You’ve probably seen Neem oil recommended in blogs, gardening forums, and on the shelf at your local gardening store. Neem is derived from the seeds of the Azadirachta indica tree, and is one type of horticultural oil that is used by gardeners looking for alternatives to synthetic insecticides. But is it effective? Is it benign? This post explores the pros and cons behind neem and other horticultural oils.
What Are Horticultural Oils?
Horticultural oils are either plant-based (like neem, canola, or clove oil) or mineral-based (refined petroleum products), and they work mostly by smothering soft-bodied pests like aphids, scale, and whiteflies.
Some plant-based oils do contain chemical compounds that can do more than smoother —neem oil contains azadirachtin, which disrupts insect development and feeding behavior.
Horticultural oils are typically considered low-toxicity for humans, and can be used on a very wide range of plants, including vegetables, fruits, ornamentals, and houseplants.
The PROS
Broad-Spectrum Neem oil is effective against a range of insect pests and some fungal diseases, yet remains relatively safe for humans, pets, and other animals. According to the Environmental Protection Agency neem oil “has been shown to have minimal impact on non-target organisms” (EPA, 2012) such as birds and mammals.
Reduced Resistance Potential Unlike synthetic insecticides that often target a specific physiological pathway (and thus promote resistance over time), neem oil affects multiple aspects of insect development, which makes resistance less likely to develop quickly.
Organic-compatible and easy-to-apply Neem and other horticultural oils are generally approved for organic gardening. interventions. They are easy for home gardeners to use since you can spray from a store-bought bottle and avoid any special equipment.
Low Residual Activity These oils break down quickly in sunlight and soil, reducing long-term environmental contamination and residue on edible crops.
The Cons
Phytotoxicity Risk If you use oils in high temperature or direct sunlight, it can lead to leaf burn and plant damage. Apply it early in the morning or late in the day to minimize this risk.
Non-Selective Action Neem and other oils can still harm beneficial insects if sprayed directly. Lady beetles, lacewings, and bees can be affected by fresh residues. You can time your application to avoid flowering periods, or spray during the evening when bees are less active to avoid non-target impacts.
Repeat applications sometimes required The effects of oils can take days to manifest and may require repeated applications (e.g. every 10 days) for best results. Gardeners expecting immediate eradication may be disappointed. Oils often work best in conjuction with other control strategies (e.g. pruning out infested areas, releasing beneficials, etc.)
Storage and Shelf Life Oils can degrade over time, especially when exposed to light and heat. They can go rancid or lose efficaciousness. Check the expiration date on your bottle and store in a cool, dark place.
Concluding Thoughts
I like horticultural oils. They can be effective tools and are safe for people to use. But they need to be used with some consideration, particularly timing to avoid non-target impacts to beneficial insects and leaf burn. Let me know your experience with oils.
References
EPA. (2012). Neem Oil; Exemption from the Requirement of a Tolerance. Federal Register. https://www.federalregister.gov/documents/2012/05/31/2012-13143/neem-oil-exemption-from-the-requirement-of-a-tolerance
Have you ever stopped while you were gardening to look at the clouds? Clouds, like flowers, come in a variety of shapes and sizes that can form beautiful patterns in the sky. But clouds are not just pretty, they can also be used to make predictions about the weather in the coming days. In this week’s post, we will look at the different types of clouds and how they relate to coming weather. You can use that to prepare for your garden work by knowing when it will be sunny and predicting when rain is coming.
Clouds were first classified by Luke Howard in 1803 in his “Essay of the Modifications of Clouds”. Howard classified the clouds by their shape, using Latin names for wisps (cirrus), lumps (cumulus), or sheets (stratus) to describe how they looked in the sky. Clouds that are precipitating either rain or snow are called nimbus clouds. Some clouds are a combination of types, such as stratocumulus clouds, which look like a layer of lumpy clouds, or cumulonimbus clouds, which are the tall thunderstorms that form in summer. You can find a great gallery of cloud photos at the Cloud Appreciation Society. Wikipedia has an exhaustive list of cloud types online as well.
Clouds are also classified by heights. Most clouds form in a single layer of the atmosphere, but you can often see multiple layers of clouds when you look at the sky. These layers may be caused by different mechanisms. The shape and height of the clouds provide clues to what is going on in the atmosphere and are especially related to the presence (and sometimes absence) of warm and cold fronts that are harbingers of precipitation and a change in wind and temperature conditions.
There are many different types of clouds, each with a unique shape and location in the sky. UCAR/L.S. Gardiner
To estimate how high the clouds are, you can use a literal “rule of thumb” that works well for cumulus-type clouds. Hold your hand out towards a cloud. If the individual cloud is the size of your fist, then it is probably a low cloud. If the lump of cloud is the size of your thumb, then it is probably a mid-level cloud. And if it is the size of your little pinky joint, then it is probably a high cloud. Note that the clouds are the same size, but the difference in height makes them look like they are different sizes.
For stratus clouds, you can estimate the height by how transparent it is. A cloud layer that allows you to clearly see where the sun or moon is located is probably a high cloud. A layer that lets you see where the sun is but shows it with blurry edges (we sometimes call this a “ground glass” appearance), then it is probably a mid-level cloud. If it is so thick that you cannot see where the sun or moon is, then it is probably a low-level cloud. But please be careful not to look directly at the sun, since it can damage your eyes!
How do clouds form?
Clouds form where moist air cools off to the point that the water vapor condenses into small drops that become visible to us. Since the atmosphere generally cools off as you go up, the clouds form where the air is rising. The rising motion can be provided by heating from the earth’s surface, lifting of the air by a mountain, or large-scale upward motion caused by warm and cold fronts, where masses of air at different temperatures interact to create areas of rising air. The highest clouds usually form in the coldest air and form as ice crystals, leading to their wispy appearance. Lower clouds appear in warmer air where the water condenses as liquid droplets, leading to their more robust appearance.
Clouds associated with cold and warm fronts, U. K. Met Office.
How are cloud shapes related to atmospheric winds and structure?
