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Saturday - January 02, 2010

From: Redway, CA
Region: California
Topic: Non-Natives, Seasonal Tasks
Title: Damage to plants after sudden freeze in Redway CA
Answered by: Barbara Medford


I live on the North Coast of California near "The Avenue of the Giants" and Redwoods State Park along the Eel River. We recently have had below freezing weather, constant rain and even snow! I have beautiful Geraniums, Creeping Charlie and Hydrangeas in a large brick planter with partial shade/sun combination. They froze on the vine, turned black and look completely dead. What can I do at this point to salvage them if possible?


We have been receiving many questions regarding what to do after the sudden hard freeze that occurred recently in Central Texas; in fact, we are still dipping below freezing at night frequently. Obviously, ours is not the only part of the country that has suffered from this unusual weather. One thing that applies in every case is, don't fertilize. Plants should be fertilized in the Spring, when you want to encourage new shoots to appear. The last thing you want to do is encourage new shoots now that will put more stress on already-stressed roots and probably just get frozen back again.

You may already know what happened; actively growing plants still have water in their upper structure, particularly the leaves. A sudden hard freeze causes that water to expand, bursting cell walls in the leaves, and they quickly turn dark and look pathetic. What made this freeze worse was that it was earlier than you ordinarily expect these conditions in the northwestern part of California, very sudden, temperatures went down very far, and remained below freezing for several hours. A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures. A sudden decrease in temperature in late fall or early winter usually results in more damage than the same low temperature in January or February.

We will address each of your plants with what information we have about their cold hardiness. The first thing we need to tell you is that being in a planter box, even one enclosed in bricks, is an added hazard during cold snaps.  A plant with its roots in the ground has the whole Earth insulating those roots; even if the top part freezes, the roots will have stored supplies to permit the plant to grow again. With the roots exposed to those freezing temperatures with only a few inches of soil around them, the roots may very well have been damaged also. 

To begin with your geraniums, there are 11 members of the Geranium genus native to North America, 6 native to California and 1, Geranium bicknellii (Bicknell's cranesbill), native to the area of Humboldt Co. on the northwestern California coast. See these pictures of this plant from Google; we're betting that's not what you have, right? Now look at these pictures of Pelargonium x hortorum, again from Google, and this article about them from Floridata. The Pelargonium genus is native to South Africa and has been so extensively hybridized, similar plants cannot be found growing in the wild. The Lady Bird Johnson Wildflower Center is dedicated to the use, protection and propagation of plants native not only to North America but to the area in which they are being grown; therefore, these plants fall out of our area of expertise. However, we can tell you that the Pelargonium is hardy to USDA Hardiness Zones 9 and 10. Your area of California is Zone 9. Obviously, the cold spell you had was severe enough to knock them down. In cooler areas, the Pelargonium is considered a tender perennial or an annual. Whether the roots managed to survive the cold in the exposed planter, we don't know. If you wish to take a chance that they will come back up in the Spring, we suggest you cut them to the ground and mulch to protect the roots. 

Next, to the Creeping Charlie, also sometimes called Creeping Jenny. The scientific name of this plant is Glechoma hederacea, a native of Asia and Europe. Most of the information we could find about it involved how to get rid of it, as it is considered an invasive weed, as seen in this University of Wisconsin article Controlling Creeping Charlie.  We learned it has very shallow roots in a mat right at the soil surface, which could well mean those roots are dead. 

And, finally, hydrangeas. There are two members of the genus Hydrangea native to North America, Hydrangea arborescens (wild hydrangea) and Hydrangea quercifolia (oakleaf hydrangea),  neither of which are native to California, being more plants of the Southeast, needing acidic soils which they wouldn't find in the normally alkaline soils of the American West. From the United States National Arboretum, here is an excellent article covering the cultivation of both native and hybridized hydrangeas,  Hydrangea Questions and Anwers, which not only covers many of the problems the plant may have, especially in alkaline soil, but also mentions that they are normally hardy to USDA Zone 6, so the damage of the cold should not be lasting. Again, we would recommend cutting the plants back severely and mulching against another Winter Surprise.



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