Spring Surprise in Ron’s Yard

By William Scheick, Contributing Editor

Meet Ron Craw, a neighbor of mine in Oak Hill. As a sideline, he once operated Tejas Native Bulbs, which some Texas Gardener readers might have seen advertised in the classifieds section. That was before recent years of drought spells and ever-increasing Austin water prices made this little enterprise financially unfeasible.

These days Ron has turned his attention to fruit-bearing bushes and trees, a big non-commercial experiment to push his front and backyard into personal productivity. He wants the fruit, for sure, but he also wants the thrill of beating the odds in an alkaline granular landscape defined by little rain, warmish winters and relentless marauding fox squirrels.

Anyone strolling past his home cannot miss the carefully pruned apple, pear, plumb, peach, jujube and pawpaw trees. Quinces, figs, loquats, guavas, persimmons and elderberries fill in nearby spaces, while a special section supports grapes.

Most of these plants flower during early spring, giving Ron’s home-scape a magical seasonal charm. A surprised passerby might stop and simply ask in amazement, “What is this place?”

Old leaves, new bees

Actually, one person, looking down at the ground rather than up at the trees, grumbled about Ron’s untidy-seeming mulching technique. Ron’s front yard does not conform to the standard suburban look. To succeed with his landscape experiment, he has covered extensive portions of his yard with leaves that he collects from others. He layers these leaves so thickly beneath his plantings that they feel cushiony underfoot. Sometimes I sink ankle-deep into them.

As this mass of leaf mulch decomposes, it nourishes the fruit trees while also retaining vital ground moisture and coolness. Maintaining a suitable pH remains a struggle for Ron, particularly since rainfall in his area is too infrequent to maintain a level of soil acidity conducive to maximized fruiting.

Given the size of his project, his rainwater reserves (pH 6.2) last only so long before he must resort to the tap. Unfortunately, his utility water (pH 7.4) tends to augment the alkalinity of the trees’ soil, which increases these plants’ vulnerability to chlorophyll depletion (chlorosis). When soil pH rises above neutral, its alkalinity restricts some plants’ nutrient uptake of minerals. Such restriction impairs the production of chlorophyll and this depletion (manifested as yellowing leaf tissue) disrupts normal metabolic activity, including fruit yield.

For fruit, pollinators remain essential. Given their declining populations, honeybees can’t be relied on as much as they once were. Consequently, some commercial fruit-growers now practice “integrated crop pollination” which deliberately attracts combinations of various native bees, sometimes in successive species sequences.

So in his backyard Ron constructed inviting native-bee nests, some made from commercial kits, others personally fashioned from medium-size PVC pipes filled with hollow bamboo stalks.

Apian squatters soon appeared “out of the blue,” Ron exclaimed one day. Blue, indeed, was the color of the orchard mason bees (Osmia lignaria) that hunkered down in their new homes. Orchard mason bees radiate electric blue legitimately, unlike those infamous Brooklyn bees that turned red in 2010 from syrup-looting at a local maraschino-cherry factory.

Other mason bees, as well, showed up in Ron’s yard. Mason bees do not form queen-centered colonies. Instead, each female deposits her eggs inside tube-shaped structures. She starts at the rear and then works steadily frontward, leaving a series of nested eggs, each deposit sealed by “masoned” mud-partitions. These deposits rest on pollen beds formed to nourish larvae hatching from the eggs. Mason bees fashion especially thick pollen beds for the female eggs at a tube/reed rear.

Ron’s kit cylinders proved vulnerable to some larvae-snatching critter and so he had to design hardware-cloth entrance covers attached by hose clamps. That did not stop parasitic cuckoo bees, which breached a few of the mason-bee chambers to deposit their own eggs on usurped pollen mounds.

Later in Ron’s “wild kingdom,” sinuous-bee flies (“humbleflies”) showed up to procreate larvae that preyed on the parasitoid cuckoo-bee larvae. Before all of that quiet carnage inside the cute avian homes, meanwhile, the busy mason bees had amply pollinated Ron’s fruit trees and bushes.

Low chill units

Pollination, obviously, requires flowers for bees to raid. Even so, flowers were not a sure bet with any of Ron’s plants that required minimum chill units.

Generally, chill units (CU) refer to the number of hours below 45º F between November and February. Just as some bulbs and seeds must be refrigerated for a certain amount of time before planting in Central Texas, many fruit trees have dormancy-inducing chill requirements. Apple trees, for instance, require more chill units than peach trees - perhaps one reason why peach (rather than apple) orchards have been popular in the Texas Hill Country.

Hoping to get trees to fruit despite the warmish winters of his 8b-hardiness-zone locale, Ron searched for cross-pollinating cultivars with the least chill requirements for flowering and also with sufficient tolerance for very hot summers.

So he planted ‘Ein Shemer,’ an Israeli apple cultivar needing only 100 chill units. His ‘Golden Dorsett’ apple cultivar (from the Bahamas) insists on 100–300 chill units, while his ‘Anna’ apple cultivar (from Israel) ranges between 250–300 chill units. All three not only suit Ron’s 8b-hardiness-zone winters, but also its springs — because each of these cultivars also needs a quick early warm-up to trigger flowering.

Apple trees, incidentally, require management (such as pruning and thinning) for maximal productivity. Left unmanaged and depending on the cultivar, they tend to yield heavily every other year.