The most interesting weather (at least to me, as a meteorologist) is where there are differences between masses of air at the surface and above the surface that are interacting. Boundaries between these air masses are called “fronts” and are named because they act like battle fronts between enemy armies. In a cold front (left side of diagram above), cold dense air near the surface pushes beneath a layer of warm, humid air, causing it to rise and cool. Clouds ahead of cold fronts tend to be relatively tall and energetic and form cumulonimbus clouds that can drop a lot of rain in a short time but generally tend to move through an area quickly unless the front stalls due to other atmospheric dynamics.
Warm fronts are large masses of warm, humid air that are pushing over the top of a layer of colder, more dense air (right side of diagram above). As the warm air rises, it slowly forms clouds as the layer cools to the temperature of condensation. Since the entire layer is rising, the clouds that form are often sheets of clouds rather than individual clouds. The higher clouds indicate that the moisture from an approaching warm front is present high in the atmosphere and shows that a warm front is likely to be approaching, signaling a change in the weather from cool to warmer conditions that could also drop rain as the warm front gets closer to your location. As the warm front gets closer, you should see the high clouds replaced by mid-level clouds like altostratus and then lower clouds like stratus clouds and nimbus clouds if they start producing rain. This usually happens over a day or two depending on the speed and strength of the surface warm front.
If you see lumpy cloud forms, then they are most likely related to the rising motion of air due to columns of warm air rising from the surface (“thermals”). Air between the columns is sinking, which leaves clear spots between the clouds. Fair-weather cumulus clouds are the tops of these thermals, when the rising air cools down enough for the water vapor in the column to condense, forming the cloud. These types of clouds usually form when the ground is heated, most often by sunlight during the day but sometimes by pavement or wildfire as well as mountainous areas. They usually form on days when you are in a mass of warm, humid air that is far from a frontal boundary, although if they grow taller, then a cold front is likely to be approaching.
If the air is very hot and humid and the surrounding air is cooler than the rising column, then the clouds can grow vertically to great heights before they hit a layer that is warmer than the rising air and stop growing. These tall clouds are called cumulonimbus clouds because they often drop heavy rain as they develop. These often form along and ahead of cold fronts and indicate that the wind is likely to shift from a south wind to a colder wind coming from the northwest (in the Northern Hemisphere). In the worst cases, the rain can cause erosion or damage to fragile plants when it is very intense. Plants that live in rainy areas evolve to have such things as pointed tips or shiny leaf surfaces that shed the water quickly. Cumulonimbus clouds are also sometimes associated with high winds, hail, and tornadoes, all of which can damage garden plants and trees as well as harm humans and animals.
Cirrostratus clouds being illuminated by the sun and forming a halo, Eduardo Marquetti, Commons Wikimedia.
How can you predict the weather by watching the clouds?
As you work in your garden, take a look at the clouds above you. If they are high and wispy, then moisture high in the atmosphere may indicate that a warm front and a chance of rain is likely in a couple of days, especially if they get lower and denser over time. The picture of daisies at the beginning of this post shows high, wispy clouds that could indicate a warm front is approaching. The glowing ring of light that appears in cirrostratus clouds in the picture just above this paragraph may also be a sign that there is moisture on the way, leading to the saying that “a ring around the moon means rain in 48 hours”.
The chance of rain may affect your plans to spray your gardens or lawns for pests or fungal diseases, since many garden treatments have weather-related requirements for when to use them. Some work better when applied to wet leaves after rain falls, but others need a period of dry weather for the chemicals to be most effective. Make sure you read the labels to know what kind of weather they need. It may also tell you that it would be a good idea to mow the grass before it rains in the next 24-48 hours.
An Intercity from Amsterdam to Den Helder passes a field in full bloom near Schagen, Netherlands, Kabelleger / David Gubler, Commons Wikimedia.
If you are already in hot and humid conditions and you see cumulus clouds getting taller and more numerous over time, a cold front may be approaching. This could indicate a period of strong winds, heavy rain, some possible lightning, and sharply cooler temperatures which could be either a curse or a blessing depending on just how hot it has been. The very shallow clouds in the picture above (cumulus humilis) are most likely seen after the cold front has passed and there is minimal lifting to cause clouds to form.
Don’t forget to look up
A good gardener should always be keeping an eye on their garden but should also be watching the environment around it to see how the conditions might be changing in the future. Who knows what delights you will see if you just look around you? But don’t forget to look up, too, because the sky is full of wonders and can inform you about the future as well as strike you with awe.
Bee hotels have become popular additions to gardens, designed to support wild bees by providing them with nesting sites. Solitary bees, unlike honey bees, live in natural and man-made cavities which can be easily provided with nesting habitats. A previously published Garden Professors blog offers valuable insights into creating artificial nesting structures for these bees, emphasizing the importance of proper design and placement. However, if you’re thinking about installing a bee hotel, I’d urge you to reconsider – some studies suggest that bee hotels, if not correctly maintained, can inadvertently harm the very pollinators they’re meant to help.
While bee hotels offer nesting opportunities, there is almost no research showing that they have a positive effect on bees. Some researchers also think bee hotels can become hotspots for parasites and pathogens (MacIvor & Packer 2015). High-density nesting sites can facilitate the spread of diseases, similar to how bird feeders can become transmission points for avian illnesses. Bees are particularly vulnerable to viruses, microsporidians, and fungal agents which can spread via exposure to feces or even through pollen left behind by bee visitors. Bee hotels can also attract bee predators – nesting aggregations of wild bees that are artificially close together might be attractive to parasitic wasps which infiltrate nests, laying their eggs inside and jeopardizing the bee larvae.
Bee hotels may still have their place, especially in community gardens where they can serve as a point of conversation and provide beauty and interest as a form of garden art. However, given the risk for disease spread, here are some tips for maintaining a bee hotel.
Best Practices for Bee Hotel Maintenance
To ensure bee hotels remain beneficial:
Annual Cleaning: After bees have emerged in the spring, clean the hotel thoroughly. Remove and replace any natural reeds or paper straws. For wooden blocks, use a thin bottle brush or compressed air to clean out debris.
Use Removable Nesting Materials: Opt for bee hotels with removable tubes or liners. This design facilitates easier cleaning and reduces the risk of disease buildup.
Proper Design: Ensure that nesting holes are closed at one end to prevent parasites from accessing the nests from behind.
