Between a Rock and a Hard Place

More knowledge makes for a better gardener. That’s what I had in mind with my most recent book, The Ever Curious Gardener, excerpted here:

With hot weather here today, and soon to be a regular occurance, I pity my plants. While I can jump into some cool water, sit in front of a fan, or at least duck into the shade, my plants are tethered in place no matter what the weather. And don’t think that plants enjoy searing sunlight. High temperatures cause plants to dry out and consume stored energy faster than it can be replenished. Stress begins at about 86 degrees Fahrenheit, with leaves beginning to cook at about 20 degrees above that.
Watering with hose
One recourse plants have in hot weather is to cool themselves by transpiring water. Transpiration, which is the loss of water from leaves, can cool a plant by about 5 degrees Fahrenheit. Over ninety percent of the water taken up by plants runs right through them, up into the air, exiting through little holes in the leaves, called stomates. Carbon dioxide and oxygen, the gases plants need to carry on photosynthesis, also pass in and out through the stomates.

All this is fine provided there is enough water in the ground. If not, stomates close, transpiration and photosynthesis stop, and the plant warms. Even if the soil is moist, stomates might close in midsummer around midday if leaves begin to jettison water faster than the roots can drink it in. This situation puts most plants in a bind. Should they open their pores so that photosynthesis can carry on to give them energy, but risk drying out, or should they close up their pores to conserve water, but suffer lack of energy?

CAM at work

Enter cacti and other succulents (all cacti are succulents—that is, plants with especially fleshy leaves or stems—but not all succulents are cacti): their fleshy stems and leaves can store water for long periods. After more than a year without a drop of water, my aloe plant’s leaves still look plump and happy.

Besides being able to store water in their stems and leaves, jade plants, aloes, cacti, purslane, and other succulents have another special trick, Crassulacean Acid Metabolism, for getting out of this conundrum.

Aloe plant

Aloe plant, more than a year without water!

They work the night shift, opening their pores only in darkness, when little water is lost, and latching onto carbon dioxide at night by incorporating it into malic acid, which is stored until the next day. Come daylight, the pores close up, conserving water, and the malic acid splits apart to release carbon dioxide within the plant, to be used, with sunlight, to make energy.

I’ve actually tasted the result of this trick in summer by nibbling a leaf of purslane—a common weed, sometimes cultivated—at night and then another one in the afternoon.
Malic acid makes the night-harvested purslane more tart than the one harvested in daylight. Try it.

No CAM? How ‘Bout C4?

Another group of plants, called C4 plants, function efficiently at temperatures that have most other plants gasping for air and water. C4 plants capture carbon dioxide in malate, the ionic form of malic acid, which is a four-carbon molecule, rather than the three-carbon molecule by which most plants—which are “C3”—latch onto carbon.

The enzyme that drives the C4 reaction is so efficient that C4 plants do not have to keep their stomates open as much as do C3 plants. The C4 pathway also does its best work at temperatures that would eventually kill a C3 plant, and cells involved in the various steps are partitioned within the leaf for greatest efficiency.

C4 plants are indigenous to parched climates, but not uncommon visitors in our gardens. Corn is a C4 plant. (Cool climate grains such as wheat, rye, and oats, are C3 plants.)
Tall corn plant
Looking at my lawn, I see another C4 plant. Hot, dry weather in August drives Kentucky bluegrass, a C3 grass, into dormancy. Not so for crabgrass, a C4 plant, which remains happily green.

I also find some other C4 plants, in addition to corn, in my garden. As many vegetables and flowers flag, all of a sudden lambsquarters and pigweed, both C4 weeds (or vegetables, for those who like to eat them), appear as lush as spinach in spring.

Gardener’s Assistance

Can I do anything to help out my plants in hot weather? Keeping the garden watered helps. (Ways to apply water and how much is needed are all-important, and topics unto themselves.)

Sprinkling or misting plants could keep them cool without their having to pull water up from the soil. But the thirty gallons of water that runs up through a tomato plant in a season, or the fifty gallons that flows through a corn plant, is for more than just cooling these plants. It also carries dissolved minerals from the soil into the plant. So it’s debatable how well a plant would grow with too much misting. And besides, wet plants are predisposed to disease.

