It's not so much the heat... Oh, wait, yes it is...

Want some perspective on global warming?  Look at overnight low temperatures, not daytime high temperatures.  The record “warmest low” (that is, the low temperatures which were the highest on a given day) for the month of July have almost all come from recent years.  In fact, 15 of the 31 “highest minimums” have come in just the last two years.  15 of 31 high min records were set in 2009 or 2010 for the month of July.

On the other side of that coin, the “low minimum”, or “record low” for each day in July has not been set since 1994.  For any of the 31 days in July.  In fact, only five of the 31 daily record lows have been set since 1990.  25 of the 31 were set before 1968.  21 of the 31 record lows were set prior to 1936.  It simply doesn’t cool down at night here any more – it’s hot during the day, but that has always been true.  What is new is that it is also hot at night..

Climate change is real.  Just ask a gardener.  Seriously, the fact that there is still anyone at all debating whether global temperatures are actually rising or not astounds us.  For anecdotal proof, ask anyone who has grown tomatoes for any length of time whether or not the fruiting season is shorter now than it was a couple of decades ago.

The answer, of course, is almost definitely “Yes, the fruiting season is shorter now than it was when I was younger.”

Why would that be, do you think?  The answer is fairly simple.  Tomatoes are dependent upon relatively cool nighttime temperatures for pollination.  In some parts of the country, this still happens well into the summer, but in most of Texas and the Deep South, there is no longer any evening relief between mid-May and late September.  Essentially, July tomatoes are now a rarity in many parts of the country, unless they are grown indoors.  Those fruits set before the nighttime temperatures soar into the upper 70s and lower 80s are all the tomatoes you are going to get.

Jennifer Freeman of the American Meteorological Society wrote an excellent piece entitled “The New Climate Normals: Gardeners Expect Warmer Nights” in NOAA’s ClimateWatch magazine, in which all the gory details of evening temperature data are summarized fairly succinctly.  Some places may be experiencing relatively cooler July high temperatures, but on average the country is warmer everywhere, mostly due to nighttime temperatures having soared.

The implications of these findings are fairly serious for society generally, but if we just restrict our thinking to what goes on in the garden, there is still more than enough to give one pause to reflect.  The bad news is that the growing season for some crops – like tomatoes – is quite simply disappearing, at least for the Spring garden.  We will probably reach a point at which we can only get Spring tomatoes for a month, maybe a month and a half each year.  In the Fall, on the other hand, we will probably be able to get good tomatoes for a three or four month stretch in a good year, because the date of first frost will probably get pushed back further and further.

There are other implications, though, for a wide variety of fruits and vegetables, which will require experimentation.  Gardening “experts” will increasingly be sought after, and their advice will be increasingly worthless, too, as changing temperature patterns make some old truisms about gardening in various places no longer true. 

Squash, for example, is an old standby in most of Texas – “The one thing you can count on in July is getting plenty of squash.”  Well, maybe… if you plant the right variety.  It never used to matter before, but it certainly does now.  Some varieties have pollen with low toleration for nighttime heat stress, you see.  Butternut squash, for example, has big, vibrant, thriving foliage and blooms during a Texas heat wave… but unless the nighttime temperature dips into the lower 70s, those big beautiful flowers are better as filling for quesadillas than as potential new butternut squash fruit.  Varieties with smaller flowers and higher-temperature pollen tolerances still produce prodigiously, but experimentation will be required to discover which varieties these are, since this is not one of the factors most seed companies advertise – it simply wasn’t a concern before, so nobody has thought seriously about it yet.

Other plants, though, thrive in the warmer weather, including the warmer nighttime temperatures.  Some of these plants are old favorites, and are welcome everywhere – cantaloupe and watermelon, for example.  We have some thriving volunteer cantaloupe – we weren’t sure at first what these plants were, and even after they started fruiting, we couldn’t be certain until the husk took on the characteristic woody muskmelon texture.  We also have some Sugar Baby watermelons which have not only produced prodigious vines, but have also produced more fruit than we ever imagined would come out of a 5’x10’ plot.