Limit Nest Density: Avoid overcrowding by limiting the number of nesting tubes. A lower density reduces the chances of disease and parasite spread
Create Habitat: Leave undisturbed, unmulched areas in the borders and corners of your garden so that bees can nest naturally in the ground. Some bees also nest in dead twigs and hollow stems and branches, so consider leaving some behind for them.
Do you have a bee hotel in your garden? What has been your experience with them?
References:
MacIvor, J. S., & Packer, L. (2015). ‘Bee hotels’ as tools for native pollinator conservation: a premature verdict?. PloS one, 10(3), e0122126.
If you’ve been paying attention to the weather across the United States this past week, you may have noticed that most of the eastern U. S. is experiencing extremely hot temperatures, especially when you factor in the effects of humidity. At the same time, in the western U. S., it has been snowing in the mountains, even though it is almost July! In this week’s blog post we will look at why this pattern of hot and cold conditions occurs so often and what is causing it.
Mount Timpanogos with wild flowers (Utah, USA), Taken on 28 August 2011, Brian Smith, Commons Wikimedia.
Heat in the East
We have talked about persistent areas of very high temperatures several times in past blog posts (for example, here and here). Those areas most commonly form in summer under stagnant areas of high pressure that have sinking motion in the middle of the high. The sinking motion of the air keeps clouds and rain from developing, leading to very hot and dry air being trapped near the earth’s surface, raising temperatures and reducing wind speeds. Often those areas also include a lot of humidity, which makes the temperatures more oppressive because sweating does not cool you off efficiently if the humidity is high, especially if winds are also light.
Formation of a heat wave : a high-pressure circulation in the atmosphere acts like a dome or cap, trapping heat at the surface, National Ocean Service, NOAA.
It is also interesting to note that in winter, areas of high pressure are often the coldest areas of the country due to the lack of cloud cover, which allows heat from the earth to escape to space, leaving colder conditions at the surface at night when no sun is there to heat the ground.
Texel – De Hors – One of the heat wave days of the Summer of 2008 – At the South Beach Corner of Marsdiep & North Sea, Txllxt TxllxT, Commons Wikimedia.
The common connection between the heat in the East and the snow in the West is the large-scale atmospheric wave that is linked to both the low pressure in the west and the high pressure in the East. The atmosphere is a fluid and is constantly adjusting its pressure fields by forming waves with ridges of high pressure as well as troughs of low pressure. Sometimes these patterns get locked in place for a few days (or even longer in rare cases), which leads to more extreme effects, but they usually move on in a few days, causing the weather to go back to normal conditions or even flip-flopping to the opposite pattern. In fact, we discussed atmospheric waves back in July 2021 following the PNW extreme heat event in this blog.
The surface weather associated with these areas of high and low pressure are what cause the big changes in observed conditions that gardeners and farmers have to deal with since they can have big impacts on flowers and crops. The atmospheric wave pattern can lock in place for a number of reasons including conditions in other parts of the earth such as unusually cold or warm water in the ocean or droughts or floods in other areas. Fortunately, these stationary patterns usually shift or break down after a few days, leading to big changes in local weather conditions that might be more welcome.
Cold spells in summer do not cause as much damage as frost and snow in spring and fall because they just don’t get cold enough to damage the plants (unless you are in the mountains and the temperatures drop below freezing), but they can slow the plants’ growth and reduce their flower or fruit production. Food crops that depend on a significant number of growing degree days (a measure of the accumulation of heat over time based on daily temperatures) will grow more slowly, resulting in late harvest of whatever crop is being grown. In the worst cases, it could delay harvest so late in fall that fall frosts become a consideration, but most home gardeners do not need to worry about this as much as commercial farmers because they are not farming hundreds of acres of crops, just their own patch of land.
I encourage you to visit some of the links in the article above to learn more about heat domes and atmospheric waves as well as their impact on your garden plants. You can also use the search field to find additional sources of information about any topic of gardening that you might want to learn more about, especially one that relates to the science of gardening. It’s a great resource!
残雪とエゾツガザクラ（Snow and flowers), pakku, taken on 31 July 2009, Commons Wikimedia.
I wish I could say I grew up with an innate fascination for the insect world—that I was one of those kids who spent hours flipping over rocks to marvel at beetles and ants. But the truth is, growing up in urban Los Angeles, I rarely interacted with nature at all. And insects? I was terrified of them. I was the last person to volunteer for anything involving creepy crawlies.
That all changed in college. I enrolled in an introductory environmental science class, and one guest lecture changed the course of my life. A campus professor gave a talk on agroecology, describing how their work wove together science, practice, and social movements to promote sustainable pest management. I was captivated. Here was a discipline that tackled food production, ecological health, and community well-being—all at once. It felt like a bold, grounded approach to conservation—not one that isolates nature in national parks, but one that weaves ecological thinking into everyday landscapes like farms and gardens.
I emailed him that very day, requesting an internship—and he agreed. My first day on the job found me in a vineyard in the Central Valley, shaking grapevines over a beating sheet, pretending I wasn’t afraid as a swarm of insects fell onto the canvas. The graduate students in the lab patiently taught me how to tell wasps from bees, bugs from beetles, and moths from butterflies. They also introduced me to the foundational concepts of biological and cultural control—how pests can be managed using natural enemies and sustainable farming practices. Later that year, one of those graduate students asked if I’d be willing to host one of his beehives in my backyard. I instantly fell in love with those bees—their complex, maternal society, their admirable work either, and the delicious honey they produced. That hive sealed it for me: I was going to become an entomologist.
After graduating, I apprenticed at the UC Santa Cruz Center for Agroecology and Sustainable Food Systems, where I sharpened my skills in horticulture, including irrigation, nutrient management, composting, greenhouse production, and pest management. I worked across a diverse range of crops, from annual vegetables to ornamentals and perennial fruit trees. During my Ph.D. and postdoctoral work, I focused on how farm and garden management practices influence wild bees and their susceptibility to the parasites and pathogens implicated in pollinator declines. My research sits at the intersection of agriculture and ecology, and I’ve always believed that productive farming and environmental stewardship should go hand in hand.
