IJ Archive

On a warm, dry summer day, the leaves on some of my plants had a sprinkling of light dots. Some were turning an unhealthy, ugly bronze color. The underside of the leaves revealed webbing, indicating that spider mites had infested the plants. Among the most common pests in the garden, spider mites can attack more than 200 plant species. Luckily, there are easy steps to take to control them.

Spider mites are not insects, but members of the arachnid class, along with spiders and ticks. They have eight legs and two body parts (insects have six legs and three body parts). 

Spider mites make fine webbing on the underside of leaves, distinguishing them from all other types of mites and small insects such as aphids and thrips, which can also infest leaf undersides. They spin webbing to protect themselves from natural enemies and the weather. Then they suck cell contents from leaves, damaging plants including roses, fruit trees, vegetables, berries, vines, azaleas, dahlias, tomatoes, houseplants and ornamental plants.

These pests increase during the dry months of June to September. They prefer hot, dusty conditions and are usually first found on trees or plants adjacent to dusty roadways or garden edges. Plants suffering from a lack of water are more vulnerable. 

To the naked eye, spider mites look like slight moving dots. I held a piece of white paper below an affected plant and shook a few leaves so some spider mites would drop off. They scooted around rapidly. They are tiny – less than 1/20” long. 

Spider mites live in colonies; a single colony may contain hundreds of pests. Populations can multiply to huge numbers quickly; a generation can be completed in less than a week under the right conditions: hot weather, low humidity, and dust and dirt on leaf surfaces. 

While a small number of mites isn’t usually a problem, high populations can damage plants, especially herbaceous ones. At first, the damage shows up as a stippling of light dots. As their feeding continues, the leaves turn yellowish or reddish and drop off. Damage may increase if plants are under water stress. 

On ornamentals, spider mites may even kill plants when their populations become very high. Loss of leaves on fruit trees may impact next year’s crop. For annual vegetable crops, the loss of leaves can significantly affect yield and lead to sunburning. Spider mites can cause direct damage to sugar peas and beans, where pods are attacked.

To control spider mites, start by keeping your plants adequately irrigated. Water-stressed plants are most likely to become good homes for spider mites. 

Spider mites have many natural enemies that limit their numbers in many landscapes and gardens, especially when undisturbed by pesticide sprays. One of the most helpful is predatory mites. Other insects are also good predators: predatory thrips, lacewings, and minute pirate bugs. 

Cultural practices can also significantly impact spider mites. Dusty conditions often lead to outbreaks. Apply water to pathways and other dusty areas at regular intervals. Midseason washing of trees and vines with water to remove dust may also help prevent serious late-season mite infestations. Forceful spraying of plants with water should reduce spider mite numbers adequately, especially on the underside of leaves. Remove fallen leaves, branches, fruit, and weeds that may serve as a host for spider mites. 

If more control is necessary, use an insecticidal soap or oil in your spray, such as neem, canola, or cottonseed oils. Be sure to test the product on one or two plants in advance to ensure it won’t damage them. If you have a spider mite infestation now, prune out damaged portions of plants and those with significant webbing. 

As the seasons change, with cooler temperatures and rain, spider mite populations should decline rapidly – until next summer. Then, take preventive measures early to keep spider mites at a manageable level.

By Julie McMillan, June 21, 2025

Image

Leaf miners are just one of the many pests that affect our gardens. The two leaf miner types most often found in Marin are the Liriomyza spp, a mostly black & yellow fly with clear wings, and Phytomyza, a blackish gray fly on top with a cream- colored abdomen. Females of both these genera insert tiny eggs into the leaves of numerous vegetables and ornamentals so that their larvae, when they hatch, can feed within the leaf tissue. The larvae use their mouthparts to rupture plant cells, weaving long serpentine trails called mines through the leaf. Adult female flies also damage plants by piercing into leaves with their ovipositor and sucking the sap. 

Identifying signs that the pest you are dealing with is a leaf miner are the presence of larval mines and, when populations are high, stippling due to females puncturing the leaves to feed and lay eggs. These mines appear as white, gray, or paler in color wavy lines, tunnels, trails, or blotches just under the surface of leaves and soft stems. In most cases, however, leaf miners cause little damage, and control is not necessary. Injuries are mainly cosmetic, so treatment decisions are based on the plant's appearance.

