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Q&A with Bob Polomski 13

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Extension Consumer Horticulturist, Clemson University

Where do the seeds of seedless watermelons come from?

-LD., by e-mail

Answer: Seedless watermelons were first developed at Kyoto University, Japan, in 1939. The parents are two seeded varieties. The male is a diploid variety—that is, a watermelon with the ordinary number of 22 chromosomes. The female parent, however, is a tetraploid variety; it has been treated with colchicine to double its chromosome count to 44. Breeders select diploid and tetraploid parent lines separately for desirable traits: taste, flesh color, size and shape, rind thickness, productivity, maturity time, and others. Once the parent lines breed true—that is, the offspring exhibit consistent traits, which may take 10 generations—the crosses are made. The resulting hybrid seed is a triploid, with chromosomes numbering 33. Although these triploid seeds will sprout, the subsequent plants are sterile. The vines flower and make fruit, but these fruit are seedless.

Growing seedless watermelons is somewhat more challenging than raising ordinary watermelons. Seed of the latter will germinate at 75°F. Seedless watermelon seeds, by contrast, need at least 80°F, preferably 85°F, to sprout. The thick seed coat sometimes sticks to the cotyledons on the emerged seedlings and must be carefully removed by hand. Sowing the seed with the pointed end down reduces the occurrence of this problem.

Seedless watermelon flowers must be pollinated to set fruit. Because the plants produce no pollen of their own, a seeded watermelon variety must be grown nearby. When you buy seedless watermelon seed, a few seeds of another variety are typically included. Plan on growing one of these seeded plants for every three seedless ones. These seeded watermelons will set fruit of their own; they are usually selected to have a different shape or rind color to make it easy to distinguish their fruit from the seedless fruit. Seedless watermelons tend to keep longer in storage because there are no seeds to serve as focal points for decay.

I bought an ant plant (Dischidia pectenoides). It's growing in what looks like small pieces of bark. How should I care for it? —K.B., Lees Summit, MO

Answer: This tropical epiphyte, an evergreen vine from the Philippines, belongs to the milkweed family (Asclepiadaceae). Its distinctive inflated leaves can store moisture for use during the dry spells that characterize an epiphyte's existence. In this plant's natural habitat, many of these leaf-pouches also serve as nest sites for ants, making the plant a myrmecophyte, or ant plant. Aggressive biting and stinging ants repel other insects, and their droppings and food residue, discarded within the nest, can provide sustenance to the plant. Secondary roots typically form from a leafstalk or stem close to the opening of a pouch and grow into it, to absorb the water and nutrients inside.

In culture, this vine is grown as a horticultural curiosity and admired for its bright red flowers. It is commonly cultivated in an orchid bark mix: redwood or western fir bark supplemented with a combination of charcoal, perlite, peat, oyster shell, or coconut husk. Keep your plant in a location with bright, indirect light and temperatures above 55°F. Mist the plant once or twice a week and apply a half-strength houseplant fertilizer monthly during the growing season. The trailing stems can be secured to a sphagnum moss-covered pole or slab.

Over the years I have built up a collection of hydrangeas, including several macrophylla types (both lacecaps and mopheads), a large number of ‘Annabelle’, and several oak-leafs. While the Annabelles have always done well, the performance of the oakleaf hydrangeas has been spotty, and the macrophyllas have rarely, if ever, bloomed. What can I do to get them to flower?

-R.B., Old Lyme, CT

Answer: Your problem is most likely a lack of winter hardiness and the fact that many of your plants flower on old wood. That is, they form their flower buds the preceding year and these must survive the winter to bloom. Both your macrophylla, or bigleaf, hydrangeas (Hydrangea macrophylla) and your oakleaf varieties (H. quercifolia) fall into this category. You've been partially successful with oakleafs because H. quercifolia is hardy to USDA Zone 5; H. macrophylla is only hardy to Zone 6. Winter injury is not of concern with ‘Annabelle’ (H. arborescens). It is hardy to Zone 4 and produces flowers on the current season's growth. (This is also true of the peegee hydrangea, H. paniculata, which is hardy to Zone 3.) Remember, however, that hardiness zones are based only on average minimum winter temperatures.

The solution for your macrophylla hydrangeas may be a little winter protection. Try wrapping your plants with burlap in late fall. You may still lose the terminal flower buds, but you should save the flower buds located further down the stem.

