Cold damage details

Its now time to get up and personal with your garden to begin to really see what specific damage our winter has wrought. This is my third and final post on the winter of 2016/17.   Greg and I have been obsessively observing cold damage now for almost 20 years as growers. I’m going to give you some tips that we’ve learned as well as a plant by plant analysis of some genera. Growing a garden is a crap shoot and if one thing doesn’t go right something else goes wrong as we say and as Greg is always telling me. “They are plants after all”. My previous post ‘Whats in a zone’ shed light on how cold affects plants. This post will be much more specific.

It could have been worse

I’ve seen much much worse freezes. In February 1989 when I was in college a massive arctic outbreak tore down from Alaska. Following weeks in the 50s and even 60s plum trees were in full bloom and spring was in full gear. The high temperature was 61ºF on Monday and by Thursday it was just 14ºF (for a high (!) and the low was 0ºF. The damage was immense- even rhododendrons and cherry laurel (Prunus laurocerasus) froze to the ground or died outright.  All roses were toast and those plums in bloom lost most of their major limbs.  Our worst freezes are when mild weather precedes arctic air and with our wet conditions plants are flush with water and haven’t entered dormancy and they are shocked by dry cold conditions. This year believe it or not we were relatively lucky. The warmest November ever was followed by a December that had 17 days below freezing but no real arctic air. This hardened plants to the more severe cold of January. If we had gone into December from November with the deep cold we had in the middle of winter it would have been so much worse.

The three stages of damage

So, the first stage of damage  occurs when we still are experiencing arctic conditions (lows below 20ºF). For instance, Abutilons will look fried immediately at temperatures in the teens. Zone 9 plants in general will die or look awful. The second stage of damage occurs about 2 weeks after arctic air has abated. That is when you see plants begin to go downhill. Browning, splitting stems, loss of leaves and frozen tips become evident. And remember that our temperatures don’t zoom back up- we gradually warm to the 40’s and 50’s and its often wet. Perfect conditions for Botrytis- rot. I’ll address that in a minute. The final stage occurs in spring with the first really warm days (above 70ºF) and plants break dormancy in earnest  and burst cells and bark split interrupts the plants ability to fully function. Whole branches die or the plant may die to the ground or die all at once. Often a plant that I’m testing for hardiness will look relatively unscathed until this first warm day. Then WHAM! Dead as a doornail- the whole thing just fails.

Take a closer look

There are some things I’d like to pass along. Not all plants respond with cold damage in the same way. Some plants its the tips- the newest foliage that goes. Cistus, Roses, many common broad leaved evergreens.  Other plants its the oldest foliage that succumbs- which can be counter intuitive. As an example,  Callistemons (Bottlebrush) show cold damage in two ways. The oldest foliage will begin to turn brown not the newest. Real trouble occurs when there is bark split. Thats when the woody cells rupture. The split shows up conspicuously if you take a real close look on the stems and trunks. The good news is they can actually recover from some bark split. So don’t give them up for dead. If the split actually girdles (surrounds) the whole stem then recovery is unlikely. In severe cases if all the top growth has failed Callistemons can be cut to the ground and a strong, woody root system will aid in fast regeneration. You will likely lose a year of flowers but you’d be shocked at how fast they can regain their former stature. Make sure they are well watered and maybe add some a compost mulch to speed recovery.



Many Ceanothus definitely didn’t appreciate our cold wet winter

Several other shrubs and trees that show (partial damage) on old foliage:

Arctostaphylos, Callistemon, Eucalyptus, Leptospermum, Agaves, Olearia, Ozothamnus, Evergreen Oaks- just a small list.

Arctostaphylos if it is cold damaged will represent with dead interior foliage while newer leaves can be unscathed. The best way to deal with this is to wait for the dead foliage to become a little dry. Then shake the whole shrub to dislodge the dead leaves. This will leave you with bare twigs which you can then remove. Remember that Manzanitas do not have dormant buds on bare stems and will not spontaneously sprout. If your Arctostaphylos is a species that forms a burl at the base (a swollen woody dome shaped trunk) you can cut the whole thing to the ground for radical regenerative pruning and dormant buds on the burl will erupt and the shrub will quickly regrow.

Rot: The final insult

Following a freeze our temperate climate that is wet is ideal for botrytis or gray rot. Its a fairly obvious mold that colonizes dead foliage and stems. There have been experiments with fungicides applied before and after freezes to combat this. To be honest we don’t use them and I don’t encourage you to- nasty chemicals mostly. There are some more benign homespun recipes for fungicides involving baking soda, toothpaste, butterfly wings and god knows what else.  Well, they don’t work.  The best prevention for botrytis is tidiness. To prevent it from infecting undamaged tissue and spreading ASAP remove dead leaves and branches. For Agaves, this is especially important. Don’t allow dead material to collect in the rosettes (deciduous leaves from other plants)- this can be the catalyst for rot that on already challenged plant tissue and once necrotic tissue begins to spread there really isn’t much you can do. Tenting plants to allow them to dry is one possibility just as long as you can maintain good air circulation. This year Agaves had a triple whammy from the the three things they despise in our climate. Freezing rain and then prolonged cold, and copious rain.  It seems only the most well established and adapted varieties came through all right. So, get out the rake and the pruners and sweep up dead material. Cut out dead branches cleanly and do not let them remain and fester rot. Tidy, tidy, tidy will increase air circulation which will deter mold  and be sure to dispose of dead material.

Agave ovatifolia

Total rejuvenation:

If a plant has frozen completely to the ground there are several things you can do to encourage recovery. First, keep them well watered all summer and add a nice mulch of compost and perhaps a little handful of organic fertilizer to give them a boost. Remember that the more established a semi-tender plant is then the larger the root system from which it can re-emerge. Plants that respond to this treatment are Melianthus, Abutilons (the hardier ones like A. megapotanmicum) Fuchsias, Phormiums, and just about any zone 8 perennial that may have lost above ground growth. A little extra care the following summer yields great rewards and you’ll be surprised, I guarantee, at the rate of recovery.

Spring is here

Spring is here, we’d love to see you at the shop. Greg and I are going to host a little symposium on plant damage so that gardeners can share information on what was a historically obnoxious winter. We’ll be announcing that via our Facebook page.


Cold damage details

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