A better alternative to sprinkling plants is to grow plants adapted to the climate and the season. My lettuce, spinach, peas, and radishes are doing fine now; despite today’s heat, it’s not really all that hot — yet. And nights are still cool. Mostly, I avoid growing these cool weather plants in summer. Except that I like my lettuce salads, so I extend its season by growing it in the shade beneath trellised cucumbers.
Lettuce & trellised cukes}
Fortunately, tomatoes, cucumbers, peppers, melons, and squashes, although they are neither cacti nor C4 plants, can take quite a bit of heat. They have very deep roots.


Don’t Go With the Gut

   “April showers bring May flowers.” Where? Not here. I had many years of gardening under my belt before I realized the falsity of that little ditty. Yes, it happens to be raining as I write these words at the end of April. But generally, April is dry, and todays’s rain amounted to a mere quarter-inch.
    For years, I would sow radishes, lettuce, arugula, and other early, cool season vegetables in April, and figure “April showers” would take care of watering needs. Which they did not. With frosts still likely around here, April was too early to get the drip irrigation going. So watering vegetable beds requires tedious lugging of hoses, making sure, as I pull on them, not to knock over plants or mess up carefully formed beds.
 Watering wand   Hand watering also required patience. A light sprinkling of the ground does nothing but wet the surface fraction of an inch. If you care to know, the amount of water needed to wet a soil about 6 inches deep is 3/4 of a gallon per square foot or, equivalently, a one-inch depth of water. And said water needs to be applied slowly enough to percolate into the ground rather than running off elsewhere.
    Fortunately, with cool temperatures and plant growth only just beginning, much of the moisture from previous months’ rain and snow still sits in the soil. The surface often looks dry while moisture is sufficient below. So all that’s needed is to keep the soil moist from the surface to a depth where the ground is still moist.


    Eyes or the gut are not reliable indicators of when to water. Sometimes I’ll just poke my finger into the ground to assess moisture level. But does it really feel wet, or does the soil feel cool because it’s April?
    All that’s needed are your hands for the more accurate “feel method” of soil moisture determination. For this method, you need to know your soil’s texture; that is, is the soil a sand, a loam, or a clay? Crumbling it, feeling its slickness, and attempting to form it into a ball or a ribbon gives some indication, depending on the texture, of moisture content (see for details).
    All sorts of high-tech soil moisture measuring devices are available: electrical resistance blocks, tensiometer, time domain reflectometer, neutron probe, and more, all beyond the wallet and accuracy required by of most gardeners.pH, moisture testor
    On the other hand, inexpensive soil moisture meters are readily available. What they lack in accuracy they make up for in convenience. Sliding the thin, metal rod into the ground gives a pretty good, qualitative measure of moisture anywhere from just below the surface to the length of the rod, which, depending on the device, is 8 to 12” inches long.
    Watering is an important key to success in gardening, and for $10 to $50, these relatively moisture meters are well worth the money. The more expensive, previously mentioned monitors can be left in the ground. The inexpensive meters must be removed and their probes cleaned after each measurement.

Indoor, April Showers Bring . . . Yes!

    Even if April showers do not bring May flowers, April — the month — does bring a few May flowers, but mostly flowers in June and beyond. That’s because April is the month when I sow many flowers for transplanting out into the garden after warm weather settles in. On the slate for this year are Lemon Gem marigold, sunflowers of all stripes, chamomile, moonflower, morning glory . . . (Where am I going to plant all these flowers? Spring’s got a hold on me.)

Tussler Was Correct After All

    The ditty “April showers bring May flowers” really does have some truth to it — if you’re in the UK, where the ditty originated. There, the northward moving jet stream picks up more and more moisture as it travels across the Atlantic Ocean; the result is rain by the time the winds reach the UK. (On this side of the “pond,” the jet stream, traveling across land, has picked up relatively little moisture.)
    So Thomas Tusser, who allegedly penned “Sweet April showers / Do spring May flowers” in the 16th century (in A Hundred Good Points of Husbandry), was not wrong, for Great Britain. Here, we have to water in April. But not too much.