Proper mulching keeps plants happy, weeds and pests away

By Tony Tomeo

Most garden enthusiasts want a well-groomed, clean landscape. Removal of fallen leaves and fruit is helpful in restricting the spread of disease and also produces a refined appearance. Unfortunately, roots just below the surface are often sensitive to the heat accumulated in the soil from direct exposure to sunlight. Exposed soil also dries more easily as moisture is drawn out into surrounding dry air. Species native to sparsely foliated environments, such as yucca and cactus, can adapt to exposure by spreading their roots deeper. However, species endemic to forested locations are sometimes reliant on the layer of organic matter that accumulates naturally over the soil's surface. Mulching is a gardening procedure used to replace natural accumulation.

Mulching helps keep moisture in the soil, shades the soil from sunlight and prevents many weed seeds from reaching the soil and germinating. However, even with mulch, sanitation is still important in situations where disease can be a problem or where certain species of insects or spider mites may overwinter or deposit eggs in leaf litter. For example, rust is quickly transmitted from fallen rose leaves of a previous summer to fresh new foliage the following spring; the disease can overwinter in rose leaves.

In conjunction with leaf removal, rust can also be inhibited by a mulch composed of material that rust will not thrive in, such as ground fir bark. Similarly, red spider mites deposit eggs on dogwood leaves before they fall off in autumn. As the weather becomes conducive to the mites, they hatch and infest the formerly occupied trees. Removal of leaf litter eliminates most of the eggs, leaving only those deposited on stems, consequently minimizing new infestation. Because mites prefer dry and dusty conditions, mulch deters new infestations.

Shredded or ground fir bark is available from most nurseries or garden centers and is one of the most useful and attractive materials for mulch. Other mulching materials are compost, sawdust, straw, leaves, lawn clippings and horse or steer manure. Even newspaper put through a document shredder may be used for small applications--just be sure to cut out the Gardening columns first. In the area of my garden used by the terrier to relieve himself, I have found shredded DeCinzo cartoons to be the most suitable mulch. If you use leaves, be certain they are not from plants infested with any pathogens, such as crape myrtle with powdery mildew or alders with spider mites. The best leaves are those dropped by maple, birch, poplar or sweetgum in autumn. Leaves like those of southern magnolias, however, are too coarse and not a desirable mulch.

Because lawn clippings are fresh when applied as mulch, it is important to spread them thin enough to dry like straw. Grass spread too deeply will smell rotten, and flies may breed in the lower layer. It is advisable to finish all jobs involving disturbance of soil prior to spreading mulch. For example, application of mulch around bearded iris would be wasted when the plants are dug, divided and replanted.

While we are discussing bearded iris, I should say that late summer is actually when they should be divided. Even if this summer exhibited normally warm temperatures, the weather still would not be hot enough to justify delaying division to autumn. Because freshly divided plants recover so rapidly, it is best to replant them early so they can grow and spread roots before winter. Division is necessary because every rhizome (the underground stem from which leaves and flower stalks are produced), splits into two new rhizomes after blooming. If flower production did not decrease as plants become crowded, the number of plants would potentially double every year if they all bloomed! The rhizomes should not be divided too soon after bloom.

Healthy plants are grown from the current year's growth, but it is best to discard last year's rhizomes, from the flower stalk back. When rhizomes are replanted, slicing off the top half of foliage minimizes dehydration while new roots are being produced. The leaf tips will turn brown anyway if not removed. Leaf shoots are arranged in flat "fans" which, when planting, should be oriented facing roughly east and west, leaning slightly west; or facing roughly north and south, leaning slightly south. You will find the fans naturally lean away from the rhizome, so you can plant them heading west or south. This positioning will best protect foliage, which was formerly growing in the partial shade of sibling shoots, from sunburn. If you do not necessarily want to produce large quantities of plants, and you do not mind if some fans do not bloom, it is not necessary to divide irises every year.

Perennial of the Week: New Zealand Flax

Most of us are familiar with New Zealand flax, Phormium tenax, which was so common in the 1950s, but too large and vigorous to use properly. In the 1990s many newly introduced cultivars not only maintain manageable proportions, but produce similar long, sword-like foliage of interesting color combinations of bronze, green, red, orange, yellow and pink. Although not normally grown for flowers, the stalks are a showy feature for many cultivars. New Zealand flax prefers full sun with moderate irrigation, but will tolerate light shade and minimal water. With the exception of swampy, poorly drained conditions, any soil is acceptable.

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Los Gatos horticulturist Tony Tomeo can be reached at 358-2574.

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