After the Freeze

From the Davis Enterprise, January 25,2007

The freeze of 2007 is now one for the history books in Yolo County. Although temperatures didn't stay as cold for as long as the freeze of 1990, nor did we reach any absolute low temperature records, we had 13 consecutive mornings below freezing in Davis January 8 through 21, tying the previous record set in December 1990.

This is all in addition to the shorter cold snap we had December 17 – 20, which did a fair bit of damage in gardens locally. In fact, we'd  already had 13 days of frost by December 31!

Some notable data from this spell of cold weather:

Temperatures in western Yolo and eastern Solano counties ranged from 1 – 9 degrees below the official readings in Sacramento. On the coldest morning, January 14, Davis was 20 degrees, Dixon was 17. Winters was 18 degrees.  Sacramento came in at 25.

Gardeners should be particularly concerned when temperatures drop below 25, as that is when significant plant damage can occur. Davis has had 3 mornings below 25, Dixon has had 6.

Humidity through this freeze has been very low. On the coldest day the humidity dropped to 16%, and only reached 52% at night (humidity is usually in the 90%+ range at night).

Esparto is the banana belt of Yolo county, with morning lows 3 – 8 degrees or more above Davis, only dropping below 25 on the coldest morning (Jan. 14), and with only four nights of freezing weather through the entire period. A quick look at the topography of northeastern Yolo County will show you how winter heat is trapped in the Capay Valley. I foresee a new real estate boom: winter condos in Esparto.

As in 1998, California's navel orange crop has been destroyed. Preliminary damage estimates range from 75 – 90%. The freeze was statewide, damaging avocadoes, cut flower crops, and the first crops of strawberries and artichokes in coastal fields. Damage to nursery stock was extensive in Southern California, where many of our favorite subtropical plants are grown. These growers supply many of the larger home improvement chains. Damage to nurseries in the north of the state will delay availability of some plant material until late spring or summer. Growers will have to cut back damaged plants, and wait for a flush of new growth, before they will be ready to sell.

Unfortunately, the damage to oranges and lemons is not obvious by looking at the fruit, and home gardeners may be in for some unpleasant surprises. Though there may be 'burnt' look on the exposed part of the peel, the damage is to the quality of the juice. As you harvest you may notice off flavors. The fruit will definitely not keep well, as spoilage will progress rapidly at indoor temperatures. If you haven't harvested already, sample some fruit and use them as soon as possible.

So what should home gardeners do about the damage in the landscape?

The simplest answer is 'wait and see.' The full extent of damage is not yet apparent, and continued frosty nights may continue to kill leaves and stems.

Continue to provide protection. Floating row covers (aka 'seedling blankets') appear to have been the most effective in preventing damage. Draped over the plants, or wrapped around them to create ghostly plant pupae, these trapped enough heat to prevent serious injury. Since they allow light to penetrate by day, they act as individual greenhouses, getting surprisingly warm during the day. Leave them on the plants for as long as frost is in the forecast.

Don't cut anything back yet. Open cut wounds are avenues for further cold damage, and the dead leaves hanging on the plant actually provide a small measure of protection simply by 'cloaking' the remaining stems and leaves. Wait until we are reliably out of danger of continued frost. Even better, wait until the plant starts to grow—probably in late March or early April. Then you'll know how hard to prune: cut back to the new growth. In many cases a severe pruning will be appropriate, but it's wise to ask a professional first.

Keep container plants watered. Desiccation is the greatest risk during freezing weather. Plants in the ground probably don't need water, except those you planted within the last 2 – 3 months. Although we are way behind on rainfall, there is enough moisture in the soil from the early winter rains that I'm not concerned about established plants. Exceptions would include plants under overhangs, where winter rainfall didn't penetrate.

Your irrigation systems will need your attention. My pipes froze on January 14 and 15, and when the water finally came back through the house there was a bunch of murky solid matter at the head which promptly plugged my sink and shower heads. Freezing the water had precipitated out the copious salts in our groundwater – calcium and boron, mostly. You can imagine what these solids will do to your drip irrigation emitters and sprinkler heads. I strongly recommend flushing out your sprinkler and drip systems before you turn them on for the season!

Australian tree ferns

Begonias

Bougainvillea

Hibiscus (tropical)

Mandevilla (evergreen types)

Serious damage, but likely to resprout:

Avocadoes

Brugmansia (Angel's trumpet)

Bananas (mostly killed to the ground)

Citrus trees (limes and young trees are most vulnerable)

Hardenbergia (Lilac vine—there won't be any of the lovely purple flowers this winter!)

Jasmine (true jasmine; Star jasmine is fine).

Palms, particularly Queen palm (outer fronds will dry up and look dead, but the growing point is protected inside the center of the tree).

Pandorea (Bower vine, considerable top damage)

Passifloras (cut 'em to the ground in late spring; they'll be back)

Potato vines (the purple ones look worse than white, but mine recovered in 1990)

Red trumpet vines (severely damaged, unlikely to flower this summer)

Any decision about replacing plants should wait until we've had several weeks of warm weather. Subtropicals can be surprisingly resilient, and may resprout as late as May. So don't be in a rush to pull them out. (Keep in mind that nurseries aren't likely to have replacement plants until summer anyway). If you're curious about a woody shrub or vine, just scratch the bark with your thumbnail: if it's still green underneath, it's still alive. A good rule of thumb is that subtropicals start

We seem to have these freezes on eight year cycles: December 1990, December 1998, January 2007. Keep the Christmas lights and seedling blankets handy for 2014'.

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