Flies emerge in spring, usually in late April or early May. The females lay their eggs typically around midday. Larvae are active for about two to three weeks. They drop to the ground next to plants to transform into pupae, surviving over the winter as pupae. Many generations can occur each year as the entire life cycle can be completed in three weeks when the weather is warm.

Leaf miners are most often found on seedlings or transplants. Inspect new plants and seedlings for signs of leaf mines before planting. And then, only plant in areas of your garden where you have not had an infestation. If you have had an infestation of leaf miners, prune and destroy any affected leaves on infected plants. Remove old plants at the end of the growing season.  

Avoid pesticide use, as it suppresses parasitoid activity. Almost always, outbreaks are associated with pesticide use, as insecticide applications differentially destroy natural enemies. If the number of leaf mines increases, Spinosad can be an effective option if applied to leaves at times that coincide with periods when egg laying occurs, usually shortly after leaves have expanded in spring. 

Notably, leaf miners are resistant to many insecticides and difficult to spray directly. It is advisable to keep your garden area free of weeds such as chickweed, lamb's quarters, nightshade, and plantain, as this reduces their availability as a leaf miner food source. Even though the mines that leaf miners leave on plants make them easy to identify as the culprit, it is critical that gardeners remember to correctly identify the insect causing the damage before applying any insecticide.

Leaf miners have many natural enemies in California, including parasitic wasps, predatory bugs, spiders, earwigs, and ants. Two of their natural enemies, parasitic wasps and hummingbirds, are particularly good at reducing the population of leaf miners. Other parasites attack leaf miners, but their feeding habits protect them from most predators. Beneficial nematodes attack the pupae of leaf miners in the soil. Birds and insects feed on them.

Plant plants that attract natural enemies or predators of leaf miners, e.g., insectary plants, which increase the abundance and effectiveness of natural enemies that help suppress pest populations. Insectary plants, e.g., fennel, buckwheat, and marigolds, can host alternate prey that will feed the natural enemies and keep them abundant locally. Avoid the use of pyrethroids to disrupt leaf miner biological control.

Liriomyza spp., commonly known as the American serpentine leaf miner, has a wide range of host plants. The measures cited above are a good place to start to prevent or address evidence of leaf miner activity in your garden.  

By Martha Proctor, June 14, 2025

Apple trees are some of the loveliest in the world, as are their close relatives, the crabapples. Their blossoms in spring are intoxicating, and no wonder—they belong to the Rosaceae family, which includes roses and strawberries.

Pollinators like bees and bumblebees are drawn to the fragrant blooms. If a blossom is pollinated, a small apple will begin to form in late spring.

Since most apple trees require cross-pollination, a crabapple tree, being botanically similar, can pollinate an apple tree—and vice versa—if they are planted within 100 feet of each other.

You might be surprised to learn, as I was a few years ago, that Northern California is home to a beautiful native crabapple.

The Pacific Crabapple

The Pacific crabapple (Malus fusca), native to the Bay Area up to Alaska, is particularly easy to grow. It is more resistant to disease and tolerant of heavy clay soil. In my garden, I've found it can even thrive in standing water during winter and endure dry conditions in the summer. This shrub can grow up to 30 feet high and 20 feet wide, but can be pruned to a smaller size. Deer might nibble the leaves, and birds enjoy the small fruit, which is about the size of a cherry. Though crabapples are sour, they are edible straight off the tree. With their high pectin content, they make a delicious jelly. Like apples, they are rich in fiber, vitamins, and antioxidants.

Apple Varieties for Marin

Choosing the best apple trees to grow requires some planning. The climate and microclimate must be considered. Apple trees need at least six to eight hours of sunlight, but they also require "chill hours"—a period when temperatures range from 32°F to 45°F. Trees that need a low to medium number of chill hours grow best in Marin.

