Citrus, Frost and Mulch: Yes or No?

I love mulch. That would be obvious to anyone who has ever talked gardening with me. Big piles, small piles, inches of mulch scattered everywhere. Love it!

 

The benefits of adding organic mulch (wood chips, shredded tree limbs, pine needles, compost, straw) to the top of your garden soil:

• retains moisture

• keeps soil temperature constant, reducing plant stress

• suppresses weeds

• gradually increases soil organic matter

• attracts beneficial organisms that improve soil fertility and porosity.

• Mulch encourages healthier plants, reducing the needs for pesticides and fertilizers.

• protects roots and plants from mechanical injury.

• On hillsides and around homes, it suppresses the spread of brush fires.

But a long-held recommendation from the University of California flies in the face of the "all mulch, all the time" rule regarding protecting citrus from the effects of freezing temperatures: "A cover crop or mulch can lower minimum temperature at night, posing an increased threat from freeze damage." 

So, our advice has been over the years, "rake away mulch from beneath citrus before an expected frost or freeze".

Now, the California Landscape Contractors Association is offering the opposite advice in a release regarding frost protection: "Mulching with a partially composted material is one the best ways to protect plant roots because it helps insulate the soil, reducing heat loss and minimizing temperature fluctuations. Protecting the roots is necessary in order for them to survive the cold." The CLCA also points out: "Be sure to check the mulching material about once a month to make sure that moisture is getting to the soil below. Avoid using weed block materials, plastic or other moisture barriers beneath the mulch so that water can get to the roots. You may also need to water some of the drier areas in mid-December or mid-January if you find the soil dry."

So, who's right? Sacramento County Farm Advisor Chuck Ingels says: keep on mulching!

"The CLCA is right on," says Ingels. "In our mild climate, mulch doesn’t protect the tree from cold because the soil and roots really don’t ever freeze. Mulch protects the soil for other well known reasons. Regarding that UC study: years ago I thoroughly researched this and wrote about it in "Protecting Groundwater Quality in Citrus Production". In a large orchard, the best orchard floor conditions for reducing frost hazards is bare, firm and moist soil. The sun hits the soil and re-radiates the heat at night, warming the air. Tall cover crops are worst because not only do those plants not hold much heat, but tall cover crops raise the level of cold air (cold air sinks), increasing frost damage potential."

"Perhaps with just a few citrus trees there may be some benefit in this regard," Ingels concedes. "But any difference is generally very miniscule. What happens on the surrounding five acres (asphalt vs. buildings vs. bare ground) affects the air temperature around your tree. So, mulch away!"

ROOT ROTS + MOISTURE + MULCH = TROUBLE FOR CITRUS TREES But wait a minute, here's another reason why mulch and citrus trees may not be the best of friends: phytophthora. The spread of this root and crown rot fungal disease may actually be aided by too much mulch beneath shallow rooted citrus trees, especially in moist, slow-draining soils. According to the UC Division of Agriculture and Natural Resources book, Pests of Landscape Trees and Shrubs, "phytophthora kills the roots and root crown area of infected plants...possibly causing mature plants such as citrus to grow slowly and gradually decline... Phytophthora may affect only small feeder roots or rootlets, major roots or all roots and the crown... Because mulch retards drying of the soil and excess soil moisture greatly contributes to the development of root rots, improper or excessive use of mulch may actually promote root rot development."
TO MULCH OR NOT MULCH CITRUS? A COMPROMISE.

 If your soil drains easily and is not compacted, you may be able to get away with adding a few inches of mulch beneath your entire mature citrus trees, but keep the mulch from touching the trunk; six inches away, at a minimum. But if you have clay soil, slow draining or compacted soil, or just want to play it safe, Debbie Flower, Professor of Horticulture at American River College in Sacramento, offers this advice to stave off decline in a citrus tree: place the mulch from about two feet away from the trunk to beyond the outer canopy of the tree.




For newly planted citrus, move the mulch to the outer edge of the canopy (and beyond).








This way, the shallow roots near the root crown (as well as the crown area itself) have less of a chance of getting root rot problems. 

Another strategy: plant citrus in raised beds or containers to improve drainage. And perhaps a string or two of the old style C9 Christmas lights for some added heat on frosty nights. And while you're at it, perhaps some insulating pipe wrap around the trunk.

Rain, Wind...Frost? Welcome to Late November, Citrus Tree Owners!

