Apr 21, 2008

A Little Cold For April

If you're thinking the last few days have been a little cold for April, you're right. In fact, yesterday San Francisco tied its record low for the date and Oakland set a new record, according to NBC11.

By itself the cold snap wouldn't be a problem; but with the weather so deliciously warm for the past few weeks, just about every last plant that had been holding on to dormancy has decided it's finally time to bust out the buds... which leaves them vulnerable to these frosty conditions.

Making the problem worse is the wind, which can chill us down another 10 degrees or more and, even worse, increases the rate at which plants transpire (i.e. lose) water. So the biggest danger isn't actually freezing: it's dehydration.

Ironically, the best defense is the same practice you would use on the hottest days of summer: water your plants, slow and steady, at least once if not twice a day. Make sure the irrigation arrives right onto the soil around the plant's dripline — ideally you have a low-flow system so there's little or no loss, but if you do have spray heads make sure they're not wasting water up into the air. The breeze is often calmest in the pre-dawn hours, so that's a good time to irrigate with little fear of the wind carrying the water away.

(On a side note, if your system is "misting" clouds of water up into the air, your system may be operating with too much water pressure — usually a result of improperly sized pipes. If you can, choke down or manually reduce the flow to your irrigation mainline to see if the misting stops.)

Also, make sure your mulch is adequate: I like to recommend a 2- to 3-inch deep layer of redwood bark or coco hulls, but I've also taken to using the coffee grounds my local java joint bundles up and sets aside. (By the way, they smell awesome for a day, then they just smell like stale Folger's.) Mulch helps the soil retain water, and it also creates the cool, aerated conditions that beneficial mycorrhizae love. No doubt the weather will warm up again soon, and your plants and soil will be prepared for their next challenge: our Mediterranean summer.

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