Then there are the plants which are not so welcome for most gardeners.  We have many times described our fondness for weeds – chicken feed, actually, is what we call them – and we will doubtless have ample opportunity to do so again.  Most varieties of invasive plant do better in high carbon-dioxide environments than do domesticated crop varieties of plant. 

We are strongly considering harvesting the chenopodia variety you see here nestled amongst our Hopi Red Dye amaranth.  The Native Americans grew chenopodium berlandieri and chenopodium album intentionally as an excellent grain source and potherb, much as we grow amaranth and quinoa.  In fact, these native pigweeds are closely related to chenopodium quinoa, and while they are not as colorful, nor as prodigious in their production of grain, they do have the advantage of growing in summertime in Texas.

Likewise, the common dandelion is an excellent salad green and forage food for chickens and other domesticated livestock.  And while its growing season is shifting also due to its particular temperature sensitivities, it has the tremendous advantage of not relying on human hands to determine when the right time for planting comes along.  We can rest easy when planning our garden, knowing that we simply harvest the dandelions when they are ready – if ever there were an easier “crop” to care for, we don’t know when it might have been.  Climate change has done nothing to make this any more difficult.

We have mentioned before our philosophy when picking fruit crops to incorporate in our garden, and it is worth mentioning again in the context of warmer nighttime temperatures.  We picked pomegranates as the most prominent fruit tree in our garden due to their ability to withstand extreme heat and cold.  The fact that this plant grows in every temperature and moisture extreme from Istanbul to Kabul makes it an excellent fruit tree for the changing climate of southeastern Texas.  Likewise, the variety of blackberries we chose to plant – Brison – was developed to be more heat-tolerant than the many Arkansas cultivars of blackberry.  Recent trials in Arizona have borne out the wisdom of this choice – the Arkansas varieties burnt to a crisp in low desert farming, but Brison berries were producing five pounds of fruit per plant by the third season.

Peaches were a problematic choice for us – the best peaches in the world, with all due apologies to Georgia, where they are rightly proud of their peaches, come from the Texas Hill Country.  We live in a region too warm on average and historically too wet to produce the high quality sweet peaches we remember eating when we were younger and lived in the western half of the state.

However, there are a few varieties with relatively low chilling requirements – Earligrand peaches only require 100 hours of temperatures below 45° in order to set fruit.  As the “new normal” begins to include less and less rainfall, the sweetening influence of water stress will hopefully improve the quality of our Earligrand crop, though we still doubt the quality will ever match that found in the high chilling requirement peaches found in Fredericksburg and Luckenbach.

Rio Grande is another lower chilling requirement peach variety – 400 hours of temperatures below 45°.  This represents something of a compromise for us, because while this sounds like not a lot of time – even calculating at just being 45° or less for only 12 hours a day, this is only about a month (33 days) of “low” temperatures – but it is nevertheless a difficult target, only likely to get more difficult as the years go by and the greenhouse gasses mount up.  To put this in context, while 90° temperatures in January are not exactly common even here, they also don’t surprise us when they do happen.

So, some years we expect our Rio Grande peaches will not set fruit.  On the other hand, when they do, they should be sweeter than the Earligrand.  Another advantage lay in the fact that Rio Grande are “mid-season” producers, meaning that in a typical year, we will likely have blackberries in May and June, with Earligrand peaches in June and July, and Rio Grande peaches in July and August.  If we stagger our plantings of watermelon and cantaloupe, we should be able to have fresh fruit clear through from the last week of April up through when the pomegranates peter out in December or January.  If we ever figure out how to produce a strawberry crop, we will be set almost year-round.

First, though, we have to just survive the heat...  A nice cold glass of water, followed by a snack of peaches sounds good right about now.

Happy farming!