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A Tropical Garden That Can Take the Frost
By Carolyn Carpino, UC Master Gardener
Many of us fall in love with lush tropical southern California landscapes and want to duplicate them here. Hey, we live in California, too, right? It turns out there's a big difference between the balmy, frost-free coast and the central valley. Most tropical plants will not survive extended cold temperatures. Some will tease you through a few mild winters and go belly-up, just when they fill an important role in your landscape. Others will survive in carefully sheltered environments, like entryways, and seduce the rest of us into planting them, too.Unless you're willing to re-plant as needed don't plant bird of paradise, bougainvillea, potato bush, tropical hibiscus, princess flower (tibouchina) or, sadly, queen palm. Although it's a favorite palm here in our valley, we all know many won't survive a hard winter.
What makes palms different from other trees? Palms are actually very primitive plants that don't have a cambium layer just below the bark like other trees do. Instead, their trunks are a bundle of tubular water-carrying vessels that support the foliage head on top.
Palms are more drought-tolerant than other trees because the tubes hold a lot of water, just like a cactus. That's also why palms can be transplanted with a tiny root ball; they can draw on their internal water reserves until roots are established.
Now think about what this means for frost tolerance. Palms are damaged when water in the internal trunk freezes, expands, and bursts, rendering them useless. The more water a palm contains, the more susceptible it is to frost damage. Cutting back on water to your palms in the fall is a good way to increase frost tolerance.
Fronds are often damaged by frost, but if the trunk is undamaged, new fronds will sprout when weather warms. Younger palms are more inclined to severe frost damage than older specimens of the same species. When an older palm freezes, the outer layer of water tubes die while those in the center remain healthy and protected. Once this occurs, the dead tubes act as insulation from future freezes. Very young palms have thin trunks. If exposed to long periods of cold temperatures, all the water tubes die and that's the end of the story.
Since they are an essential component of a tropical landscape, we're lucky palms differ in their degree of hardiness. There are several hardy, tough, relatively small palms that will work well in our home landscapes.
The hardiest single-trunk fan palm (to 5 degrees) is the Chinese windmill palm (Trachycarpus fortunei). It grows to 20 feet and has a brown trunk covered with hair and stiff fan-shaped fronds. Another good single-trunk palm is the pindo palm (Butia capitata), hardy to 15 degrees. It grows slowly to 15 feet with arching, feathery gray-green leaves. Another very hardy palm is the Mediterranean fan palm (Chamaerops humilis) which has survived 6 degrees. It grows slowly, about a foot a year, to a maximum height of 20 feet. This is a clumping palm, which means it forms numerous off-shoots at the base. You can split off the suckers, which easily root into new plants.
Many other tropical-looking plants can take the frost, too. Some plants in the hibiscus family have flowers similar to tropical hibiscus but are much hardier. Try perennial hibiscus (Hibiscus moscheutos) which dies to the ground each year but springs back with bright flowers 8 inches across. Hibiscus mutabilis has flowers almost exactly like tropical hibiscus on a tough shrub that, as a bonus, doesn't attract aphids.
For a spectacular tropical flowering plant that's a hummingbird magnet, look for Caesalpinia gilliesii, sometimes called the bird of paradise bush. It's a deciduous shrub or small tree with finely cut filmy foliage. It blooms from April to December in my garden with clusters of yellow flowers adorned with bright red 5-inch stamens. This plant is tough. Ours survived the 18 degree winter after being dug up and moved across the yard in late fall. Caesalpinia mexicana has orange red flowers but is not reliably hardy - make sure you're buying the right one!
May and June are actually the best times for us to plant these sub-tropical plants. They are readily available and establish better in the heat. Get them in soon, though, to allow them time to root well before winter. Stop fertilizing in late summer and reduce water to frost-tender palms. We may not be in LA, but we can still have a tropical California garden if we choose carefully!
June 22, 2000
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