Sunburn

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Out with the old, . . .

There are two types of horticultural sunburn. Sun scald is what happens to formerly shaded bark if it suddenly becomes too exposed. (As I mentioned in my gardening column for this week, which posted last Monday here, sun scald that occurs during winter in colder climates is caused more by frost than by sunlight.) Scorch is what happens to overly exposed sensitive foliage.

The picture above is an example of scorch on a significant scale. The bigleaf maple was formerly shaded by a big Douglas fir that fell last May, leaving the maple both severely disfigured and very exposed. Such exposure would not have been a problem it the foliage had always been so exposed. The problem was that it developed in shade, so could not adapt to the new exposure.

The exposed foliage survived for a while, but eventually succumbed to warmth, sunlight and aridity (minimal humidity). Deterioration accelerated as the weather became warmer in just the last several days. Because the foliage scorched rather than succumbed to cooling autumn weather as it would have done a few months later, it remains attached to the stems that it grew on.

That is actually an unsightly advantage for the bark of the stems that are shaded below. If the bark suddenly became exposed too, it would be susceptible to sun scald. This tree knows what it is doing. Deteriorated foliage that does not get dislodged by later winter weather will be dislodged as new foliage develops next spring. The new foliage will be adapted to the new exposure.

The picture below shows how new grow that is adapted to the new exposure develops adventitiously from the exposed trunk. It does more than just exploit the increase of sunlight. Ideally, it shades suddenly exposed bark to protect it from sun scald.

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. . . and in with the new.

Sun Scald Happens Here Too

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Sun scald ruins otherwise good stems.

Those who enjoy gardening where winter weather is harsh likely know what sun scald is. It happens late in winter, if exposed bark warms enough to prematurely resume vascular activity during the day. Vascularly active tissue then succumbs to hard frost at night. Wintry sunlight is not sufficiently intense to scald bark; but the damage suggests otherwise. Glare from snow enhances exposure.

Of course, without hard frost or snow, this sort of sun scald is not a concern here. However, there is another sort of sun scald that happens during the warmth of summer. It truly is scald, caused by exposure to sunlight that is sufficiently intense to literally cook vascular tissue just below thin bark. Although induced by opposite extremes of seasonal weather, the damage is remarkably similar.

Since even deciduous trees are foliated during summer, most bark is safe from summertime sun scald. Bark becomes more exposed and susceptible if deprived of some of what shades it. That can easily happen if aggressive pruning diminishes the foliar canopy above. Removal of a nearby tree also eliminates significant shade. Painting an adjacent wall a light color can enhance glare.

White paint applied to the trunks of susceptible orchard trees reflects most of the damaging sunlight, but is too unsightly for landscape trees. Stubble of small twiggy stems can shade the trunks of some young trees until their canopies are broad enough to provide shade. Sun scald typically develops on southwestern and upper exposures that are more exposed to the most intense sunlight.

Maple, oak, ash, birch, flowering cherry, flowering crabapple, English walnut and almost all deciduous fruit trees are innately susceptible to sun scald. The interior stems of privet, holly and English laurel are more resilient to sun scald if exposed by major pruning during late winter rather than during summer. Although, with only a few exceptions, any thin bark can be susceptible to sun scald.

Foliage of many plants can be damaged by enhanced exposure too, but that is known as ‘scorch’, and is another topic.

Horridculture – Green Is The New Black

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Foliage does not get any blacker than this.

This is not another of my many racial slurs for the renowned Southern Californian landscape designer, Brent Green. Believe or not, I endure many more of such slurs from him; so will not even bother putting something else out there that compels his retaliation. This is about Japanese laurel, Aucuba japonica, which is incidentally rather yellowish with rich golden variegation.

Japanese laurel, which is known as gold dust plant locally, is happy in partial shade, and will tolerate rather significant shade. That is a distinct advantage in landscapes that are dominated by so many big redwoods. Even without significant bloom, the bright yellowish foliage is an asset in visually dark parts of the landscapes. There probably should be more of it here than there is.

It is not one of my favorites though. It does not cooperate with pruning, and often produces overly vigorous growth that flops over in response to aggressive pruning. It shelters proliferation of snails in warmer climates. What I dislike most about it is the prominent blackening of some of the foliage that is too exposed to direct sunlight. It is so unsightly in front of the cheery gold.