Now, I serve as an Extension Advisor with the University of California, based in Ventura County. As an entomologist, my primarily focus is on integrated pest management of insect pests, including identification and monitoring, developing cultural and biological control methods, and evaluating new chemical control products. One of the best parts of my job is the incredible variety of crops I get to work with, including avocado, citrus, strawberries, cole crops, and greenhouse ornamentals – working with so many different commodities means that I’m constantly learning. In my work I aim to provide science-based, practical recommendations that help farmers, pest control advisors, and others in the industry to effectively manage pests while safeguarding beneficial insects and human health. For example, I’m currently leading several projects that explore how small-scale habitat enhancements—like hedgerows or cover crops—can improve pest control and support wild pollinators in citrus orchards.
I didn’t set out to become an entomologist. But through a mix of chance, community, and unexpected inspiration, I’ve found myself surrounded by insects—not in fear, but in deep fascination and appreciation.
I am traveling in Colorado this week, so my thoughts naturally turned towards the mountains. Mountains affect gardening in a number of ways, many of which include a weather or climate component. They also provide some special challenges for gardeners because of the harsh conditions and short growing seasons that are often found in and near mountainous terrain.
Red Rocks Park in Autumn, MichaelKirsh, Commons Wikimedia.
How the mountains affect weather and climate
It is said that mountains create their own weather, and there is a lot of truth to that. Mountains interact with the atmosphere in several ways, and that interaction can change both the atmosphere itself and the conditions on the surface of the mountain that is being affected by the air.
If the air is diverted sideways around the mountain it can block the wind from hitting some locations downwind of the peak. Our fearless leader Linda told me she experienced this just a few weeks ago when a strong low-pressure center moved into the Northwest, bringing strong winds to the region. However, Linda noted that in her location, those winds were blocked by Mount Rainier, resulting in much lower local wind speeds due to the shelter from the massive mountain.
Wind flow up and down the mountains due to temperature variations
Mountains can heat or cool the air around them, depending on the time of day and the season. In summer, the peaks warm up and provide a heat source that helps lead to the formation of thunderstorms. You can see this almost every summer day in the western US with storms that develop over the mountains as the sun warms them. Those storms then move out over the prairies, leading to scattered rain or even virga, rain that evaporates before it falls to the ground. At night or in winter, the air near the peaks cools quickly and the denser air flows down the mountains into the valleys, resulting in katabatic winds that can cause freezes in low-lying areas when the coldest air reaches the lowest elevations. In this situation, the valley floors may experience frosts while areas on the slopes remain above freezing because the dense air drains through them relatively quickly. The winds can also increase evaporation rates, limiting the amount of available moisture and causing water stress on garden plants.
Orographic rainfall diagram, Encyclopedia Brittanica, Inc.
Temperature variation with elevation and orientation
The surrounding atmosphere also affects conditions on the mountain terrain. Atmospheric temperatures decrease with height, so as you go up in elevation on a mountain, the temperature will drop. This can lead to cooler climates and shorter growing seasons due to the increased likelihood of frost with the colder temperatures. This limits the types of plants that gardeners can grow because the climate of that location has limited suitability for plants that grow well on the flatlands.
Another aspect of mountainous terrain is the number of microclimates that are present in the rocky, uneven landscape. Mountaintops and sides offer a range of microclimates, from sunny, well-drained slopes to shady, cooler areas, influencing plant growth in different locations. North-facing slopes get little direct sunlight and can pool pockets of cold air that result in frosts every month of the year, while sunny south-facing slopes could be much warmer and more suitable for a variety of plants. Any mountain gardener has to be especially aware of the local microclimates in their area and account for them when choosing what and when to plant.
Alpine garden, Montreal Botanical Garden, Thomas1313, Commons Wikimedia.
Of course, there are other characteristics of mountains that can also affect garden success. Soils are often shallow, rocky, and low in organic matter. Lower pressure and humidity may cause problems with plants’ ability to thrive in the harsher conditions. Sunlight can be very intense and shade from taller plants may be limited. Some areas may experience extensive periods of snow, which can provide insulation but can also damage plants when it slides downslope.
Gardening in the mountains can provide challenges for many gardeners due to the difficult environment that the mountain air provides, but it can also allow gardeners to create unique collections of alpine vegetation that can provide enjoyment for years to come, all set in a diverse and scenic landscape. Be sure to look for additional information on alpine or mountain gardening on the web to make sure your garden is well-suited to your local conditions.
Alpinarium w Bydgoszczy, Pit1233, Commons Wikimedia
Deciphering fact from fiction for one of the most infamous plants in the world.
Dandelion in a lawn. Photo: Abiya Saeed
Dandelions can be a bit of a polarizing subject for gardeners. Some absolutely love them, while others may despise seeing these bright yellow bursts of unconformity in an otherwise ‘pristine’ lawn and garden landscape. Many often find themselves somewhere in the middle of this spectrum. Dandelions are also used as a symbol for resilience–growing despite all odds in some very harsh and unforgiving environments–often ones where few cultivated plants would successfully grow. Some people enjoy eating them, while others embrace them as a source of nectar, pollen, and food for other critters in the landscape. There is a lot of wonder in the way that they disperse, and many kids and adults alike have enjoyed blowing on dandelion seed heads (with or without making a wish) to watch them float away in the wind, drifting to new locations that they can conquer as their own. There is even an annual festival devoted to dandelions in Carbondale, Colorado (located in the same county where I served as an Extension Agent several years ago). When I attended years ago, it was such a cute and unique event, where people were wearing dandelion flower crowns, sharing art and artisan products associated with dandelions, and enjoying music and merriment in the spirit of the whimsical yellow plant. There is a lot of myth, magic, and majesty associated with dandelions, that it can be hard to decipher fact from fiction.
I get asked about dandelions more than any other plant combined– especially pertaining to pollinators, but also many other things. These queries come from people of all backgrounds and viewpoints, ranging from: how to get rid of them, to: how one can encourage and/or intentionally grow and cultivate them. These queries are especially common as spring rolls into Montana with the classic yellow bursts of color being some of the most visible flowers at this time of year, especially in the colder climates of zone 4 in the greater Rocky Mountain region. I have found myself endlessly poring over research to try and answer some of those questions, that I thought it would be fun to ‘dig in’ (pun always intended) to the lore of dandelions and the science-based information that we have on this notorious plant.
