue flower spikes. It’s hardy to about zero degrees.
There are lots of other besides these, but hopefully, that will get you started.