Today, it was an inch of rain. Tomorrow, the rain clears out and it's supposed to blow to 40 mph in the Sacramento area. The next night...frost? The National Weather Service is predicting overnight lows Friday and Saturday to dip into the mid-30's. But depending where you live, it could be lower. All gardening is local. 

We are now officially entering the shivering season for the Central Valley, Bay Area and low foothills. Late November through mid-February is the most critical time here for protecting frost-susceptible plants.

Frost Cloths Protecting Lemons, Mandarins, Oranges

This is especially true for citrus tree owners, who are anxiously keeping an eye out on the upcoming weather forecasts.

Several days before an expected frost (temperatures dipping down to 32 degrees) is the time to gather the necessary implements to protect your citrus trees, including giving the ground beneath them a good soaking (moist soil is better than dry soil at moderating the temperature beneath the tree).

Most gardeners first thoughts about protecting their citrus trees during a frost or freeze is, "protect the fruit!"

Four Winds Growers, the Winters-based wholesale grower of many excellent varieties of citrus, offers the Citrus Variety Information Chart at their website, FourWindsGrowers.com. 

Included in that chart is extensive information about each citrus variety, including suitability for indoor growing; its bloom and fruiting seasons; its recommended summer heat level to produce good fruit; and, its minimum tolerable temperature for preservation of fruit quality.

  The chart points out that lemons, limes and citrons are most sensitive to frost, while sweet oranges, grapefruit, tangerines and calamondins are intermediate. Kumquats and Owari Satsuma Mandarins are the most frost-tolerant, braving temperatures into the twenties (that would classify as a freeze).

From that chart, here are the temperatures (in degrees Fahrenheit) at which citrus fruit damage may occur.

Sweet Oranges

Washington Navel Orange  28
Trovita Orange  28
Cara Cara (Pink) Navel Orange 28
Lane Late Navel Orange  28
Robertson Navel Orange  28
Shamouti Orange (Jaffa Palestine)  28
Valencia Orange  28
Midknight Valencia Orange  28  

Blood Oranges
Moro Blood Orange  28
Sanquinelli Blood Orange  28
Tarocco Blood Orange  28
 

Sour Oranges
Bouquet De Fleurs Sour Orange  28
Chinotto Sour Orange (Myrtle-Leaf)  28
Seville Sour Orange  28
Bergamot Sour Orange  32
 

Mandarins
Gold Nugget Mandarin (Patented)  26
Tango Mandarin (Patented)  32 
Owari Satsuma Mandarin  24
Dancy Tangerine  32
Clementine Mandarin (Algerian)  28
Murcott Mandarin  32
California Honey Mandarin  32
W. Murcott Mandarin  32
Kinnow Mandarin  32
Kara Mandarin  32
Page Mandarin  32
Piie Mandarin  32
Kishu Mandarin  32
 

Lemons
Improved Meyer Lemon  32
Eureka Lemon  32
Lisbon Lemon  32
Ponderosa Lemon  32
Variegated Pink Lemon  32
Yen Ben Lemon  32  

Mediterranean Lemons
Villafranca Lemon  32
Genoa Lemon (Gea)  32
Limonero Fino Lemon  32
 

Limettas
Millsweet Acidless Limetta  32
Marrakech Limetta  32
 

Limes
Bearss Seedless Lime (TahitiPersian)  30
Kaffir Lime (KiefferThaiWild)  32
Meican Lime (Key)  32
Thornless Meican Lime  32
Meican Sweet Lime  30
Palestine Sweet Lime  30
Rangpur Lime  32
 

Grapefruits
Oroblanco Grapefruit  32
Rio Red Grapefruit  28
Star Ruby Grapefruit  28
Chandler Pummelo  28
Cocktail Grapefruit  28
Chinese Grapefruit  28
Melogold Grapefruit  28
 

Kumquats
Meiwa Kumquat  28
Nagami Kumquat  24
Indio Mandarinquat  26
Centennial Variegated Kumquat  30
Nordmann Seedless Nagami Kumquat  28
Marumi Kumquat  26
Eustis Limequat  32
 

Citrons
Buddha's Hand  Fingered Citron  32
Etrog Citron (Ethrog)  32
 

Other Interesting Varieties
Minneola Tangelo  28   
Australian Finger Lime  32   
Yuzu  24   
Calamondin  32 
Variegated Calamondin  32

But what about the overall health of the citrus tree? How low can temperatures go during a freeze event before the tree is toast?

