Reprinted from Southern Fruit Fellowship Newsletter, Fall 2012
Temperatures and Semi-Tropical Fruit:
More “Fruitful” Ways to Predict the Weather
By Bob Randall
Gardeners, orchardists, farmers and landscapers are some of the planet’s great weather forecasters. We fearlessly plant vegetables, fruits, and other plants in the belief that we can predict that the weather they will be dealing with in the months and years ahead will be agreeable to them.
How do we do this? Not with a bunch of weather satellites and complex computer models, but mostly by having learned a lot about the needs of the plant and the place where we will be planting it. This strategy works well as long as we didn’t just move to the place, as long as we are growing plants that have been in the area for a long time, and as long as the climate doesn’t change.
I moved to Houston and the Gulf Coast 33 years ago after having lived and gardened in seven other places that either were hotter, colder, equally humid, or much dryer, so I have been noticing climate issues and their effects for a long time. Today, in addition to our large vegetable garden, Nancy and I grow about 130 distinct fruit varieties on our quarter acre suburban lot. About 30 of these are citrus.
So we have many decades of experience growing some things, while other plants, like lychees, mangoes, star fruit, avocados, longans, grumichamas, sugar apples, jaboticabas, papayas, and tropical guavas are relatively recent trials. Will they work here or is it too cold, or too something else?
Many home growers might be satisfied with reading a bit about these plants and trying those that might work, but I am in a very different position. For the last 25 years, I have been professionally in-volved in advising gardeners and farmers what and where to plant. And I have been doing this all over the Southeast Texas Gulf Coast from areas just feet from the Gulf to dense urban areas of Houston, to farms in the middle of forests five counties and 150 miles removed from the sea.
My best selling book Year Round Vegetables, Fruits and Flowers for Metro Houston (from http://www.brazosbookstore.com) is now in its 12th edi-tion; I teach seven fruit classes for Urban Harvest (www.urbanharvest.org ) yearly, and am part of the fruit tree ordering committee for an annual fruit tree sale that sells about $130,000 worth of fruit trees in three hours each January.
So my problem is not just what will grow in my yard, but also what will grow in everyone’s yards and farms. As well, unless you have a good understanding of weather conditions where someone else is growing a fruit, you can’t easily learn from their good results or failures.
For example, many of us are growing Opal, Wilma, and Fantastic avocados. These are said to be the cold hardiest avocados on the planet because they have survived 14˚ in Devine, southwest of San Antonio. But how does Devine compare with the Houston area? In 1989, we had 7˚ here. To answer the question, we need to know not what USDA planting zone they are in (its officially Zone 9 as are we), but far more about their weather and ours.
Confronted with this situation over the last 25 years, and plenty of evidence that some areas are much warmer than others, I have made every at-tempt to understand how Metro-Houston weather varies from one location to another and one year to the next, and what this means for growing fruits and vegetables. In preparation for the next version of my book, I have studied the last 20 years of temperature data for every National Weather Service station in Southeast Texas.
For perennial fruits, by far the most important issue is winter temperatures. For tropical, semi-tropical, or semi-temperate plants that might get killed or damaged by cold, how low temperatures might go in the winter is a key question. “How often these lows occur” is equally important, because a few trees can be protected for a few days every 20 years.
For temperate plants that go truly dormant in winter, the issue is how much chill does the winter provide, how do you measure this, and what are the likelihood of late freezes after a plant has leafed out. I will discuss winter chill another time. Here I want to look at winter low temperatures, and how it is easiest to describe them and use information about them.
What I have found generally is that:
(1) Temperatures are very different from one Southeast Texas location to another. Generally, areas within a few miles of the Gulf Coast or Galveston Bay or in dense urban areas shielded from north winds are much warmer than other places.
(2) There are substantial differences between the cooler winters of El Niño years and the warmer La Niña and La Nada years; and there are espe-cially cold winter temperatures when the arctic vortex collapses.
(3) Rural places with few forest trees not protected from north winds are colder than average, and in low spots are especially cold.
(4) All other things being equal, temperatures will be colder the farther you get from the Gulf Coast.
(5) The coldest weather in the period from 1962-1992 was much colder than either the previous 60 years, or the last 20.
(6) On the coldest, windiest winter night of the year, there can be as much as a 7-degree difference between the warmest, most-protected place in a yard and in the most open, unprotected place. This difference drops to 5 degrees on still nights.
Give the importance of temperatures to the sur-vival and good growth of plants, and the importance of agriculture and eating to the general public, it is surprising that the USDA produces agricultural zones that are of so little help in actually deciding what to plant. The latest zone definitions are based on 1976-2005 “average annual extreme minimum temperature” for an area, (http://planthardiness.ars.usda.gov/PHZMWeb/AboutMapMaking.aspx).
According to them, all but a tiny area of coastal Southeast Texas is in zone 9a (average yearly low 20-24) with the more inland areas in zone 8b (lows averaging 15-19). This is completely at odds with what has been happening in Southeast Texas in the last two decades. Where I live is in what they say is Zone 9a. Actually, over the last 20 years, it has been in zone 9b (25-29˚F) or higher 70% of the time, has been in zone 9b+ or higher (28-29˚F) 50% of the time, and zone 10a- (30-32˚) or higher 30% of the time. My part of Southeast Texas has been in zone 10 as often as zone 9a and has not been in zone 8 (below 20˚) even once.
These numbers are typical of large portions of Metropolitan Houston. There is even one zone 10 site in Matagorda County that has been in Zone 11 (above 40˚) as often as Zone 9. For all of Southeast Texas and some other Texas sites, I have posted an-nual winter low temperature data for the last 20 years at my website: https://yearroundgardening.wordpress.com/2012/09/01/annual-winter-low-temperatures-in-southeast-texas-1992-2012-2/ . This summarizes low tempera-ture data for every National Weather Station with at least 10 years of data in the last two decades. And it lists each zone from 7 (0-9˚F lows) to 11 (40-49˚F lows), not by averages, but by the percentage of years the lows occurred in these zones and in 2-3 degree divisions within them.
What this means is that if say papayas need lows above 30˚, guavas above 28˚, mangoes and Meyer lemons above 25˚, oranges on trifoliate above 22˚, and so forth, it should now be possible using the table to tell a fruit grower at a specific site how protected their planting site needs to be for a fruit to live, and how often they will need to take serious protective measures to keep it alive.
Using everything I have learned, it is now possi-ble to grow the delicious Barbie Pink guava where I live. 90% of the time, it needs no help because it is in a sheltered part of my yard, and 10% of the years, it needs heavy protection for a few days. We ate a few off the new plant several years ago. But took hits in 2010 and 2011 when the coldest temperatures since the mid 1990’s dropped the area into the low 20’s and our guava’s location to 26˚. But we kept the guava alive with coverings, and this summer had a nice crop.
In reading recommendations in the Southern Fruit Fellowship newsletter, it would be interesting to know what temperatures people are growing tropical and semi-tropical temperatures under. If you are interesting in making a table like the one on my web site, or just get information for your garden or farm, one good place to look is the monthly tempera-ture records for all NWS sites. Just pay a small fee and plug in a zip code or place at http://www.weathersource.com .