The Dandelion
Dandelions (Genus: Taraxacum) are a widespread genus in the Aster family that can be found in most parts of the world, either as a native plant or naturalized through intentional and unintentional introductions. Although there are around 250 species in the genus, the most widespread dandelion species is Taraxicum officinale, also known as the ‘common dandelion’, which originates from Eurasia but is now naturalized in many parts of the world (and can be found on on every continent except Antarctica). For the purposes of this Blog post, I will be focusing on information pertaining to the common dandelion (Taraxicum officinale). This is considered a short-lived perennial plant that can reproduce sexually (through seeds) and asexually (through the roots), can withstand a wide variety of climates and soil conditions, all of which can contribute to the fact that dandelions are so prolific and widespread.
Starting out as a rosette of lance-shaped leaves, it shoots up the characteristic yellow flowers in early spring. Each ‘flower’ is actually an inflorescence that consists of several ray and disc florets clustered together (similar to its relative: the sunflower). The length of the flower stalk is extremely variable, ranging from a couple of inches (in frequently mowed areas such as lawns) to multiple feet in length. I once saw a dandelion curiously poking out through the top of a boxwood shrub that was nearly 3 feet tall, and upon investigation, measured the stalk at a staggering 35 inches (which is half the length of the tallest dandelion stalk on record found in Ontario, Canada). The flowers are followed by the very distinct seed-heads which contain individual seeds, each of which are attached to a fine tuft of hairs (a pappus) that act as a parachute to aid in wind dispersal. Dandelions are well-known for their tap-root which can contribute to the plant’s drought tolerance, and allows them to compete well with other vegetation for limited resources.
The fact that dandelions are edible is not a topic that is frequently debated. Many know this to be true. Almost every part of the common dandelion from the taproot to the flower heads is all edible (with the exception being the stems which contain a milky latex that can be very bitter). Plenty of dandelion recipes can be found with a quick Google search. There is, however, much debate about their taste. To some, dandelions are a whimsical treat, where you may enjoy the flowers or young leaves in a salad, while others choose to steep parts of the plant to make teas, or enjoy the fermented products as delectable dandelion wines. Furthermore, dandelions can be found in many skincare products, salves, lotions, herbal remedies, and more.
In North America, European settlers intentionally brought dandelions for their nutritional and medicinal value. They were intentionally grown alongside vegetable and herb plantings, and used to remedy a variety of ailments. The nutritious properties of dandelions are comparable to salad greens such as spinach and arugula. Leaves are high in potassium, calcium, and iron, whereas the roots can have diuretic and laxative properties. For more information on dandelions as food, check out the link to the publication from University of Wisconsin in the resources.
Although I have yet to find a dandelion recipe that I thoroughly enjoy (besides recipes that try and mask the flavors using in intense array of herbs and spices, or baking them into treats, brewing them into teas, and fermenting them into wines). I am not personally a fan of the bitter and earthy flavor of the raw plant, though I admit, I haven’t tried all variations of cooking or flavoring the parts of a dandelion. If you have a recipe you love and swear by, feel free to share it with me, and I will give it a try (as long as it isn’t too time and resource intensive). Regardless of your taste preferences, if you do choose to eat dandelions, collect them from a safe location, make sure that they have not been treated with any chemicals, and wash them thoroughly to remove any soil, debris, or insects.
Dandelions and Pollinators
Many embrace dandelions because of the associated value to pollinators. The science behind this, however, is not as black and white as some may think. Although dandelions can be convenient sources of pollen and nectar for pollinators in highly urbanized landscapes, especially early in the season when very few other plants may be flowering in some of these densely populated areas, they are not the highest quality source of food for many of our pollinators. Some claim that dandelions are the earliest flowering plants blooming in the spring, which is also untrue in many parts of the world. That being said, dandelions are among the most widespread and consistent sources of nectar and pollen in some landscapes (such as urban areas with fewer flowering plants intentionally incorporated to support pollinators all season long) and also some of the earliest blooming plants visible in these types of landscapes.
A lot of research has been done that shows dandelions attract a wide array of pollinator species, and can therefore be a critical source of food for pollinators in urban areas, where these plants are widespread and can act to bridge the gaps between other areas of more diverse floral resources (Larson et al., 2014). Research also shows that dandelions do not have the most nutritious nectar and pollen, lacking in certain important amino acids, making pollinators (such as honey bees) unable to survive on dandelions alone (Loper and Cohen, 1987). We also know that a dandelion-only diet can impact the ability of honey bees to rear brood (Herbert et al., 1970). Unfortunately, our research is usually restricted to managed bee species such as honey bees, so we have far less information on how most of our pollinator species (including the rest of our 20,000 species of bees) would respond to a dandelion-focused diet. What we do know is that pollinators need a varied diet with floral resources available all season long, including early and late in the growing season when some nectar and pollen collecting species have to begin provisioning their nests and when some species are getting ready to overwinter. Although there are countless flowering plants that are better for supporting pollinators, dandelions will always have a place on that list until more intentional pollinator-friendly plantings are incorporated.
Dandelions can also support caterpillars of a variety of butterfly and moth species. These caterpillars will use the rosette of leaves produced by dandelions as their primary source of food, or as part of a wider diet consisting of a variety of plants. These caterpillars, in turn, can be an important source of food for animals higher up on the food chain (including birds). This can make dandelions important for ecosystems beyond just their nectar and pollen for pollinators.
Dandelions as a Weed
Common dandelions are undoubtedly a resilient plant in many landscapes, as all of us have seen them popping up in lawns, through dense vegetation, in gravel roads and driveways, sidewalks, roadsides, and so on. In fact, it would be difficult to imagine a landscape without dandelions. Because they can grow in conditions that may not favor some other cultivated plants, and therefore may be found taking over areas where you wanted to grow something else, they are commonly considered a weed. In fact, they are probably the most famous weed you can think of, and the poster plant for many lawn care companies, herbicides, and other garden products aimed at controlling or limiting their abundance in our managed landscapes.
