Winter palm protection

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A decade ago, Northern California froze. A weeklong period of snow and frost tested every exotic plant grown in local landscapes. It was a time for learning through observation. The way plants behaved during this time taught volumes about how various forms and species survive a rare deep freeze.The palms told us, by brown heads and fallen fronds, the limits of each species. In this group were two closely palms, the California fan and its cousin, the tall gangly Mexican fan palm. Both were planted heavily in the northern part of the state, often as a Victorian parlor palm moved into the garden. Others were planted after surveys to mark property corners with a tree that stood out from all the other vegetation. It had to be drought-resistant and long-lived to mark that point for posterity. Still other tall palms were used as visual reckoning because they rose well above the orchards of the flat valley floor.During and after the freeze, Mexican fan palms illustrated the greatest frost damage. This was the tall thin trunk form. The California species is not as tall, with a much thicker trunk and a larger foliage head. This one fared well in that year of abnormal cold. This difference is related to origin, but there's another factor: trunk size.The palm is like a bundle of water-conducting tubes bound in an outer skin. When exposed to cold, the tubes in the outer layers will suffer frost damage. The dead remnants then act as insulation to protect the inner bundles during future freezes. But when a palm is very young, the trunk is not large enough to sacrifice bundles to frost protection. The young trunks can freeze right through in a single cold night.The story of these old palms and frost tolerance helps us understand how to protect all our garden palm trees from winter cold. Those growing in the ground require protection, while young, from that occasional frosty night.The most vital part of the palm is the heart, which is located deep inside the very center of the foliage head, buried in protective leaf stems and bark. This is the meristem.In a light frost, you'll see the existing fronds experience browning around the edges, and some may die out entirely.In a moderate frost, you may lose all the green foliage at the top of the tree, but it will grow back from healthy meristematic tissue in the heart.In a more severe frost, the meristematic tissue freezes and dies, killing the heart of the tree.Fall is the time to protect young palms from freezing until they grow old enough to insulate themselves. Your goal is to protect the heart. Since they are still young and relatively small, the best way to retain the plant's natural warmth is to throw a bedsheet over the foliage head. This will trap the air and keep it from rising out of the foliage to be replaced by sinking cold, freezing air. As soon as the sun rises in the morning, remove the sheet for a quick warm-up.Do not use sheet plastic for this purpose. It does not breathe and may develop condensation overnight, which can freeze in the coldest hours of early morning. Cold can also move right through plastic, so wherever it's resting on the foliage it transfers through to freeze the leaf. Beware of heavier quilts or comforters because their weight can cause the palm head to become misshapen. For larger applications, purchase a frost blanket, which are designed to provide optimal frost insulation without weight. These are available at garden centers and home-improvement stores.The palms of Northern California showed us how far we can push the frost line. The key to anyone who loves these beauties where winters are cool is to protect the young so they can optimize their own system of insulation.(Maureen Gilmer is a horticulturist and former host of "Weekend Gardening" on DIY Network. Her blog, the MoZone, offers great ideas for cash-strapped families to live more richly on less this year. Read the blog at www.MoPlants.com/blog. E-mail her at mogilmer(at)yahoo.com .)