After pruning a few overly vigorous stems that became floppy, I noticed how quickly the lush and fresh new foliage blackens from exposure. The pictures above and below were taken about two hours after the stems were pruned. The stems grew in a notably shaded situation, and were then left out on a hot black bed liner without shade, which of course accelerated the process.

I should have gotten a picture of the foliage as I found it, with all exposed surfaces blackened, as if spray painted where they were on the black vinyl. The portions of the leaves that remain green were shaded under other foliage.

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Shaded parts are still fresh and gold dusted green. Exposed parts are roasted to a crisp.

Gravitropism Gets Germinating Seeds Oriented

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Toward the light. Away from gravity.

Germinating seedlings know which way is up. Perhaps they just know which way is down. They can not see, hear, taste, smell or feel anything like we can. Nonetheless, they know which direction to extend their first root and stem. One thing that they can perceive is gravity. Gravitropism, which was formerly known as geotropism, is how they respond to gravity, or the Earth that generates it.

Positive gravitropism is why the first root to emerge from a seed extends downward toward gravity. Negative gravitropism is why the first stem to emerge from a seed extends upward away from gravity. Positive is toward. Negative is away. Roots and stems that develop after the first, disperse in other directions in response to other stimuli, but never really forget where gravity comes from.

Once a primary stem of a seedling emerges from the soil, it immediately responds to sunlight. Just as it exhibits negative gravitropism to grow away from gravity, it exhibits positive phototropism to grow toward sunlight. Since sunlight comes from above, positive phototropism is compliant to negative gravitropism. Branches will later disperse laterally to avoid the shade of other branches.

While branches are finding their way in the World, roots are doing the same. All of them can not always reach for the center of the Earth. They branch and disperse laterally as they sense that they are sufficiently deep in the soil. Those that venture too deeply sense an inhibition of gas exchange. Those that are too shallow sense if they get too warm or dry. There are a few types of tropisms.

Roots respond to moisture, nutrients, chemicals, temperature and mechanical stimulation within the soil. Branches and foliage respond to humidity, wind, temperature variations and air pollution. In order to function within their dynamic situations, plants somehow coordinate their responses to all of the many stimuli they experience. They are impressively perceptive, as well as responsive.

Even celery, green onions and leafy tops of carrot in a refrigerator can reach upward away from gravity.

Shade Can Be An Asset

90911thumbJust about every home garden has some sort of shade. Even if there are no substantial trees or shrubbery, there are northern walls of homes and garages, and they likely have eaves that extend their shadows a bit farther. Fences to the south create shade to the north. Gardens of modern homes are smaller, and surrounded by higher homes and fences, so are shadier than older gardens.

Those who enjoy gardening tend to enjoy more trees and substantial shrubbery than those who do not enjoy gardening, so generally contend with more shade. It is both and asset and a liability. Cooling shade makes outdoor living spaces more comfortable in the heat of summer, but limits what we can grow. With very few exceptions that are not worth mentioning, all plants need sunlight.

Fortunately, many plants need less than others. Of these, many are understory species, which live in the partial shade of larger plants in their natural environments. Not only do they naturally need less sunlight, many prefer to be in partially shaded or sheltered situation. Their foliage and bloom can be scorched by sunlight if too exposed, especially while the weather is warm, windy or arid.

Plants that prefer partially shaded and sheltered situations are characteristically different from those that prefer more exposure. Their leaves tend to be bigger and darker green to absorb more sunlight. Those that are sensitive to frost may prefer shelter from evergreen shade. To compete for pollinators with bloom above, flowers may be either bigger and more colorful, or more fragrant.

There are, of course, many exceptions. Ferns are probably the most familiar foliar plants for shade, but provide no bloom. Cast iron plant is comparable to fern for providing rich green foliage, but with insignificant bloom. Caladium, coleus and hosta are grown for lush foliage that is strikingly colorful instead of rich green. Hosta contrarily blooms with pastel flowers that are not even fragrant.

Kaffir lily, calla, hydrangea, azalea, rhododendron and impatiens provide more color for partial shade.

The Wrong Time For Pruning

80801thumbNot many plants are sensitive to mere heat alone. Actually, many plants prefer warm weather. The difficulty that some plants have with heat locally is that it typically accompanies aridity, and often accompanies afternoon breezes. As appealing as breezes and minimal humidity are to us while the weather is warm, they promote and accelerate desiccation of exposed sensitive foliage.

