Humidity And Wind Affect Heat

Heat is more than mere temperature.

Gardening is not so much fun when the weather gets as warm as it has been recently. It is more comfortable to stay inside with air conditioning, or at least where it is shadier. The plants out in the garden are on their own. Except only for those that are potted, they do not have the option of coming in out of the heat.

Most plants actually do not mind the sort of heat that is uncomfortable for us. Some actually enjoy it. The problem is that heat often occurs in conjunction with other weather conditions that can collectively become really unpleasant for plants.

Minimal humidity makes otherwise unpleasantly warm weather more comfortable for us, but can desiccate foliage that prefers more humidity. Japanese aralia, fuchsia, rhododendron, split-leaf philodendron and many other plants that should not mind warmth can get roasted if warm weather is also too dry. Because sunlight enhances the process, exposed foliage is much more susceptible to damage. To make matters worse, sunlight is more penetrating through clear dry air.

Wind that makes us feel a bit cooler in warm temperatures can likewise cause desiccation as it draws more moisture from foliage. Finely textured plants like Japanese maple, many ferns and some grasses, are particularly susceptible. Offshore wind, like the famous Santa Anna Winds of Southern California, are the worst, because they come in both hot and dry from more arid inland areas, combining all three factors of minimal humidity, heat and wind.

Many of the plants that are susceptible to damage from heat happen to be tropical or subtropical plants that typically enjoy heat as long as the air is humid and still. Others are ‘understory’ plants that naturally live in the shelter of higher trees, so do not like direct sun exposure or wind. Yet, even substantial trees, like fern pine (Podocarpus spp.) and even redwood, can get a bit roasted if the weather gets hot, dry and windy enough.

Conservation of water makes warm weather even more uncomfortable for sensitive plants. They really want more water to keep their foliage and stems well hydrated. Hosing ferns and grasses when things get really hot helps to cool the foliage, and briefly increase the ambient humidity. Because thin young bark is more susceptible to sun-scald in hot weather, pruning that would expose more bark should be delayed until the weather turns cool again.


Humidity Is The Other Weather

Some delicate foliage prefers more humidity.

It is difficult to always ignore the weather. Regardless of how pleasant it typically is here, it sometimes gets warm or cool. It occasionally gets hot or cold. Rain is wet and perhaps messy. A breeze is comforting while the weather is warm. Strong wind can be damaging. However, humidity is one major component of local weather that gets little consideration. 

Humidity gets more consideration in climates that are either uncomfortably humid or arid. Some parts of Florida get famously humid and hot simultaneously during summer. Some flora and insects enjoy such weather. Unfortunately for the rest of us, humidity enhances the already unpleasant heat. Locally, hot or warm weather is rarely bothersomely humid.

Similarly, local weather is rarely unpleasantly arid (lacking humidity). This is a chaparral climate, which is ‘semiarid’. Relatively minimal humidity makes heat a bit more tolerable than it would be with more humidity. Yet, humidity is generally sufficient to sustain foliage that would desiccate in a more arid desert situation. Actually, this is an excellent climate.

Although, it is not perfect. Flora and fauna have different standards for exemplary climate and weather. The relatively minimal humidity that makes uncomfortably warm weather a bit more tolerable for people and animals is much less appealing to some plants. Except for those that are native to desert or chaparral climates, most plants prefer more humidity.  

Many popular plants are understory plants, that naturally live in the partial shade of taller vegetation. With shelter from desiccating arid wind and harsh sunlight (to enhance heat), most do not mind heat. Otherwise, foliage might roast. Those with finely textured foliage, such as astilbe, ferns, grasses and some Japanese maples, are particularly susceptible.

Some tropical and subtropical plants, such as split leaf philodendron and fuchsia, prefer to be understory plants here, even if they would prefer more exposure within their natural ecosystems. The shelter provided by more resilient vegetation compensates for deficient humidity. Furthermore, adequate irrigation promotes healthy hydration of delicate foliage. 

