Month: July 2023

My news feed has been filled in recent weeks with many stories about the unusual heat that has affected many parts of North America, Europe, and the Atlantic Ocean. High temperature records are being broken at an amazing rate, and while we don’t expect every day or season to break a new heat record, the trend towards warmer global temperatures overall from greenhouse warming makes periods of extreme heat more likely.

Today I want to talk about the term “heat dome” and explain what they are, how they affect gardeners (and other humans) as well as the plants in our gardens and agricultural fields. We had a great introduction to the physiological impacts of heat a few weeks ago in Jim Downer’s post “I’m hot! So are my plants!” on how high temperatures affect plant growth and respiration. If you have not already done so please read that for more details about heat’s impacts on garden plants.

What is a “heat dome?”

A “heat dome” is an expansive stationary area of high pressure that is associated with unusually hot temperatures. While slow-moving high-pressure areas are often seen in summer months, the size and unmoving nature of a heat dome coupled with the extremely high temperatures in the area beneath them make “heat domes” especially dangerous for humans and animals. They can also cause detrimental effects on many garden and agricultural plants because they are also associated with long dry spells along with high rates of evapotranspiration. The term “heat dome” is a phrase that has been popularized by the news media as a way to explain extreme heat events across large regions and is more jargon than a true scientific term, but it is certainly descriptive!

Any high-pressure center in the atmosphere is characterized by sinking air, light winds, and relatively cloud-free skies. The sinking air heats up as it compresses near the surface, and the rising temperatures are enhanced by the lack of clouds, which lead to more incoming sunlight and even warmer air in summer. Pavement also absorbs sunlight and radiates it out at night, leading to overnight low temperatures that can be in the 90s in extreme cases.

When the center of high pressure is locked in place over one area over a long time period, it tends to divert cooler, moister air away. That makes it appear as though there is a glass dome overlying the area blocking rain from falling. The hot and dry conditions tend to get worse quickly, leading to temperatures that in the worst cases can break records. This week many daily high temperature records were broken in the western United States as well as in parts of Europe. If the air is already warmer than average to begin with, that makes it even easier to reach record high temperatures. Often, a very wavy but stagnant pattern in the upper-level winds causes a series of heat domes to form across the globe, leading to heat waves at several locations at the same time.

What danger does a heat dome cause to gardeners?

Air in the center of the high pressure that makes up the heat dome tends to trap pollutants in a shallow layer near the surface of the earth. This can lead to episodes of smog and high surface ozone that can cause health effects on people, especially children and people with poor lung function. It can also trap smoke from wildfires, as I discussed last month. The trapped pollution makes it hard to breathe and can cause lung damage as well as leaf damage on plants. Temperatures that stay in the 80s or higher overnight do not allow human and animal bodies to cool down to their normal temperature, resulting in health issues that build up over several days as the heat wave continues. Phoenix has experienced every day this July with a maximum temperature of 110 F or higher, and has seen deaths due to heat increase, especially in homeless populations that have little access to cooling. Deaths from heat spells increase after several days of extreme temperatures when the body is not able to cool itself down and the heat causes physiological changes that can lead to severe impacts. While drinking a lot of water is important for hydration, it is not enough to fend off the impacts of the high temperatures. You must find a way to cool off or your health will suffer.

Humidity is also a factor in heat-related illnesses. Humans cool their bodies off by sweating. The moisture is evaporated from the skin, leading to an energy transfer that makes the skin cool off as the sweat is changed to water vapor. But if the humidity is too high, evaporation is so slow that it can not provided the needed cooling, and the body stays hot. That can eventually lead to death. In fact, it is not just humans and terrestrial animals to suffer. The water temperature near southern Florida this week was near 100 F, leading to predictions that the coral reefs in the ocean there would soon die because it is too hot for them to survive, much less thrive. This is also happening in other ocean locations around the world. Since coral reefs provide food and shelter for many marine species like fish, it is likely to cause dire consequences for the oceanic food chain, including those of us who eat shrimp, fish, and other seafood.

What do gardeners need to do when a heat dome is forecast?

When a heat wave or heat dome is predicted, gardeners and farmers should recognize that a protracted period of extremely hot, oppressive conditions is likely to occur. The first action they should take is to make sure they are protected from the effects of the heat. That means working outside early in the day when it is cooler, drinking plenty of water, wearing light-colored clothing to reflect sunlight, and taking plenty of breaks in the shade or air conditioning. They should take care of pets and livestock as well and consider keeping them inside or in shady places with plenty of water for drinking. They should monitor their plants carefully for signs of heat stress and water regularly to reduce the impacts of the hot, dry conditions. Trees should also be watered, especially if they are not well established or if the dry period is especially lengthy. If the humidity is high, watering incorrectly can increase the likelihood of fungal diseases, so read Linda Chalker-Scott’s article Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases – The Garden Professors™ to learn the best ways to water your plants and preserve soil moisture. Saving Your Trees From Drought! – The Garden Professors™ also provides information about watering trees during dry periods.

Eventually, all heat domes and dry spells do pass, but it is always good after one is over to assess how your garden did and to plan ahead to protect soil moisture using some of the techniques mentioned in the links above.

One of the most annoying weeds in garden and landscape beds is horsetail (Equisetum spp.), a genus native throughout North America and most of the rest of the world. They have survived since prehistoric times because they are highly adaptable to their environments and are almost impossible to eradicate. There is great debate among gardeners on whether to pull or cut horsetail. Online you can find statements such as this:  “…each time you break the stem, little portions under the soil regenerate new plants. Essentially, you will be creating more horsetail.” This and many other websites recommend cutting instead.