 "I consider 22 degrees to be terminal for citrus tree cambium cells," says Cedar Seeger of Four Winds Growers. The cambium layer is the growing part of the tree, the cells that are producing new wood and healing wounds. It is located just beneath the bark.

And that's for a citrus tree in tip-top shape: good health, with moist soil around it during a freeze. Cedar uses the example of a Meyer lemon tree:

Blanket + Tomato Cage for Citrus Protection

"We often have a two to three hour dip to 28 degrees after storms; and if the above conditions are met, even Meyer lemon trees can survive, albeit not to happily, without protection. 28 degrees for four hours probably won't kill the tree, provided the rootstock cambium doesn't freeze. It will defoliate and lose twigs. At 24 degrees things start to get dicey. That's when the blanket, frost cloth, bonnet and/or the old-style, large outdoor Christmas lights that give off some heat will work wonders. Remember, those blankets and bonnets need to go to the ground in all cases."

Chandler Pummelo, Pummeled by 2010 Freeze

You may recall Thanksgiving Week of 2010, when morning low temperatures dipped well below freezing for six days in a row, led by a citrus-killing 27 degree morning on Thanksgiving.








When a large, cold-air mass moves in from the north after a storm in the winter, that is called an advective freeze. The one that sticks out in most gardeners' memories here was the freeze of mid-December 1990, when nighttime temperatures fell into the teens for several days in a row, with a couple of days that didn't climb above 32 degrees. To add even more injury, a second cold snap hit near the end of the month, with temperatures dipping into the mid-20's. Many of the most susceptible (frost intolerant) landscapes were completely lost; some nurseries never recovered.

"Our first year in the citrus business here in Winters was 1990-91. My wife, Mary Helen, and I have a Masters in Disaster," says Cedar Seeger. 

Which is why Cedar is an adherent of watching the dew point, the temperature at which saturation has been reached, when water vapor condenses into water. The lower the dew point, the more danger of cold damage to your plants. One good online source for dew point temperatures is the National Weather Service's Tabular Forecast Page  , which offers a forecast for two days in advance (that link is for Sacramento).

"A good watering going into an advection night is mandatory. If the ground and surrounding grass is wet, it creates a micro dew point environment around the trees. In a dry, cold La Nina winter such as we're about to get, it is important to remember the dew point concept. At 22 and below, it's full on emergency response, pile straw, hay around trunks, anything, lights, covering," says Cedar, a man who learned these lessons the hard way.  But he is not an adherent of running sprinklers during a freeze.

"My experience with overhead sprinklers is that they more often than not freeze up, and then it's all over. And you are risking branch breakage on that ice-entombed citrus. Yeah, it can work, but screw it; it's messy, risky and a lot of work. Use the large Christmas lights and mid-weight frost covers. But pay attention to the trunk / rootstock. If that freezes, it's a goner."


Citrus trees most at risk to fatal damage from a frost or freeze are the young trees. It is vital that they be covered completely when a heavy frost or freeze is predicted, and provide protection for the trunk, bud union and rootstock area. That can include trunk wraps, newspapers, old carpeting. If possible, move small containerized citrus closer to the house, preferably next to a south or west-facing wall to maximize reflected heat.

In the book, "All About Citrus and Subtropical Fruits", author and grower Lance Walheim also suggests applying a copper-based fungicide to the trunk and then mounding or banking soil against the trunk and lower limbs. Just don't leave it on too long; fungal rots can develop (that copper can only work for a limited time). He advises leaving that soil next to the trunk from Thanksgiving until February, or March, in colder areas.


And Thanksgiving, by the way, is next Thursday. Welcome to Turkey Weather season!

Gardening Shoes...NOT "Garden Shoes"

GARDENING SHOES OR "GARDEN SHOES"?

          



VS.

 

It's time to revisit the pros and cons of the various styles of gardening shoes here.

Note that I said "gardening shoes", not "garden shoes". 

There is a difference. 

"Gardening shoes" imply footwear that combines function, comfort and safety while working outdoors. 

"Garden shoes" are an amalgam of style and color, with an overabundance of plastic. Their job: make the gardener look good, as in "Don't these purple sandals complement my English Lavender? And they go so well with the mauve wall color in our dining room!"