Six on Saturday: Ill Wind

There are no flowery pictures here this week. Nor is there a picture of Rhody. I know that everyone loves Rhody. Also, I had been trying to include something flowery as everyone else does. Instead, I got only pictures of damage that was caused by very strong wind that blew through here on Monday night and Tuesday. I missed it while at my other work, but now have a major mess to contend with. Redwoods are very big and very messy, even without wind. With wind, they are very dangerous too. No one can remember stronger wind here.

1. Electricity sometimes gets disabled prior to strong wind. This wind storm disabled the electricity first. Debris such as this needed to be removed before the electrical service was restored.

2. Decayed dead trees blow down easily, even without much drag. They are not as heavy as viable trees, but are not as flexible either. A few stubs of broken limbs perforated this lodge roof.

3. Stairway to photinia was too silly to not get a picture of. The photinia looks as if it had always been there. I certainly did not expect it to fall over. We took the necessary steps to remove it.

4. Redwoods are hundreds of feet tall. Even small limbs that fall from such heights come down with significant momentum. This limb punctured the roof and this plywood porch ceiling below.

5. Several limbs such as this perforated this same roof. Abundant other debris was raked and blown off before these limbs could be removed, and the roof could be patched. Rain is expected.

6. This roof, as well as the house below it, got the worst sort of damage when this big fir came down. Sadly, this is not the only home that was destroyed by falling trees. Several cars died too.

This is the link for Six on Saturday, for anyone else who would like to participate:

Blow Out

Wind is messy!

While strong Santa Anna Winds were blowing through Los Angeles four hundred miles to the south, and Storm Ciara was arriving in Scotland and Norway, we were getting some remarkably strong winds of our own. They were not nearly as strong as winds that were causing so much damage in Europe, and involved no flooding rain, but they were dangerously messy nonetheless.

We live and work among dense forests of coastal redwood, the tallest tree species in the World. Beyond the upper edge of the redwood forests are more forests of huge Ponderosa pine. Huge Douglas fir are mixed throughout. Their understory includes trees that would be considered to be massive anywhere else, such as coast live oak, tanoak, Shreve oak, bay laurel and madrone.

Such big trees drop big limb, and in abundance. Furthermore, limbs that fall from such great heights are significantly more dangerous than those that fall from smaller trees that are closer to the ground. They gather major inertia on the way down. They do not necessarily fall straight down either, but can get blown significant distances to where falling limbs may not be expected.

While the winds were blowing through, I could hear crashing of falling limbs and entire trees from the mostly deciduous riparian forest outside. I know that many of the big cottonwoods, box elders, willows, alders and sycamores are deteriorating, but did not expect so many to be blown down while bare. I suspected damage would be worse among the bigger and evergreen trees.

The pile to the left in the picture above is just the debris that was collected last Monday (while I was conveniently not here to help). It is more spread out but at least twice as voluminous as the pile on the right, which is pruning debris that took me several days prior to the wind to collect. The green cargo containers in the background demonstrate how big the piles of debris are.

More debris was collected on Tuesday (while I was still doing other work). The mess was not the worst of it. The roofs of a few buildings were impaled by falling limbs. Some of the damage is significant. Fortunately, the only big trees that fell did so into forested areas where there are no buildings, and electrical service was disrupted for less than a day. No injuries were reported.

Pasadena Windstorm


The weather in the parts of California that most of us are familiar with is generally rather mild. Some of the hottest temperatures every recorded were in the Mojave Desert, but not many of us even know how to get there. Some of the heaviest snowfall ever recorded was near Tahoe, but many of us think of that as almost Nevada. San Jose, Los Angeles and the most populous regions enjoy mostly comfortable weather throughout they year.
‘Drought’ is often an inaccurate description of the naturally prolonged dry chaparral and desert weather, as if it is abnormal. There would be no chaparral or desert if it rained here as much as it does in other climates. What is considered to be normal rainfall in some regions would be disastrous to regions that do not normally get so much precipitation. Drought does happen here sometimes, but it is not as common as outsiders believe it to be.
Once in a while, we get something that really is strange. The floods and mudslides of the Winter of 1982 were disastrous. The wicked frosts of late 1990 were the worst in recorded history, even though they would not have been much of a problem in most other climates farther inland. On the morning of December 1 in 2011, Pasadena and the surrounding regions of the San Gabriel Valley experienced historically strong and destructive winds.
When I went to Los Angeles shortly afterward, I was amazed to see that pieces of the glass facades of some of the skyscrapers had been stripped away. Thrashed fronds of queen palms hung limply as if a hurricane had gone through. My colleague got these startling pictures of destroyed Canary Island date palms, which are famously resilient to wind, in Leimert Park, about fifteen miles southwest of Pasadena.