Unfortunately, this is bad advice. The trick to eradicating any perennial weed without chemicals (or at least bringing them to manageable levels) is to starve them to death. Plants depend on their roots (and rhizomes in the case of horsetail) to survive, so anything that reduces root resources is going to eventually kill the plant. Obviously the more above-ground material you can remove, the less photosynthesis occurs and fewer resources are transported to the roots. Pulling weeds, especially if done with a forked weeder (also used in this post), is going to remove far more material than simply cutting weeds off at the surface.

Once you start a weed removal project, you have to keep after it: once is not enough. There will be rhizomes or roots left underground to support new stem growth, and once they reach the soil surface they will start producing resources to send to the roots. “Constant vigilance” is needed to keep these shoots in check. You can significantly reduce the repeated pulling by adding a thick layer of arborist wood chips to the newly weeded site. This forces the roots to put even more resources into stem growth to reach sunlight, meaning fewer weeds and more successful, desirable plants.

There is one caveat for controlling any weed that spreads underground. If you can’t control the spread from adjacent properties, you will not be able to eradicate the problem. In such cases, you may want to install a root barrier along the edges of your gardens. You simply dig a trench and install the barrier of your choice, making sure there are no gaps between the sections. Treated timbers, concrete pavers, and other materials that are slow to degrade can be used. The depth is going to depend on your soil conditions and the weeds of interest; some preliminary digging to determine the depth where you find weedy rhizomes and roots will help. Keep in mind that root barriers will also interfere with the root spread of your desirable plants.

If root barriers are not an option, the other method you can try is to densely plant low shrubs and perennials along the property line to create a competitive line of defense. The roots will compete for space, water, nutrients, and oxygen; the crowns will create a shaded environment where invading stems struggle for space and sunlight. You will still have to watch for invaders, but the amount of weeding needed will be far less than it was before. And don’t forget the mulch, both for the benefit of your barrier plants and to force invaders to use more resources to get their stems to the surface.

This method works for ALL plants – not just horsetail. (Plant physiology is funny that way.) Bindweed, English ivy, Himalayan blackberry, and Canada thistle are all weeds that I have personally controlled through physical removal and deep mulching with arborist wood chips. If you’ve had success with this method on another aggressive weedy plant, be sure to post a comment!

We are again in the midst of excessive heat events in many parts of the United States. Records were broken for the highest temperatures ever recorded just a few days ago. This is also a time when the days are at their very longest, so high temperatures have large impacts on plants in landscapes.

High temperature can have immediate (acute) and continuing impacts (chronic) on plants. When temperatures get much over 90F photosynthesis becomes less efficient and in some plants may stop all together. As temperatures increase beyond 90F photosynthesis shuts down and transpiration may also stop to avoid breaking the chain of water molecules that plants must have to move water. When this happens heat builds up in the foliage leading to cell death and eventually symptoms (acute response). These may initially show as wilting, loss of color in the leaf and rapidly within days show as yellowing and then necrosis. This is usually seen in the center of the leaf first as the edges of leaves dissipate heat faster and more efficiently than around the mid vein area of leaves.

Chronic effects of heat are related to the poor efficiency of photosynthesis at high temperatures. When plants are hot and the photo systems that capture sunlight energy are impaired, or not working, the plant must still use energy in all its cells for respiration. Stored carbohydrates are not available for growth as cell maintenance (respiration) is the first demand for energy. When temperatures are high for long periods, stored carbohydrates in roots and stems are depleted. Since energy for growth is not available, slowed or stopped growth is the biggest chronic effect of hot days on most plants. This is why even hydrated plants just seem to stop growing in hot weather.

What can be done to mitigate high temperatures? First, never let plants dry out during high heat events. Evenly moist soil (but not saturated) will allow plants to absorb water and cool themselves as much as their physiology will allow. If soils are dry the damage of high heat events is “magnified” many fold and foliar damage will increase. Irrigate late in the day or early to avoid evaporation of applied water. Get your plants ready for high heat by irrigating before it hits. We usually have good weather prediction a few days ahead of high heat events.

Another way to mitigate high heat is to avoid plantings in “high albedo” environments. Albedo is the reflection of sunlight. Low albedo surroundings abosorb sunlight energy, high albedo environments reflect it. Plants exposed to reflected sunlight will be more readily damaged by sunlight during high heat events because they can not transpire enough water to cool their leaves. Reflective soils like decomposed granite, or some kinds of rock will damage young trees during heat events. Cover the soil with arborist wood chips which have a relatively low albedo. Young plantings can be protected by placing shade cloth over their canopies until the high heat subsides. If you don’t have shade cloth, a white sheet will do fine as it will reflect heat away from the canopy.

Ensure that the mulch or soil is moist before the heat of the day starts so humidity increases during the day. This will reduce the demand on transpiration and and the possibility of cavitation (the disruption of water chains in the plant and introduction of air which stops water movement), thus preventing a catastrophic heat death event.

A final word of precaution- Never fertilize during high heat events. Even when watered this changes the osmotic potential of water in soil making it harder for plants to pull water in. Adding fertilizer is like adding salt and this is a big NO during high heat events. Try to ensure that plants have all the mineral elements they need before heat becomes an issue.

You might think that during heat events its a wise idea to prune. This is not the case! Avoid pruning, especially thinning, as the removal of leaves will increase the impact of heat on the remaining canopy. Pruning and removing leaves will decrease the humidity around a plant and the remaining leaves will have to transpire more to cool the plant. This can be a disaster during a high heat event.