Some research shows that dandelions can compete with native vegetation for resources. Research in Japan on native Taraxacum spp. and the impact of growing alongside Taraxicum officinale showed a reduction in seed production for the native species (Kandori et al., 2009). The authors hypothesized that the more attractive flowers of T. officinale may deprive the native species of pollinators, resulting in a reduction of pollination services. They also stated that the transfer of pollen from the non-native species could interfere with the successful reproduction of the native species, however, this was disproven by hand-pollination experiments (Kyogoku, 2021).
In many home garden settings, whether or not something is a weed, is usually very subjective and dandelions are no exception. I, personally, don’t mind dandelions growing opportunistically in my landscape, and my stance on many persistent (non-noxious) weeds in the garden is usually a variation of ‘may the best plant win’, but I know that not everyone feels this way. Although some embrace the odd dandelion speckled in a lawn, when you have large swaths of dandelions in the place of what used to be turfgrass, the problem usually extends beyond dandelions themselves.
One of the best ways to combat dandelions in a turf lawn setting, is to make sure that your turfgrass is healthy and able to form dense coverage on the soil. Healthy turf can often outcompete weedy vegetation that can opportunistically take advantage of open spaces for establishment. Addressing soil compaction, nutrient needs, and responsibly caring for your turf lawns can all play a role in reducing weed issues, including dandelions. For situations where competing vegetation may not be an option, mulches can be used to reduce the presence of dandelions that may be found in flower beds or veggie gardens. Mechanical removal is also very effective for controlling dandelions, depending on the scale of the issue. Using your favorite tap-root removal tool (such as the aptly named dandelion fork) can remove plants without too much exertion. Pulling out as much of the root as possible will offer the best control, as dandelions are less likely to propagate from smaller root fragments.
Several herbicide options are also available for dandelion control. Broadleaf herbicides such as 2,4-D, dicamba, and MCPP (commonly found in ‘weed and feed’ fertilizer products) can be effective for dandelion control in lawn settings. Spot-treating individual plants using an appropriately labeled broadleaf weed killer is usually more effective than broadcast application across larger areas. Herbicides with the active ingredient Glyphosate are not as effective for long-term control of perennial weeds like dandelions, because they often knock back leaves without killing the roots, which allows the plants to regenerate. These products can be more effective if a plant is cut or mowed and the herbicide is carefully painted on the fresh cut, which can facilitate movement into the root system. Late summer and early fall is the best time to control perennial weeds such as dandelions. This is because these plants are moving resources from the foliage to the root system in order to prepare for winter, which can also help to transport systemic herbicides to their roots and/or deplete their root systems of energy for more effective and longer-lasting control. (Remember that herbicides may kill desirable plants, and not just weeds- so use them responsibly and sparingly while taking precautions not to apply them near susceptible plants). Always, always, always read and follow label directions, and if unsure: reach out to your Extension resources for assistance.
Whether you are a big fan of dandelions, or the opposite of that–I hope that you learned something new from this post–and continue to be curious about the plants that surround us!
Larson, J. L., Kesheimer, A. J., & Potter, D. A. (2014). Pollinator assemblages on dandelions and white clover in urban and suburban lawns. Journal of Insect Conservation, 18, 863-873. https://link.springer.com/article/10.1007/s10841-014-9694-9
Herbert, E. W., Bickley, W. E., & Shimanuki, H. (1970). The brood-rearing capability of caged honey bees fed dandelion and mixed pollen diets. Journal of Economic Entomology, 63(1), 215-218. https://academic.oup.com/jee/article-abstract/63/1/215/798721
Kandori, I., Hirao, T., Matsunaga, S., & Kurosaki, T. (2009). An invasive dandelion unilaterally reduces the reproduction of a native congener through competition for pollination. Oecologia, 159, 559-569. https://link.springer.com/article/10.1007/s00442-008-1250-4
In the last few weeks NOAA has declared the end of the weak La Niña that has been present in the eastern Pacific Ocean and the return to neutral conditions. I want to take time today to discuss what it means for our summer growing season in the United States. I will also provide some links to guidance for how it might affect conditions in other parts of the world for our non-US readers.
We track ENSO carefully because it is an internal oscillation of the atmosphere-ocean circulation that has big implications for variations in climate across the globe. If we know what phase of ENSO we are in, we can make some statistical predictions of what the climate is likely to be at our locations as well as others around the world. This is because the ocean temperature is linked to vertical cloud growth, which can act like a rock in a river, diverting the high-elevation winds north or south. Those winds push around the storms that bring clouds and precipitation to the regions where they are located, so that gives us some knowledge of what weather we might generally expect to experience. Of course, each El Niño and La Niña are different because the atmosphere is constantly adjusting in relation to other factors like droughts and floods, heat spells and cold outbreaks, so every event is unique, but statistically there are known relationships that give us some confidence in how our coming climate for at least the next few months might be.
What do we expect this Northern Hemisphere summer?
The strongest statistical relationships between the ENSO phase and climate conditions occur in the winter months, so the climate pattern this summer will be affected in some ways by the neutral conditions but will be more affected by other factors. The biggest impact will come during the Atlantic tropical season, since neutral years are linked to higher than average numbers of named storms. And sure enough, this year the early seasonal forecasts are for more storms than usual. The number is likely to be a little lower than last year because the Gulf and the Atlantic Ocean are not as warm as they were last year, although they are still warmer than average. Tropical cyclones in the Eastern Pacific Ocean could be fewer than normal because that is typical in a neutral year, but the water there is pretty warm so that would likely lead to more storms.
NOAA’s seasonal forecasts
NOAA’s Climate Prediction Center has issued their 3-month forecasts for the early summer (May-July) and late summer (August-October). The maps are shown below. For the early summer period of May through July, most of the country is likely to be warmer than normal with the exception of the Northern Plains, which has equal chances of above, below, and near normal temperatures. Note that the depth of the color shows how high the probability of that condition is, not how hot it will be. The rainfall this summer is expected to be drier than normal in the western half of the United States except for southern Arizona and New Mexico, which has a chance of experiencing a wetter than normal monsoon. The northern Gulf Coast and the Eastern seaboard is all expected to be wetter than normal, at least in part because some of that rain will be coming from the tropical storms that are expected to occur during the hurricane season from June through November.