Motion As A Landscape Element

60120+Landscapes appeal to our senses. The colors and textures of blooms and foliage are visually appealing. Floral fragrance and foliar aroma appeal to the sense of smell. Fruits and vegetables can provide flavor. Wind chimes, fountains and birds visiting bird feeders might add a bit of delightful sound and motion. Yet, the motion of certain plants in the breeze is rarely considered.

Weeping willow is famous for the way it blows softly in even a slight breeze. Unfortunately, it is also famous for thirsty roots, structural deficiency, and getting too big too fast. Mayten can do the same on a smaller scale, and in drier situations. California pepper is somewhere in between, since it can eventually get too big, and might have structural problems, but takes a while to do so.

Some of the various eucalypti have softly pendulous stems as well. Lemon gum and red ironbark are two of the better known specie that know how to blow in a breeze. Silver dollar gum is nice too, even though it is not as pendulous or as dynamic in the wind. Eucalypti that get too big for home gardens can be seen blowing in the wind in larger landscapes, such as parks or on freeways.

Two of the best trees for motion in the wind are not as popular as they used to be. European white birch that was so popular in the 1970s has become considerably less common that the less pendulous (but brighter white) Himalayan birch. The elegant and formerly popular Chinese elm has been replaced by modern ‘improved’ cultivars with stiffer stems, like the Drake Chinese elm.

Almost all bamboos, most big clumping grasses, and many palms are ideal for taking advantage of breezes. Mexican weeping bamboo really is comparable to weeping willow, but on a much smaller scale. Taller bamboos can catch a breeze on top even if lower foliage is sheltered. Pampas grass, although certainly not for every garden, has both dynamic foliage and dynamic blooms.

Of course, plants that move in a breeze can only do so with a breeze. They can not do much if sheltered by larger trees or buildings.P81222++++

Santa Ana Winds

P71221+They have been a part of life in Southern California longer than anyone can remember. The Santa Ana Winds have been blowing down from the high deserts to coastal plains long before people arrived in the region. They are arid and usually warm before they leave the Great Basin and Mojave Desert, and they get even warmer as they flow downhill through mountain passes. That is what makes them so dangerous during fire season. Wind alone accelerates wildfire. Warming arid wind desiccates fuel, making it more combustible before wildfire arrives.

Santa Ana Winds are so regular that they affect how tall trees grow within the regions of the mountain passes where Santa Ana Winds move the fastest. Tall Mexican fan palms that grew up straight where sheltered from wind near the ground innately lean with the prevailing wind as they grow up and become more exposed. Those closer to the narrow canyons lean the most. It is something that arborists recognize everywhere around the Los Angeles Basin. They can tell how strongly the Santa Ana Winds blow in any particular neighborhood by how Mexican fan palms lean.

Santa Ana Winds can be strong enough to break tree limbs, and blow trees down. In the past few days, they have been making quite a mess in the Los Angeles region; although not as much of a mess as they are making in conjunction with the Thomas Fire in Santa Barbara and Ventura Counties, which may already be the biggest wildfire in the modern history of California.

Overnight, Santa Ana Winds blew this California pepper tree onto a swimming pool in Los Angeles. Tree services, landscapers, gardeners and those who enjoy gardening will be busy cleaning up such problems for the next few days, as Santa Ana Winds continue.