"Gardening shoes" are worn by horticultural heroes for whom the term "sweat shirt" means "cool weather warmth" or "The stinky, wet short-sleeve tee with the 1977 Lynyrd Skynyrd Tour Schedule on the back that gets worn every Saturday".

These are the gardeners who are dripping perspiration by 10 a.m., after spending the early morning shoveling, wheelbarrowing and transplanting.

"Garden shoes" are on the feet of rose snippers, leaf fondlers and blossom sniffers who wander through their garden after Sunday brunch, a copy of Sunset magazine in one hand, a Mimosa in the other. Their main job: straighten up the area so that their landscape service doesn't think they are slobs. Their shoes were not constructed to traverse any of these outdoor conditions: mud, weeds, snails or walking any distance more than the 70 feet between the front porch and the back fence. 

And that's assuming they have a "backyard". Sorry, but that patio/outdoor kitchen (complete with overhead fan and pizza oven) inlaid with the 400 square feet of travertine tile that supports six oversized terracotta planters from Pottery Barn doesn't count. This group can take a swiveling seat in their Martha Stewart Bistro table and chair setup and go back to reading the artichoke quiche recipes in Sunset while us gardeners talk practical footwear.

What do hard working gardeners want in a shoe? A shoe that doesn't distract from a day in the yard. A shoe that a gardener can spend the entire day wearing without a complaint, racking up the miles and the fingernail dirt. A worry-free shoe that doesn't make you stop in mid-pruning to take care of blisters, cramped toes or sticky weeds that work their way into the socks. A shoe that repels water during a broken sprinkler repair job. A shoe with a solid sole for shoveling and a reinforced toe for when you drop your shovel. A shoe with no laces to come undone. A shoe with a sole that has some grip on slippery surfaces but doesn't track excess mud into the house.

No single shoe meets all those requirements. Some, though, come close. Among the popular gardening shoe brands mentioned by the radio listeners over the past couple of years are Bogs, Sloggers, Muckboots, Crocs, Blundstones. Me? I prefer either the Blundstones or Merrells. My wife likes her Birki-style clogs.

A review, based on 30 years of yardwork, as well as input from other gardeners. The grading system:

**** Great for gardening use. You won't think about them at all.

*** Recommended, with certain limitations. Good backup pair.

** Feels good when you put them on... you'll regret it later.

*  Great while sitting on the patio. Hey, is there something burning in your outdoor pizza oven?

RUBBER BOOTS

Function: *** Does it all for Pacific Northwest gardeners.

Comfort: ** Fine, for about 15 minutes.

Safety: **** Ain't nothin' gonna attack you through that!

HALF BOOTS

Function: **** Good for hauling, climbing, shopping!

Comfort: **** All day gardening, any you don't think about your feet!

Safety: *** Stiff toe for when you miss with the shovel.

HIKING SHOES

Function: *** Does it all; laces attract sticky weeds and mud.

Comfort: *** With the right socks, yes.

Safety: *** Won't stop the mud in the house.

SLIP-ONS 

Function: *** Hard sole is good for digging.

Comfort: **** Wear 'em all day!

Safety: *** Clean before going inside, or you'll never hear the end of it.

GARDEN CLOGS

Function: *** Not for digging hard clay soil.

Comfort: **** Yes, until the insides are gravelly.

Safety: *** Watch your ankles for insect bites.

SNEAKERS

Function: *** A harder sole is better for digging.

Comfort: *** Excellent, until wet.

Safety: ** Foxtails stick in laces.

SANDALS

Function: * Have you ever tried digging in sandals?

Comfort: ** Fine, til the bottoms of your feet get sweaty.

Safety: * Choose nail polish to match stubbed toes.

"GARDEN SHOES"

Function: * Matches blooms of "Cherish" rose.

Comfort: ** While sitting, drinking Mimosas...sure!

Safety: * "Mary Jane" style subjects foot to hot, dropping ashes.

AERATOR SHOES

Function: * Compacts soil. Twists ankles.

Comfort: * If you like the feel of nipple rings, you'll like these.

Safety: * Not for use during backyard games of "Twister".

NO SHOES

Function: * Slug squishing keeps feet cool!

Comfort: ** Fine, until you step on a rock.

Safety: * At least your socks won't bunch up.

Avocado Trees in Sacramento? Good Luck.

