Reading the Leaves

Culture

by Ken Slump

Posted by Sys Admin over 1 year ago.

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Examining Plants Closely Offers a Clue to Health


ONE OF THE MOST IMPORTANT habits that an orchid grower can develop is the practice of regular and careful observation of his or her plants. Each part of the orchid — from the root to the pseudobulb, leaf, bud and flower — displays information about the plant’s general health and condition. But there is probably no part of the plant that can tell you more at a glance than the foliage.
Leaves are the primary repositories of chlorophyll, the green pigment that is the driving force of plant growth. Excepting a few examples that are naturally leafless, without healthy foliage, you cannot have a thriving orchid that will reward with flowers.
Most of us appreciate that the orchid family is large and diverse. Just about every imaginable version of a simple monocot leaf is expressed somewhere in the clan. Many epiphytic orchids have somewhat thickened leaves that store water and nutrients to help see the plants through dry spells. Shade-loving orchids tend to have broader leaves. Some orchids that thrive in high light have thin, narrow foliage. Terete-leaved orchids have foliage that is rounded in cross section. While most orchids have solid green foliage, there are those with mottled or patterned foliage. There are even orchids with variegated leaves.

Obviously, to recognize abnormalities in orchid foliage, you should have an appreciation for the range of healthy normal leaves. Generally, orchid leaves are medium to light green. Orchids grown in maximum light for their type sometimes exhibit some yellow pigment. Some orchids show a red pigment in high-light situations too, but this can be the result of cooler temperatures as well. I am told that some vanda leaves exhibit a purple tint in cooler weather and that many Brassavola nodosa crosses have “blushing leaves” in spring, a condition (if it bothers you) that may be remedied with a watering containing 2 tablespoons of Epsom salts (magnesium sulfate) per gallon (3.8 L) of water. Very dark green leaves may indicate that the plant is receiving insufficient light. Conversely, yellow leaves can tell you that a plant is receiving too much light.

It is normal for evergreen orchids (those plants with living leaves at all times) to lose their older foliage. Growers should not panic if an older leaf yellows and dies as long as new healthy growth replaces it. On the other hand, some orchids are deciduous and go through a stage when all of the leaves drop and the plant rests. Usually, this is a signal that watering should be curtailed for a time and that temperatures ought to be lowered a bit as well.

If the leaves or new growths on a particular orchid are smaller than previous ones you should look to cultural problems. Ask yourself if you have drastically changed the environment for the plant since its previous growing cycle. Possibly there is a buildup of fertilizer salts in the growing medium if water has not been thoroughly flushed through the pot regularly. Perhaps you have neglected your fertilizer schedule, but it is more likely to be that repotting is overdue. There could be root damage due to the breakdown of the growing medium or from overwatering. Tweaking the growing environment or feeding a declining plant can be a fairly easy fix, but if root damage is suspected, immediate action must be taken if you intend to save the plant.

Foliage can be instructive about watering practices too. Wilted or limp leaves may be another indicator of root damage due to overwatering or a sour growing medium, a bad sign. Immediate repotting may save the plant, but it will be a slow road to recovery. Equally distressing is the discovery of a mushy brown leaf on a monopodial orchid plant (e.g., phalaenopsis). If a single leaf is affected, you may be able to remove it and treat the plant with a fungicide before the problem spreads. If crown rot has occurred, the plant will have to produce a side shoot or division in order to survive. There are many fungal diseases that can affect orchids but most of them primarily affect stressed plants or are the result of poor cultural practices. Generally, they do not occur if care is taken to water plants early in the day and to avoid letting water stand in the crown of the plant.

At the other end of the spectrum, wrinkled or pleated leaves, particularly on some oncidiums and their hybrids, is an indication that insufficient moisture was available when that leaf was emerging. It is also said that the upper leaves on vanda orchids will fold together more tightly during periods of heat and dryness in order to reduce water loss due to transpiration. More frequent watering will help prevent this as well as avoid the excessive loss of a vanda plant’s lower leaves.

The sudden appearance of rather large yellow, brown or black areas on the leaves may indicate sunburn. This is particularly likely if the plant has been recently moved to a much brighter location. Be especially careful of this if you transfer plants outdoors during the warmer months. Blackened areas can be the result of excessively high temperatures too. I once had that symptom appear on an orchid purchase that was left too long in the trunk of a car on a warm day.

Brown or black leaf tips may come from several causes. In cattleyas, they can indicate a calcium deficiency that can be rather easily remedied with crushed eggshells or Tums sprinkled on the surface of the growing medium. Sometimes darkened leaf tips occur when fertilizer salts build up excessively. Brown tips on thinner-leaved indoor orchids may simply be the result of insufficient humidity.

Tiny dark spots on orchid leaves are usually of minor concern. Often, they are the result of bacteria. Many thin-leaved orchids seem to have such spots as a matter of course and their harm seems to be mostly cosmetic.

Conversely, dark streaks, patterns and large sunken areas on the leaves may indicate the presence of virus. There are a number of viruses that affect orchids. Viruses cannot be cured and can only be verified through laboratory tests, some of which can now be conducted by the hobby grower. Unfortunately, I have heard that the tests are not always accurate. Most large established orchid collections likely house at least a few virused plants. Some orchids with virus seem to be asymptomatic. Virus may be transferred from plant to plant with infected pruning tools or by sucking insects.

The problem of virus in orchids is serious and certainly justifies an article of its own. I do not pretend to be an expert on the subject. My current practice, which may seem harsh to some, is to dispose of any suspicious plant. Often they are plants that have not been thriving and flowering well anyway, which may be further indication of their likely infection.

Insect pests, too, can be discovered through a careful examination of the leaves. Among the most commonly encountered on orchid foliage are scale, mealybugs, mites and aphids. Orchid growers should learn to recognize each one as well as their symptoms. These pests tend to do their dirty work out of sight and are often discovered only with a careful examination of the undersides of the leaves as well as the leaf axils.

Slug and snail damage is more likely to be first noticed on orchid flowers but may appear on foliage too. Watch for slime trails that indicate their presence.

How you deal with any particular pest infestation will depend on its severity as well as your personal comfort level with various treatments. Nearly every pest may be combated with a variety of treatment options that range from simple mechanical removal to environmentally friendly sprays and home remedies, as well as the options that are offered by horticultural chemicals.
Remember that stressed plants are more prone to problems. Plants may be stressed by temperature extremes, poor watering practices and insufficient light, to name but a few causes. Also keep in mind that crowded plants are more susceptible to problems and that good air movement helps keep many types of orchids happy and healthy.

A bit of attention to your orchid plants’ foliage is bound to provide you with considerably more accurate information than you will receive from the average gypsy reader of tea leaves.

Acknowledgments
I extend my personal thanks to Dot Henley of Ft. Lauderdale, Florida, who predicted that I would (pushed me to) write about this and provided some ideas.
 

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