This happens because new buds form shortly after fruit-set, with the fruit given priority over the buds. So a particularly fruitful year means fewer buds for the next year. That next year, in turn, will yield little fruit but plenty of buds, readying the way for fewer buds and more fruit during the third year. Strategically reducing (thinning) much of the set fruit early on disrupts this biennial tendency by fostering ample budding for a decent yield every year.

Besides apples, Ron staked a ‘Methley’ plum. This small-fruited Japanese cultivar falls at the low end of the winter-chill scale (100–300 CU) and needs (like his apple varieties) a swift spring-heat prompt for flowering. His planted pears - Orient and Ayers (an old Southern favorite) - take 300–350 CU. With a culture similar to apples and pears, Ron’s ‘Pineapple’ quince should receive more than 150 CU to flower and it withstands Central Texas heat. His Japanese persimmons, including Chocolate blossom after less than 100 CU.

In short, Ron tries not to exceed a winter-chilling requirement of 300 units, roughly equal to 13 days of temperatures below 45º F. Since the 300 hours do not need to be consecutive, cold-enough winter nights for a month or two beneficially add up. That’s true as long as no abnormal heat spells occur during winter to snap Ron’s trees out of dormancy and as long as no plunging freezes inflict budded-limb damage.

Ron doesn’t play favorites. He wants all his plants to excel. Yet recently, there is one that particularly gripped his attention. Its performance in Ron’s enchanted orchard amounted to a spring surprise that felt like a mystery, too.

Pawpaw

Ron’s response to this spring surprise parallels a scene in Richard Powers’ mystically strange The Overstory (2018): “Look at this! … Pawpaw! The only tropical fruit ever to escape the tropics,” a father excitedly tells his daughter. “Biggest, best, weirdest, wildest native fruit this continent ever made. Growing native right here … and nobody knows!” When it comes to pawpaws (Asimina triloba), that dad and Ron share the same enthusiasm wavelength.

While the pawpaw prevails in the American wild (especially in the Midwest), Ron’s parched setting amounts to a challenging environment for this tree. That was particularly true last year, Austin’s third hottest with more than 50 days of triple-digit heat worsened by severe drought.

In nature, pawpaws benefit from “a minimum of 32-inches of rainfall spread rather evenly throughout the year,” Kentucky State University researchers report. The long taproot of Ron’s pawpaws ensure its endurance during bad times, but he wants the mango-shaped fruits, not just a good-looking tree. Since pawpaw fruiting requires plenty of available moisture, the odds for success with his tree in droughty Oak Hill remained poor.

Another limitation loomed, as well. Even if Ron’s mature pawpaw blossomed, fertilization seemed improbable. Although the complicated reproductive behavior of pawpaws remains under-researched, the standard view holds that these trees exhibit a floral sequence from female to male stages (protogyny).

The flowers of any particular pawpaw are supposed to be self-incompatible (SI) even if its blooms open at different times, as Ron’s did. When pollen from any bloom of an SI plant attaches to any stigma (female organ) of the same plant (or its clone), the resulting fertilized embryo usually stops developing at some point - meaning finally, no fruit or seed.

Pawpaws are not only identified as self-incompatible but also noted for opening blooms at different times, with each flower morphing from mature female-organ stage to mature male-organ stage.

The female receptors (stigmas) mature and then shut down well before the same flower’s anthers release their pollen. This floral behavior (dichogamy) combined with self-incompatibility (SI) can amount to a double whammy preventing self-fertilization. Both deterrent behaviors probably enhance genetic diversity in pawpaws.

Fruit feast

So Ron seemed out of luck for pawpaw fruit and never bothered to hand-pollinate. He needed more pawpaws with genotypes different from his mature one. He had planted other pawpaws (Potomac, Mango, KSU-Atwood), but at this point they were still immature and (unsurprisingly) none produced any flowers.

The mason bees kept busy visiting the blossoms of Ron’s other plants, while beetles (the good, the bad and the ugly) showed up, as expected, on the mauve pawpaw blooms.

Beetles are attracted by these flowers’ yeasty odor, which intensifies during the day by means of thermogenesis - the rare ability to increase floral warmth a tad higher than the surrounding temperature. Besides beetles, a variety of other accidental pollinators (such as lacewings) protectively sheltered overnight inside the comfy pendulant pawpaw flowers.

None of this critter activity should have made any difference with Ron’s sole mature pawpaw growing in a non-native habitat with no other pawpaws in sight. And yet it did.

To Ron’s mystified delight, more than five-dozen mango-shaped fruits formed and slowly began to swell. Netting the tree saved the fruits from birds and squirrels, and the same netting caught the melon-tasting fruits for easy collection when they ripened and detached on their own in August.

While Ron gazed in wonder at these fruits, I revisited the research we both had delved into previously. How did Ron’s presumably self-incompatible tree bear fruit? I needed a clue, any clue, even if only a mere wiggle-word buried in some report.

It turned up in a Purdue University document, the only instance (to my knowledge) that says pawpaws “are usually self-incompatible.” Usually! It seems that blooms maturing at different times on Ron’s pawpaw and the haphazard behavior of accidental pollinators resulted in the seemingly magical outcome of self-fertilization and actual fruit.

That unusual pleasant surprise, against the odds, still feels like a mystery.