What does root hardy mean?
If the plant tag says that it’s hardy to 32 degrees, is that air temperature or soil temperature? Well, hardiness does refer to how well a plant tolerates cold temperatures. This is most often discussed in terms of hardiness zones, developed by the USDA. The smaller the zone number, the colder the temperatures in that region. For example, Central Texas a zone 8 area, and the coldest parts of Alaska are zone 1. When describing a plant’s hardiness, the soil temperature is very important, but the air temperature is what is used for reference. Most people don’t have soil thermometers, but we can all check the weather to see what our area’s low temperature for the night is going to be, so the air temperature is more useful to us in practice. And we know that if the air is a certain temperature, then the soil will be a comparable temperature, but not as cold. And even if the air is below freezing, the soil probably won’t be. When night time temperatures reach mid 20’s, the temperature of the soil may still be at 40 degrees. Because the air is colder than the soil, many temperate zone plants have developed the strategy of dropping their tender leaves, or even sacrificing their entire body, to hunker down into the soil, where it’s relatively warmer. Plants that have this strategy are called perennials if they’re relatively herbaceous, like our native Echinaceas and Gazanias, and root hardy if they’re woody, like Lantana and esperanza. And if a plant is listed as “hardy” to a certain temperature, it is likely to be killed if temperatures drop below that number. To protect your perennial and root hardy plants in the winter, be sure to mulch very well before the first freeze, piling the mulch up much higher around the root zone than you normally might.
When can I prune perennials?
If you have perennials that have browned or are dead on top, you can cut them to the ground now. Those above-ground plant parts, which may have looked completely lifeless, had sugars and other plant nutrients in them that may take a while to make their down into the roots. They also serve as a small amount of protection to the soil around the roots of the plant, and those are two reasons why it’s really best to leaves those unsightly “sticks” alone until we are into late winter. Another reason is that pruning stimulates growth. And when a plant is trying to “go to sleep” for the winter, you need to go ahead and let it do that. But then, when the plant is truly dormant, many plants do need to be pruned, to clear out all the dead growth and make way for the growth. How to prune trees may be a little more obvious than it will be on shrubs and smaller woody plants, both those that have gone dormant, and those that have stayed evergreen, but stopped growing in the winter. To renew evergreen Salvia greggiis, this is the time to cut them back several inches to encourage compact, fuller growth. If left unpruned, they’ll continue to grow, but will get leggy and just look unkempt. You should remove back to the source (the main trunk or the ground) any woody stems that are obviously dead. As many of you know, it’s also a great time to prune trees, especially trees that are susceptible to oak wilt. We want to prune them by mid-February at the absolute latest. But one group of plants we should hold off on are the evergreen shrubs, at least until we’re closer to the last freeze date. Pruning now will encourage tender new growth that can be damaged by a late winter freeze, or even a frost. Also, wait until mid-February to prune your roses. You can go ahead and cut back ornamental grasses now, or you could leave them another month, since they provide habitat for overwintering butterflies and other garden creatures.
Will freezing temperatures be a concern for bluebonnet rosettes?
This really is a great question, since it gives us an opportunity to talk about our wonderful native wildflowers. As more people begin to plant wildflowers in their landscapes, they begin to notice them at different times of year, other than just at spring flowering time. Most of our wildflowers sprout in the fall and spend their winters as rosettes, which are plants with shortened stems that grow very close to the ground, but not towards the sky. This strategy allows them to survive and even thrive through the worst cold that Central Texas can throw at them, so they shouldn’t be at all damaged by freezing temperatures, or even snow, should we happen to get any. And the way to ensure great flowers this spring is to water your wildflowers in the fall and winter, when the rosettes are actively growing and building up the energy to flower. Depending on your soil type and the amount of natural rainfall we get, and also how sunny we are, you might need to water as much as once a month on clay soils, or a couple of times a month on really well-drained soil, rocky soils. But if we have a cool, cloudy winter, with even one or two rainfall events, you may not need to water at all. If you do water your wildflower areas, be sure to give them a deep, thorough soaking.
Why do some plants die in freezing weather?
There are actually a few ways that freeze damage occurs on plants, but most commonly it happens when the frozen water in the plant thaws out. Jerry Parsons, one of my favorite retired Extension specialists always explained this question with the challenge of an experiment. Fill a glass half-full with water and put it in your freezer. Take it out the next day, once it’s thoroughly frozen, and immediately place it under a warm stream of tap water and watch what happens. You’re not a science nerd like me? Well, then I’ll spare you the experiment and tell you what happens, but I bet you can guess. The glass will shatter as soon as the ice starts to thaw and crack, exactly as thawing plant cells will do as soon as the temperatures warm up. Some plants contain more water than others, especially in their leaves. A pine tree has very little water stored in its leaves, so pine trees don’t generally freeze. But tropical plants are from climates where the humidity is high and rain is usually plentiful, so they have lots of water in their leaves, making them very susceptible to cold temperatures. Many plants also retain high amounts of water in their fleshy stems, so those may freeze too. Plants have different strategies for dealing with cold. Some die completely, leaving the next generation to carrying on in their seeds. Others only lose their tender leaves and perhaps any tender new stem-growth, while other die all the way back to the ground, but have hardy root systems, allowing them to emerge after the harsh cold weather has passed.