Alternatively, you can plant reblooming H. macrophylla cultivars that produce flowers on current season's growth. These include ‘Bailmer’ (‘Endless Summer’), ‘David Ramsey’, ‘Decatur Blue’, ‘Luvumama’, ‘Madame Emile Mouillere’, ‘Oak Hill’, and ‘Penny Mac’ among the mop-heads, and the lacecap ‘Lilacina’.

The United States seems to be one of the few countries in the world whose populace does not use urine as fertilizer. Urine from a healthy person is sterile, high in nitrogen, and easy to apply. What is your advice on its use?

-C.P., Molalla, OR

Answer: Innumerable gardeners have performed their own experiments and discovered that, yes, human urine is an excellent nitrogenous fertilizer. Indeed, more than one gifted horticulturist has mumbled something about “maiden's water” being the secret to horticultural success.

The nitrogen in urine is a by-product of protein digestion, so the precise quantity of nitrogen to be found in urine will depend on a person's diet. Nevertheless, a typical N:P:K analysis might be 10:1:2.5. Urine can be applied at full strength to soil or compost piles. If it is to be applied directly to plants, it is best diluted. Carol Steinfeld, author of Liquid Gold: The Lore and Logic of Using Urine to Grow Plants (Chelsea Green Publishing, 2003) recommends a ratio of one part urine to eight parts water. Human urine is much safer to use than human feces, but it is still wise to be cautious. Steinfeld recommends applying it primarily to the soil of fruiting crops or crops that will be cooked. Done carefully, we ought to be able to return this nitrogen stream to the land from which it came.

The branches of my peach trees are oozing gummy blobs of what I think is sap. A few limbs look like they're dying. What's wrong with my trees?

-J.H., Abanda, AL

Answer: The answer depends on whether the gummy secretions are amber colored or clear. Amber-colored gum can have several causes. If the sap is mixed with frass, sawdust, and pieces of bark, you can suspect peach tree borers, which are capable of girdling and killing peaches. The greater peach tree borer typically attacks the lower trunk; the lesser peach tree borer attacks higher limbs. Both are best controlled with insecticides applied when the eggs are laid. Established borers can sometimes be reached and killed with a thin, sharp wire.

Clear gum, on the other hand, can result from mechanical injury, such as the tree being struck by a lawn mower or string trimmer. It can also be a sign of cytospora canker or fungal gummosis, both peach diseases. Cytospora canker is more common in the North and is controlled by pruning after bloom, keeping large cuts to a minimum, and spraying cut surfaces with fungicides used to control brownrot. Fungal gummosis disease is of more recent occurrence and has spread in peach orchards throughout the Southeast. Removing and destroying gummosis-infected wood reduces inoculum, but the best control comes from planting gummosis-resistant varieties, such as ‘Harbrite’.

Any peach tree that is poorly managed and water-stressed is more likely to suffer disease and insect attack. Peach trees grow rapidly, bear early, and die young. The secret to keeping them healthy and productive, as is true with so many plants, is to see that their basic needs are always met.

My neighbor has a weeping willow tree and its roots grow under my above-ground swimming pool liner. Last year I dug a ditch along the property line and cut off the roots. I also put some stuff down that was supposed to stop roots in pipes, but the roots are growing back again. What can I do to stop them?

-W.S., Round Rock, TX

Answer: Just cutting a tree's roots, as you've discovered, is effective only if it is repeated at least annually. Next time try filling the trench you have made to cut the roots with three-quarter-inch or larger gravel mixed with cobblestone-sized rocks.This “exclusion zone” should help keep the willow roots at bay because the large pore spaces in the trench's backfill will not hold water.

Alternatively, you might consider installing a commercially available root barrier. These are made of rigid plastic or geotextile fabric. Some of the interlocking plastic root barriers have ribs that direct root growth downward.

Install the barrier at least a foot away from your swimming pool. The barrier should project at least an inch above the ground and extend 24 to 30 inches deep. To reduce the likelihood of roots growing underneath the barrier and coming up on the other side, compact the soil tightly against it, eliminating pore spaces. For further insurance, install the same barrier horizontally. If possible, use one piece of material or join the seams tightly together.

Geotextile fabrics are an alternative to rigid plastic root barriers. Biobarrier, for example, is made of spunbonded polypropylene impregnated with the pre-emergent herbicide trifluralin. Install the fabric so it extends above the soil surface and down to a depth below where existing roots can be found. Again, when filling the trench be sure to firm the soil against the fabric. Fabric barriers will not work indefinitely, but various studies have shown an effectiveness lasting from 3 to 10 years. H