Pruning, which obviously becomes necessary while warm weather promotes growth, can make plants more sensitive to damage caused by warm, sunny, arid and perhaps breezy weather. It exposes formerly sheltered stems and inner foliage, which are more sensitive than outer foliage is, to more sunlight and drying breezes. Exposed foliage can either desiccate or roast, or both!

A bit of unsightly but relatively minor foliar damage on the extremities of the outer canopy might be only superficial, but major damage can be dangerous. Superficial damage often gets replaced by fresh new growth before it deteriorates enough to expose more foliage and stems below. However, recovery from major damage can be delayed by the distress associated with the damage.

Japanese maple, aralia, philodendron, rhododendron and all sorts of ferns can easily get damaged by increased exposure. Low ferns are not likely to become too exposed by any loss of their own foliage, but often become more exposed by the pruning of plants above them. Like frost damage, foliar scorch might need to be left to shelter remaining foliage until new growth develops.

The bark of many plants, although not susceptible to desiccation, is very sensitive to sun-scald if too exposed. Young and smooth bark is the most sensitive, particularly if it had always been shaded. Scald kills bark and the vascular tissue below. As it decays, it exposes interior wood to more decay that is likely to compromise the structural integrity of the affected stems and trunks.

Pruning during relatively cool weather and while there are a few relatively cool days in the forecast allows foliage a bit of time to adapt to a new exposure before the weather gets dangerous. Through summer, pruning should not be so aggressive that too much sensitive foliage or bark are exposed, even if it is necessary to leave a bit of unwanted sloppy growth to partly shade bark. Aggressive pruning of exposed and sensitive plants should be delayed until autumn, when sunlight is not so intense, and weather is cooler and wetter.

Krispy Kritter

P80715KThis really is the best climate here. Winters are just cool enough for many plants that require a bit of a chill, but not unpleasantly cold. Summers are just warm enough for most plants that need warmth, but the weather does not stay too unpleasantly hot for too long. Warm weather here typically lasts no more than a week, and is accompanied by light evening breezes and cooler nights. Minimal humidity typically makes the worst of the heat a bit more tolerable.
While much of North America and parts of Europe were experiencing abnormally and uncomfortably warm weather earlier in summer, our weather somehow stayed relatively mild. For a while, it was significantly warmer in Portland than here. It certainly was nothing to complain about. Vegetable plants that crave warmth were not too inhibited by the mild weather. Flowers that typically deteriorate in warm and arid summer weather lasted a bit longer than expected.
Then the weather changed. It did not get too unseasonable warm. In fact, the weather merely did what is typical for this time of year. The problem was that it happened so suddenly. The weather went from pleasantly mild and somewhat humid, to more seasonably warm and arid overnight. Flowers faded and warm season annuals wanted for more water.
The worst of it was that exposed foliage of some plants got roasted. English laurel and rhododendron were particularly susceptible Because we had no way of anticipating the sudden change of weather, we had just shorn a large hedge of English laurel, just in time for the warm weather to cook the freshly exposed inner foliage.
At about the same time, new plants were being installed into a small newly landscaped area. The transition from the cool and comfortable nursery on the coast to the warm and arid landscape where they were surrounded by black groundcloth (prior to the installation of chips) was too much of a shock for some of them. We could not water them enough to prevent some of the foliage from desiccating.
This crispy barberry is fortunately not as dead as it seems to be. Only the outer foliage is roasted. The stems and buds do not seem to be damaged. It will probably foliate again before defoliating in autumn. Even if it induces premature dormancy, it should recover as next winter ends.
Nonetheless, such damage on new plants is disconcerting.

 

Things Heat Up In Summer

70705thumbThis does not seem like such a mild climate when it is difficult to distinguish between the time and the temperature on a local bank clock tower. You know; when punctuation is the only difference between four minutes past one, and one hundred four degrees. Fortunately, like mild frost in winter, hot weather does not happen too often, which is why this climate really is milder than most.

Most of us know what to do for the garden when the weather gets warmer. Obviously, many plants want more water. What we do not often consider is that there a few things that we should ‘not’ do. Unlike us, the plants in the garden can not find shade when the weather gets warm. Those that are exposed find creative ways to provide their own shade. We really do not want to mess with that.

By this time of year, outer foliage of exposed plants is mature enough to tolerate heat. Only foliage of plants that prefer to be partly shaded is likely to be damaged. However, inner foliage of even the toughest plants is not as resilient as outer foliage is. Simply shearing a hedge exposes inner foliage that can be scorched by overexposure. Sunlight enhances the effects of heat and aridity.