After the holidays, what should we do with our beautiful plants? Can we put them in the ground? Well, although you can’t; put them in the ground, I wouldn’t recommend it. But, once we’re past freezing weather, you can keep your poinsettia in a pot outdoors, but be sure to protect it from the full sun, especially harsh afternoon light, and mulch it well, to keep the air around the plant moist. It will need to be cut back and fertilized through the summer. But to get it to bloom again, you’ll need to provide at least 14 hours of darkness starting in September. Even then, the colorful bracts may not be as rewarding as those on a new plant would be. But here’s a success story from our friend in Leander. Last Easter, he planted his two Christmas poinsettias in the ground. They’re in well-prepared and well-mulched soil near a live oak tree, so they get shade about 50 percent of the time. He watered them twice a week this summer. In November, they bloomed again! Although they’re much smaller than his original plants, his experiment just goes to show that gardeners can do anything. And, for a poinsettia that will return for you, without much effort at all, remember our Texas native, Euphorbia cyathophora. It goes dormant in winter, but will give you Christmas color all summer long.
Pineapple guava, Acca sellowiana
This evergreen shrub is listed as hardy to 10 degrees Fahrenheit, and has very attractive light, slightly bluish green foliage. It can take full sun, but also does fine in light shade, although it will likely be less bushy if it’s not getting enough light. It has really nice, large blooms in late spring, which emerge as white to light pink buds, opening to reveal showy deep red centers with lots of red stamens with bright white tips. This is a great wildlife plant that attracts bees, birds and butterflies. Pineapple guava needs about an average amount of water, about once a week after establishment but be careful not to overwater it, especially in heavy clay soils. If given too much water, it will respond with yellowing leaves that drop off pretty quickly. You can either leave this plant bushy, as a small shrub, or you can train it, with selective pruning, to be a small tree. It makes a nice addition in areas where you may have perennials or other plants that die back during the winter, since it has that nice blue-green sheen all through the dreary cold months.
Winter edibles in the landscape
Most Central Texas gardeners plant winter edibles from late summer through fall, but if you got a late start or want to plant a second crop, there still time to plant in mid to late winter. Most local nurseries carry these cold hardy vegetables all winter long, so you don’t have to worry about sprouting seeds. Winter vegetables such as lettuce, spinach, Swiss chard, cabbage, kale, and broccoli, not only provide you with food, they also look beautiful planted right in the middle of your established landscape. These days, there’s really no need to plant vegetables in rows unless you just want to. A grouping of strategically placed oak leaf lettuces look right at home beside the fall aster and Salvia greggii in your front flower bed. The leaves of winter edibles, which are what we eat on many of them, are truly striking and provide brilliant color to what might otherwise be a drab winter landscape. Try planting them among your perennials, which will live below ground all winter anyway, so there won’t be any competition for space. Winter vegetables are naturally very resistant to cold temperatures, but an added benefit of having them planted among your landscape is that they’re more protected if we get any truly cold temperatures, especially if they’re well mulched, so there won’t be much need to protect them on cold nights, even if we’re down in the 30’s. But if you want to be extra careful, protecting with light row cover should do the trick. If you like, you can leave a few of leafy greens in your landscape instead of harvesting and eating them. As they prepare to flower and ultimately die, they shoot up into the sky, which is a process known as bolting, and provide a striking, architectural element to the garden for a brief period.
Heartleaf skullcap, Scutellaria ovate
Scutellaria looks a lot like many of our blue-flowering Salvias. And there’s a good reason for that: they’re in the same plant family, the mint family. Even though heartleaf skullcap may look tender, it’s actually a very cold tolerant native plant that is normally evergreen in Central Texas. You may even see them putting on new growth now, at the coldest time of year, preparing to produce their lovely, towering purple flower spikes in spring. In fact, it prefers the cold so much that it may actually go dormant to avoid our overbearing Central Texas heat. (I don’t know about you, but I sure wish I could do that.) Heartleaf skullcap grows vigorously via its fleshy roots and it will creep around your garden. So be prepared to tame it back if it moves into unwanted areas. The oil in its leaves also confers some deer resistance and the nectar in its flowers serves as a hummingbird attractant. The heart-shaped leaves are also very pretty, with purplish stems and a fuzzy surface. Heartleaf skullcap prefers to be kept a little on the dry side, in dappled sunlight. It can be a little challenging to find in nurseries, but if you know anyone who has some, just ask and I bet they’ll dig up a chunk for you.