For later summer (August through October), all of the country is expected to be warmer than normal, with some areas more likely than others. Rainfall continues to show a wetter pattern than usual in the Southeast and in the monsoon region but is likely to be drier than normal in the northern Plains.
You might ask why the maps show a tendency towards above normal temperatures. That is mainly due to the rising temperatures associated with global warming. As temperatures continue to rise due to the greenhouse gases being emitted into the atmosphere, on the average each year will be a little warmer than the last, although of course there are year to year variations due to ENSO and other variations like ocean temperatures and drought that affect individual years’ statistics. But in the absence of detailed information about the coming year, it is a better bet that we will get a warmer than average year than a colder than average one.
How can gardeners use that information to plan this year’s gardens?
If you are just starting to plan or plant this year’s garden, you can use the information from the CPC to help determine what kinds of flowers and vegetables to put in. If you know that the summer is likely to be warmer than usual, then you can purchase plants that love the summer heat! Be prepared to water them, though, unless you are planting succulents or cacti. This will be especially true if you are also predicted to be drier than normal over the summer, because your gardens are likely to demand a lot of water if there is no rain to quench their thirst. You can help keep moisture in the soil by adding arborist chips as mulch over the surface of the soil to minimize the loss of soil moisture from the root zone.
Fountains at Balboa Park, San Diego, Jon Sullivan, Commons Wikimedia.
Since neutral conditions, where neither El Niño nor La Niña are present, are associated with more tropical storm activity in the Atlantic, if you are living in an area that is normally affected by those storms, you should prepare ahead of time by storm-proofing your garden and home and watching the forecasts carefully once we start to get into the active season. It is never too early to prepare!
I hope you have a great growing season and enjoy the fruits and the scents and sights of your garden if you are in the Northern Hemisphere or planning for the next growing season if you are in the Southern Hemisphere far from the equator. We love to see your photos on our Facebook page!
Zinnia Elegans, Muhammad Khaikal Al-Akbarsyah, Commons Wikimedia
There are many popular songs about fire. Those of you who are fans of Bruce Springsteen will recognize these lyrics from “Dancing in the Dark”. They popped into my head when I was driving home from Asheville NC to Athens GA this past weekend and noticed plumes of wildfires punctuating the air along the highway. That inspired me to write this post on wildfires, which are affecting the Southeast this spring but also affects many areas of the United States and the world too, especially when those areas are in drought. In this post I will discuss how wildfires start, how the local environment may help them spread, and what you can do to protect your properties and gardens from the impacts of wildfires in your communities.
David Sands / Rosebay willowherb leads the way to a ruined building / CC BY-SA 2.0
Wildfires versus prescribed burns
Fires in the environment can be caused by natural events like lightning or can be sparked by human sources. Some fires are set on purpose to clear land and reduce fuel loads so wildfires are less likely to occur and some are caused by ignition sources like sparks from dragging chains, a carelessly tossed cigarette butt, or an untended campfire. Some are set deliberately to cause damage and chaos by arsonists or are the result of careless children or adults. According to Earth.org, “40% of wildfires that affect British Columbia in an average year are human-induced. In the US, the amount is more than double, with nearly 85% of the nearly 100,000 wildland fires that affect North America every year caused by human activities, according to data from the National Park Service.”
The fires that are set to reduce fuel loads and remove overgrowth from land are called “prescribed fires” and they are regulated by most states. Farmers sometimes use controlled burns to remove cover crops and prepare for spring planting. Those who want to set a prescribed fire usually have to file a form or follow a procedure to indicate what they are going to burn and when and what the weather was at the time of the burn. They are also expected to file all the necessary permits and notices for smoke and fire hazards. In some states you must be certified to conduct a controlled burn. If the weather is too windy or the humidity too low, they are generally not allowed because the chance of a fire getting out of control is high in those atmospheric conditions. There have been instances of prescribed fires escaping their planned burn areas or causing significant hazards, including a number of deadly multi-car accidents when the aerosols from the fire attracted enough water vapor to form a “superfog” that moved across a busy highway and caused visibility to fall to near-zero feet which blinded drivers speeding down the roads. “Superfog” is especially dangerous if it occurs overnight when the humidity is the highest which causes it to be more dense.
What are the causes of wildfires and how do they spread?
The number one cause of wildfires from natural causes is lightning strikes, especially in areas of drought when the vegetation is very dry and moisture is scarce. Volcanoes can sometimes cause fires by dropping hot embers onto flammable land cover and buildings. Strong winds can quickly spread the fires to new areas downwind and provide a source of oxygen that helps them continue to burn. In areas with a lot of fuel like drought-stricken national forests or open grasslands the fires can rage out of control and cover large areas in short amounts of time.
Lick Fire on the Umatilla National Forest burning at night, U.S. Forest Service- Pacific Northwest Region, Commons Wikimedia.
How to know when you are threatened by wildfire
When conditions are ripe for wildfires, government agencies such as the National Weather Service and state forestry departments will often put out watches and warnings to notify people in areas that are vulnerable to wildfires to be aware of threatening conditions and prepare to evacuate if necessary. If you live in an area that is prone to wildfires, you should make a plan for how to get out quickly and safely and share it with your family and colleagues. If an evacuation order is sent out you should be prepared to move quickly to safety carrying necessary documents, medicines, and valuable property with you in a “go bag” which is assembled ahead of time.
Zones 1 and 2 make up the area immediately surrounding structures on your property. These areas must be well irrigated and consideration must be given to the types of plants used, and the clearance between them | Photo by Courtesy CAL FIRE (from Firesafe Landscaping: Defensible Space – This Old House)
What you can do to your gardens and properties to minimize damage from wildfires
Hazards caused by fires continue even after the flames are out. Burned-out trees have lower strength and may be prone to dropping limbs or even falling over, creating hazards to anyone or anything beneath them. Ashes may have toxic components that can be carried long distances by the wind or by vehicle tires. In one of my favorite books, The Control of Nature, author John McPhee discusses the debris flows that can occur out west when heavy rain falls on recent fire scars, leading to the destruction of buildings and blockage or destruction of streets and other infrastructure by mud and boulders.
