Growing Plants in Times Like These

Unexpected Heat kills.

So does cold.  From 1962 to 1989, temperatures where I live in Southeast Texas dipped to 10 or below on an average of every 9 years.  Since then, in the southern half of Houston, there haven’t been any such temperatures.  This last February, the 15˚F temperatures where I live were the coldest in any month since 1989.  It was 35˚F below normal in early spring when plants were actively growing. It was also the coldest February day in 70 years, the first February freeze where I am in several years, and the coldest temperature that low after mid-February since 1899. 

But it wasn’t as cold as the 1980s subteens, and it wasn’t every 9 years. However, it killed perhaps 200 people and broke thousands of water pipes.  At our house, we had irrigation system damage, lost 26 of 31 citrus trees, and 8 other subtropical fruit trees, because our fruit trees had broken dormancy.  Several temperate trees like plums were crop failures.  

Unexpected cold kills, too.

The climate and rain

Just a few years ago during Hurricane Harvey, Nederland, TX (between Beaumont and Port Arthur), got over 5 feet of rain in 4 days. During that same rainstorm, based on calculations from more than a century of experience, the southeast of Houston’s Harris County had a rainstorm that should happen “once in 20,000 years.”

Just 10 years ago, a Texas Hill Country site had what was estimated from tree rings, the driest spring growing season for trees since the mid 1500’s. Despite the terrible drought of 2011, the collapse at that time of Houston’s backup water supply at Lake Conroe, or the fire threat to our largest urban park, droughts are not becoming more frequent, but because of heat, they are getting more dangerous.  

Except for coastal islands, yearly rainfall here has increased a little, but it often comes in the form of multi day storms that saturate the soil, breed root damaging pathogens, and are harmful rather than beneficial. Five of our area’s 10 wettest years and most of our wettest storms have been in the last two decades. In this century, a third of the years have had a month with more than 12 inches of rain, and ¾ of the years have also had a month with 10-12 inches. 

But 70% of years have also had a month with less than one inch  and sometimes, this is with temperatures in the 90’s.

Our temperatures are increasing too.  Most of the 10 hottest years and hottest summers have happened at Southeast Texas weather stations since 2000.  

So, evaporation is increasing.  And in dry spells between the gully washers, thirsty plants will have ever increasing problems.  For information on US trends, see https://www.weather.gov/tbw/newnormals .

The plants

When we talk about climate change, the focus is often understandably on people, their suffering, and property loss. And much of that loss is because they are poorly prepared for unprecedented or rare events. But plants too need stable weather, are poorly prepared for rare or unprecedented events, and they suffer badly.  And unfortunately, some disasters are a challenge to design for.

We all know plants need water, and everyone knows what wilting in hot weather leads to.  Plants use water to fix carbon, to transport minerals and sugar to their leaves and cells, and those that lack wood use water to keep themselves upright.  They also use a lot of it to cool themselves on a hot day.  

Plants transpire and once temperatures get into the 90’s, most plants use their water to cool themselves rather than to grow. If this evaporation is needed more rapidly than the roots can get it, leaves die and sometimes plants do too.  Water shortage has other profound effects, but my point is this: if the temperatures rise into the 90’s, most plants if they survive will stop growing and in the 100’s almost all of them will.

Those of us who grow plants in subtropical areas are aware of the cold temperature tolerances of plants we grow whether they be native or exotic, for habitat, food, or other reasons.  What most growers, wherever they are, have been less aware of, and need to learn, is the hot temperature tolerances of plants and the likely hot temperatures in the months and years ahead.  

Basically, all plants and animals have optimum temperatures in which they thrive, and a wider range both colder and hotter where they survive but don’t grow. At even colder or hotter temperatures, they struggle and have pest and disease problems, and at still colder temperatures or hotter ones from optimum they are heavily damaged or die.  These temperatures are related to the biome where they evolved and to a lesser extent to later efforts to adapt them to other places.