Can you grow avocados in the Sacramento area? Maybe. Probably not. It depends. Good luck. 
People loooovvvve avocados. And that love overrides the commons sense of intrepid area gardeners. Admit it: you've enjoyed that Hass avocado from the grocery store; saved the pit; suspended it on toothpicks over a glass of water; then, planted it outside when that pit sprouted roots.
How'd that work out for you?

The deck is stacked against growing fruit-bearing avocado trees here: 
• Choosing the wrong variety. The Hass avocado tree, for example, becomes avo-mush after a season of our typically freezing December-January mornings.

• Wrong location. Avocado trees need nearby reflective heat surfaces for best survivability: south or west facing walls, nearby concrete or brick patios, shelter from our hot, dry winds. Although they will grow in the shade, they need full sun to fruit.

• Wrong soil...aka, heavy, wet, clay soil.

Yet, despite these natural hurdles, the most asked question in the last two years has been: "Can I grow avocados here?"
I've stopped saying, "just go to the grocery store", after seeing pictures of producing avocado trees in Sacramento. Still, they are the exception (quick: how many avocado trees are in your neighborhood? Zero? Listen to your brain.)
But for those still attempting this Sisyphusian task, I offer some guidance. 

For example:

From the garden email bag, Nelson wants to know:
"Hi Fred, I had a chance to hear your show for the first time on Sunday while driving. I had no idea that a show like this was going on in Sacramento. I wanted to ask you a question because none of my friends seem to know. I live in Elk Grove and have an avocado tree that I planted early this spring. Its grown about a good 8-12 inches in the past few months and has been doing well. In October, I started noticing the leaves begin to brown. The tree sees about 4 hours of sunlight a day due to the location of the tree and surrounding shade trees and home. Any suggestions on what this browning and how to take care of it would be great!"

Watering may be the culprit. Avocados need moisture, but they need excellent drainage. If the soil is too moist for too long, that can cause leaf drop due to any number of soil borne diseases such as verticillium wilt or root rot (phytophthora). Check the moisture level around the root zone at the outer edges of the tree, about 10 inches down, before you water. A moisture meter can aid you in this. Planting on mounds can alleviate the problem of poor drainage, as well.

Good water management is essential to reduce the threat of rot. It prolongs the life of diseased trees and can prevent spread to other trees. Moreover, the only way to know how much water is correct for your soil is using that moisture meter or digging down to the root zone and grabbing a handful of the soil. If that handful is muddy, don’t add more water yet. If that soil is bone dry, then definitely increase your watering frequency. The ideal moisture level for that handful of soil: you should be able to form a clod easily with your hand, yet break it apart easily. There should not be water running down your arm.


For those interested in trying to grow avocados here, choose varieties that have a chance to survive our cold winters. 
Among the avocado varieties that may succeed here are the Bacon, Zutano, Mexicola, Fuerte, Pinkerton, Jim, Sir Prize and Stewart. These have better cold tolerance; however, success here with avocados is iffy. Placing the tree where it can get reflected heat in the winter from a south-facing wall may help.

What about the Hass Avocado?
That tasty grocery store staple prefers to grow in the most mild areas of California, where frosts are very rare: San Diego and Ventura counties are prime Hass avocado growing regions. Try to grow a Hass here, and it will probably die. Probably. I used to never say "probably". Then, this past summer, listener Gayle of Dixon (west of Davis) sent in this:
 
Yep, a Hass avocado tree, albeit a small one.  
ALL GARDENING IS LOCAL (Farmer Fred Rule #1). Get back to me, Gayle, when that tree gets bigger than the house. Kids, please don't try this at home.

More Advice for the Brave Sacramento Gardener 

Another tip to get avocados to bear fruit: plant two different varieties. Avocado pollination is increased when you have two types of avocado trees, one with an "A" flower and one with a "B" flower. These flower types open at different times of the day. According to the California Rare Fruit Growers Association, avocado flowers are either receptive to pollen in the morning and shed pollen the following afternoon (Type A), or are receptive to pollen in the afternoon, and shed pollen the following morning (Type B). The latest edition of the Sunset Western Garden Book lists the “Type A” flower varieties to include the Mexicola and Pinkerton. The Bacon, Jim, Zutano, Fuerte, SirPrize, and Stewart are Type B flowers. Stewart, actually, may be both Type A and B, depending on where it is grown. These Mexican avocados mature 6-8 months after flowering.