Smoke Plume from the 2024 Adams Fire, US Government, Commons Wikimedia.
Wildfires are a hazard which can affect any place that has flammable vegetation. Take the time to understand what your risk from wildfire is, plan your landscaping accordingly and you will be better protected from this dangerous natural (and sometimes human-made) disaster!
“Put rocks in the bottom of pots for drainage” is one of the most pervasive gardening myths, because it makes intuitive sense (as discussed in this earlier post). Understanding the science behind capillary barriers (what gardeners call perched water tables) is not only more mentally satisfying than the faulty belief but it can help you avoid other gardening practices and products that inhibit water movement within the soil (see earlier posts here, here, here, and here).
Classic image of capillary barriers forming between two different soil textures. From Hsieh and Gardner, 1959.
Science is not static however, and new research can change our understanding of soil-water relations. A new article was recently published in PLOS One that contradicts the well-established science about layered soils and other media impeding drainage. It’s important to give these contradictory viewpoints careful scrutiny before any deciding whether they represent a true paradigm shift or if they are fundamentally flawed. This article falls into the second category, primarily due to flawed experimental design.
The methods sections of scientific articles are, unfortunately, the least exciting. My approach to reading scientific articles is to read the abstract first and then the conclusion. (Kind of like having dessert before the salad course.) Then I read the introduction and discussion and finally the methods and results. Getting the big picture first prepares me for the nitty gritty details of how the work was done.
Red flags appeared as I read the introductory section: the author compared research on textural barriers between different soil types with their research, which was organic potting media overlaying drainage material. Differences in adjacent soil textures cause perched water tables, or as soil engineers like to call them, capillary barriers. Their presence is a well-established fact. But even given the faulty comparison there is substantial research showing that organic matter can also create capillary breaks with the underlying soil. This means there is a very large body of literature that supports the presence of capillary barriers between soils of different textures and between soils and other materials.
Root ball left in potting media and installed into native landscape soil experiences capillary break, which impairs root establishment into the surrounding soil.Image courtesy of Tammy Stout.
Eventually I got into the methods section. While reading about how to dissect methodology is not the most exciting thing in the world, learning how to do it is exciting – you never know what you will find. Ideally, methodological errors are found during peer review but sadly peer-review is not always of the highest quality. In any case, a careful reading of the methods section of this article generates more questions than it answers.
Without going into excruciating detail about experimental design (there are textbooks written on this), it’s important to understand what you will find in a rigorously designed experiment. There will be an underlying hypothesis (or research question) to explore, a well-designed experimental methodology with sufficient replicates and controls, and appropriate and objective statistical analyses. Paramount to all of this is that variability must be controlled, which means all replicates must be treated identically except for the variables being studied (in this case different drainage materials and their overlying potting media).
Below are many of the problems I identified, along with a brief explanation of why.
“For trials with sand, a piece of fibreglass mesh (16 x 18 mesh count, approximately 1 mm spacing) was placed over the drainage hole to prevent the sand escaping.” Mesh should have been placed in ALL treatments, regardless if they were needed or not. The presence or absence of mesh has now become an uncontrolled variable (an unforced error to use sports terminology).
“For each potting medium, trials were conducted with a layer of each drainage substrate to depths of 30 mm and 60 mm…” Weights of the materials (drainage as well as media) should have been made for each treatment replicate so that there are no differences in material weights among the replicates for each treatment. Each drainage substrate should then be shaken to eliminate any large gaps before adding the media on top. Differences in weights of the drainage material is another uncontrolled variable.
“For each of the three potting media, a baseline moisture level was defined according to the mass of a fixed volume of medium.” These baseline moisture levels need to be reported in the methods. Furthermore, it would have been better to use fully hydrated media, as the wetting time for the different media were likely different.
“Each filled container was irrigated from above using a watering can with a rose until the water level reached the top of the pot.” There should have been a fixed volume of water added to each container. Differences in how much water was added is another uncontrolled variable.
“The containers were monitored until water was no longer visibly draining, which took between one minute and three hours.” The differences in time is another uncontrolled variable, as they do not appear to have been used in any of the data analysis. Among the questions one could ask is if there were differences in drainage times within the same treatment?
“The saturating and draining process caused all the potting media to compact.” Given that the media compacted to different levels, this introduces another uncontrolled variable. A fully hydrated media could have been compacted uniformly. Furthermore, adding water to a dry medium results in an unknown amount of water running down the inside of the plastic containers (which do not bind water), which is a loss unrelated to the experimental goal. Media hydration time varies among media. Unintended water loss is another uncontrolled variable.
“The compacted depth was measured from one representative sample for each medium with each depth of drainage substrate…” This is a fatal methodological flaw. The depth should have been measured for each container and the average should have been calculated statistically. Furthermore, how was the “representative sample” chosen?
I did read the supplemental files (linked at the end of the article) which I hoped would contain much of the missing information I noted above. They did not – and again I had more questions arise than were answered.
Lack of drainage is probably due to multiple capillary barriers created during sod installation.
The article was supposedly about drainage – and I expected there to be data generated that looked at drainage times and water loss. If a fixed volume of water was added to each container of fully hydrated substrate, it would have been easy to measure water loss over time. (You can do this at home using university extension information, such as this website from Iowa State University). Instead, the article focuses on water holding capacity -which really doesn’t look at whether or not a perched water table exists -and on developing models.
If you’ve made it this far through my post, congratulations! Here are the takeaways:
There were numerous design flaws and methodological errors, which introduced uncontrolled variables and created high levels of uncertainty. The data cannot be analyzed statistically and any discussion of the results is pointless.
This is a good example of insufficient peer review. Had this been submitted to discipline-specific journal (like a journal in the soil or compost sciences), many of the problems I found would have been flagged. But the journal is a general interest journal, meaning that peer reviewers may have had little to no expertise with the science.
The cautionary tale here is don’t be a victim of SSS – single study syndrome. One contradictory article does not dismiss decades of peer-reviewed research and publications unless it meets a very high bar, which this one does not.
Tried but not true. Rocks don’t help pot drainage. Original photo source unknown.