If possible, it is best to delay such pruning and shearing until after unusually hot weather. No one wants to be out working in the garden on a hot day anyway. More typical seasonable weather may not seem to be much cooler, but a few degrees can be a big difference to plants. Once exposed, inner foliage should adapt, and hopefully be resilient to heat before the weather gets hot again.

While young and thin, formerly shaded bark that suddenly becomes exposed can be damaged by sun scald. (Deciduous trees do not get scalded while defoliated in winter because the intensity of sunlight is diminished at that time of year.) Sun scald of bark is much more serious than foliar scorch because it kills bark, leaving open wounds on main limbs and trunks. Decay within these wounds compromises structural integrity, and can ruin otherwise healthy trees.

Although rare, spontaneous limb failure can occur in some trees during warm weather, particularly if humidity increases and breezes remain minimal. It sounds silly, but warmth accelerates vascular activity, possibly until foliage becomes too heavy for the limbs that support it. If limbs break, they can cause major disfigurement, and detrimentally expose bark of inner limbs and trunks.

Tropism Gives Plants Some Direction

80523thumbWhen a seed germinates, the roots know that they want to go down, and the new stem knows that it wants to go up. Going up sounds simple enough. Stems just go towards the light. How do they know where the light is while they are still under the soil? Well, new stems go upward for the same but opposite reason that roots go downward; gravity. Roots go toward it. Stems go away from it.

Tropism is how plant parts respond to a variety of stimuli, particularly gravity and light, but also water, chemicals, mechanical stimulation, trauma and electricity. Response to gravity is geotropism. Roots exhibit positive geotropism by growing towards gravity. Stems exhibit negative geotropism by growing away from gravity. Stems exhibit positive phototropism by growing towards sunlight.

Roots are always figuring out where to go next by prioritizing their innate positive geotropism, their tropism for or against certain chemicals, and their tropism for moisture but against saturation. Until we see them migrating into lawns, displacing concrete or getting into a septic systems, their work underground is unseen. Tropisms above ground are quite visible and perhaps informative.

When a tall herbaceous plant falls over, it tries to get up. If unable to, it can at least curve new grow upward. Even cut flowers and vegetables in the kitchen can do that much. Snapdragons that are initially arranged leaning outwardly from the center of a floral arrangement can go vertical within a day or two. The fluffiest houseplants regularly get turned so than they do not favor one sunny side.

Trees are too big to move, but on rare occasion, it happens. The crossed pairs of Mexican fan palms commonly planted outside of In-N-Out Burger restaurants get planted at a lean while mature, but then grow vertically after installation, leaving an angular kink where the direction changed. Such a kink in an otherwise straight trunk of a tree that was not planted at an angle might indicate a sudden destabilization. A curved trunk indicates either a slow destabilization, or tropism to escape shade.

Shade Is Not For Everyone

70426thumbThere is no way around it. Just about every garden has some degree of shade. Even low profile single story houses without eaves or fences are shaded on the north side. Eastern exposures get the cooler morning sun. Western exposures get only the warmer afternoon sun. There are vacant and treeless parcels out in the desert that lack shade, but not many of us are gardening out there.

Shade is very often an asset, which is why shade trees are so popular for shading both gardens and homes. Eaves and awnings are architectural features that shade windows and doorways. Arbors, lath roofs and patio umbrellas provide shade for patios where trees are lacking or insufficient. Without shade, garden spaces can get too uncomfortably warm to be useful during summer.

The problem with too much shade is that, although it makes the garden more comfortable and useful for some things, it also makes the garden less useful for gardening. Roses, vegetable plants and most flowering annuals need good exposure. Lawn, the carpeting for some of the more useful of garden spaces, can be sparse where it is too shaded. Sunlight is as important as shade is.

This is one of the many reasons it is so important to select the proper trees for each application. Big trees are nice, but might shade too much area. Evergreen trees that are good for obscuring unwanted views at a distance, will prevent warming sunlight from reaching parts of the home through winter if they are too close. Neighboring gardens and homes need to be considered as well.

Planning functional gardens is of course not always simple. Most of us contend with trees, shrubbery and vines that are too big and shady, either in our own gardens or in neighboring gardens. Climbing vines like wisteria, honeysuckle and trumpet vine, are notorious for growing far beyond their intended applications. It sometimes becomes necessary to remove overgrown or crowded plants, or prune them for confinement. Big plants that can not be contained will limit the choices for other plants that share their space.