The California Rare Fruit Growers advise avocado tree owners to use a balanced fertilizer, four times yearly. Older trees benefit from feeding with a nitrogen fertilizer, applied in late winter and early summer. Yellowed leaves (chlorosis) may indicate iron deficiency. A chelated foliar spray of trace elements containing iron can usually correct this. Mature trees often also show a zinc deficiency.

More about the flowers of avocados, from the
Avocado Information website of the University of California Agricultural and Natural Resources: 

The avocado flower has both functional male and female organs. The male floral organ, which produces pollen, is comprised of the anthers and stamens. The female floral organ is comprised of the stigma (which receives the pollen), style and the ovary. The avocado exhibits a type of flowering behavior known as "synchronous dichogamy". An individual flower will be open for 2 days, however the timing of the male and female phases are distinct. When the flower first opens it is in the female phase and the stigma is receptive to pollen. At the end of the female phase, which lasts 2 to 4 hours, the flower will close. On the second day the same flower re-opens in the male phase and sheds its pollen.

"A" Avocado Flower Varieties

 
Mexicola 
Mexican varieties have an anise or fennel-like smell to the leaves. 
This is a very typical Mexican variety. Originated about 1910 as a seedling at Coolidge Rare Plant Gardens, Pasadena, CA,  Fruit season, Aug.-Oct.; color, black; weight, 3-5 ozs.; shape, spherical to pyriform; skin, thin, smooth; flavor, excellent.  One of the first varieties to be commercial, planted by Mr. Henry Huntington around 1915-17. It has an anise flavor & is quite good tasting.  Bears early & regularly; very heat & cold resistant (down to 20 degrees...maybe).


Pinkerton
Commercial variety. Harvest begins in January in some commercial areas. 
High oil content. Medium slightly spreading tree. Hardy to 30 degrees.


 





"B" Avocado Flower Varieties
 
Bacon
Originated in Buena Park, California, by James Bacon. Introduced in 1951. 
Flesh has an unusually pale yellow/green color. Oil content high. Matures Nov. to Jan. in Orange County and Dec. to Mar. in Ventura County, Ca. Consistant,heavy producer; frost tolerance excellent. Bacon has a commercial background. Large trees.


Jim
Was commercial at one time. 
Color when immature is a very light avocado green. Some fruit has a rosy blush that does not appear to be caused by thrips.

 





Zutano
Originated in Fallbrook, California, by W.L. Ruitt. Introduced in 1941 from a selection made in 1926. 
Ripens in Oct. to Dec. in San Gabriel Valley, Ca.; and Jan. to Feb. in Ventura County, Ca. Tree; consistent producer; hardy to 24 degrees; more hardy than Fuerte. Commercial variety. Fruit are oval/pear in shape with waxy bumps on the skin.

 



Fuerte
Introduced as budwood in 1911 from Atlixco, Puebla, Mexico. A hybrid Mexican variety that is ready to pick in November and is good through March. 
Hangs on the tree well. A long time California commercial variety valued for its winter season and its B blossom type. Skin thickness is medium thin. Seed size is medium large.

 



SirPrize
Season of maturity averages 6-8 weeks earlier than Hass. Mexican-race type avocados are typically more cold resistant than 'Hass' so this tree is being tried in inland valleys and other regions unsuitable for Hass. Early results do not indicate it is any more cold tolerant than Hass. 
Fruit shape is pear with distinctive ridge along one side. Skin texture is not truly pebbled like 'Hass' but does have tiny islands of varying yellowish shades giving the illusion of 'Hass'-like pebbles. 'Sir Prize' is upright in tree form. Peak bloom period is earlier than 'Hass' by several weeks.

 

Stewart  (A or B flower) 
Originated in Mentone, San Bernardino County, Ca, on the Stewart Ranch. 
Introduced in 1956. Flesh clear, bright, light yellow shading to green toward skin, firm, but melting, excellent quality; ripens from early Oct. to mid-Dec. Tree: spreading, strong, vigorous; bears good crops. Although listed as an "A" type flower, it typically shows "B" flowering characteristics at South Coast Field Station in Irvine, Ca.

Bottom Line
If you want to experiment, are willing to be patient, and have a wind-sheltered area with many surfaces of reflective heat, try the Mexicola and the Zutano varieties of avocado trees here in the Central Valley. And good luck.

Pictures courtesy of the Avocado Information website of the University of California Agricultural and Natural Resources...
except for the pictures of Nelson's suffering avocado tree in Elk Grove and Gayle's magic Hass avocado tree in Dixon.