Plants often have different optimum growing temperatures for day air, night air, soil germination, pollination, harvesting, and for perennials, winter and summer averages, lows, highs, as well as decadal highs and lows.  Asparagus, for example, needs below 50˚F soil temperatures for three months to do well; tomatoes have different nighttime and daytime temperature ranges for pollination; papayas crater at 30˚F; and most apples are in trouble when winter temperatures are too warm in mid-winter or too cold during flowering.  

Plants also have specific tolerances for moisture—too much or too little spell trouble, especially combined with unsuitable temperatures.

What to do?

What does this mean to growers when temperatures are now increasing rapidly and somewhat unpredictably?  And in times where the arctic vortex may be losing its ability to keep the jet streams regular while the oceans, large lakes, and gulfs continue to heat thus increasing the potential for torrential rains, violent storms, and wild temperature swings?

It means that in addition to doing everything we can to slow and reverse climate change, we need to be good at observing our local climate to guess the future temperatures and rainfalls for plants we want to grow.  We need to learn what we can about the big pattern—global warming—and as much as we can about its local patterning.  We also need good information about the temperature and moisture tolerances of specific plants. And we probably need to be lucky.

A good start is knowing both what temperatures you have had in the last 15 years where you are growing your plants and what hot and cold temperatures your plants thrive in and can tolerate.  If you are planting 5-month annuals, you need to plant them when this year’s temperatures where you are will be optimum for the plant. And if the temperatures are usually not what they were 30 years ago, you need to change your planting date.  

And if you are planting fruit trees or other perennials that you expect to live for 1-3 decades or more, they will need to tolerate longer summers, hotter temperatures, shorter winters, more drought and flood, and the likely temperature extremes. You may or may not have fewer cold threats, but your average lows will probably get higher.  

You also may need to use raised beds, rain gardens, swales, rain tanks, ponds, drains, swales, and Keyline approaches to reduce flooding, improve water retention, and improve drainage.  Also, you need to use shade, trees and prairie, biologically rich mulches, and microbe friendly organics to increase the number and length of beneficial root-microbe associations, and to encourage deep water reserves.  Trees transpire, but forests cool, and shade reduces surface evaporation.

I have explained how to do this for most Southeast Texas food plants in my book Year-Round Food Gardening for Houston and Southeast Texas (see http://yearroundgardening.ME for sources).  Broadly I have provided lists of temperatures that more than 140 annual food plants need to thrive, as well as temperatures that bother or kill them. I did the best I could on this despite the poor availability of such data in the literature. 

For short-lived plants like lettuce, we growers also need to know where they are, what the temperatures will probably be in the next months.  And for longer lived plants like oranges, papayas, or raspberries, we need to guess likely temperature spans for many years based on recent observations like averages and extremes for the last 15 and 30 years.  

These data are being collected all over the US, Canada, and other nations, but are not always easy to find. If you have trouble online using links like http://weather.gov or http://climate.gov , ask a weather or climate specialist at a university or an NWS site. They will know where the data is if it exists.

For Southeast Texas, I have provided a planting schedule for mainly vegetables based on vegetable planting zones A-G that differ in seasonal low temperature averages of 2˚F (about 1.1˚C) per zone. This will help someone know, even as climate warms, when to plant based on local temperature observations in Southeast Texas.  It also provides fruit tree low-temperature tolerances, so growers can adjust their planting to their location’s warming, and their own guess work.  

Outside of Southeast Texas, there is even more work to do.  This basic work is missing in most places even though the weather is changing. And unfortunately, good temperature tolerance data is not systematically published or verified.  So, there is much to learn, and another reason to slow greenhouse gas usage and take as much as you can out of the air using plants.

By Bob Randall

Bob is a food systems anthropologist and longtime food systems community organizer in Southeast Texas.  He is the Secretary of PINA and has Pc Diplomas in both Design and Education, and a Ph.D. from Cal-Berkeley.