A good starting apple to grow here is the Gravenstein. This heirloom variety was once the most popular fruit grown in California before grapes took over. Still widely grown in Sonoma, it's known for its satisfying crunch and flavor and makes excellent pies.

Other varieties that thrive in Marin include Anna, Fuji, Gala, Granny Smith, and Jonagold. Be sure to check your local nursery for information on chill hours and whether the variety is suitable for your specific microclimate.

You might want to consider a multi-graft variety if you have a small space. One tree in my garden has three different types of apples. This tree, now over 30 years old, is incredibly productive. Multi-graft trees are more expensive but work well for cross-pollination in smaller gardens.

Growing a Little Apple Tree

Emily Dickinson once said that heaven was an apple just out of reach. Nowadays, it's possible to grow apples within easy reach. Pruning techniques help keep trees small, and you can buy trees grafted onto dwarf rootstock (8–10 feet tall) or semi-dwarf rootstock (10–20 feet tall). A dwarf tree typically takes only two to three years to bear fruit.

Planting an apple tree

In our Mediterranean climate, an apple tree can be planted in the fall, winter, or early spring. Fall planting gets the roots established early. Winter planting means the tree is dormant, and you can find deciduous, bare-root trees that are less expensive and easy to plant. Just remember not to plant in soggy soil. If planting in spring, make sure to plant before the last frost.

Apples are easy to grow, but there are some diseases and pests. Many of these can be avoided by pruning the tree so that it gets good airflow. Another tip is to thin your apples on the branch to one every six inches. This will lessen the chance of coddling moths and give you fewer but larger apples to enjoy.

Visit the UC Marin Master Gardener website for more information on planting, pruning, and maintaining apple trees. 

By Pamela Noensie, June 7, 2025

Master Gardeners enthusiastically support finding homes for succulents in your home garden. Once reserved for arid climates, succulents have proven resilient and adaptable. Including succulents in your water-efficient garden makes creating a dynamic, biodiverse, and pollinator-friendly garden easier. The Falkirk Demonstration Gardens include a stunning Succulent Garden showcasing many species and their uses. The garden also features bromeliads, a sub-category of succulents.

Most succulents originate from low-rainfall regions, making them ideal for Marin gardens. Their ability to store water in their thick, fleshy leaves, stems, or roots allows them to survive prolonged droughts, making them perfect for water-wise gardening in California’s Mediterranean climate.

Designing with succulents follows the same principles as planning any garden space. Consider soil, irrigation, lighting, and thoughtful design.

Soil  While succulents are highly tolerant of poor soils, they love a nurturing home like any plant.  Much of our local soil is clay, and succulents do not like to sit in clay.  You can amend your soil with organic matter or plant in raised beds and pots.

Watering Succulents can survive long periods without irrigation, they appreciate occasional deep watering for a lush look. Water deeply but infrequently, allowing the soil to dry between waterings. When integrating succulents with other plants, ensure their neighbors have similar water needs. 

Light Needs Like all plants, succulents have specific light requirements. Aloes and many crassulas thrive in full sun, but too much intense sunlight can burn them. Shady spots suit crassula multicava, haworthias, and gasterias, but not most other succulents.

Design Tips  As you plan succulent placement, ask yourself about colors, size, textures, and contrasts.  You would not want to put a spiky succulent next to where you sit or walk.  The variety of colors in succulents is abundant, so have fun with them. Scale is important.  If you are planting in a landscape, consider larger varieties in the back, medium-sized plants in the middle, and maybe a creeper in the front.  In pots, thrillers (larger or taller plants) go in the middle, chillers (medium-sized plants) go around them, and spillers (hanging plants) go in front so they can drape over.  Smooth or spiky leaves may like a more textured plant next to them to help show them off.

Maintenance Succulents are low-maintenance, but a few tasks help them thrive. The UC IPM suggests removing dead leaves to prevent pests and rotating potted succulents for even growth. If a succulent becomes leggy, prune it back to encourage fresh growth. The best time to trim your succulents to keep a more compact shape is in early spring, so they can quickly restore themselves during summer’s rapid growth.  You can also take a cutting from the stems and create new plants by re-rooting them. 

Join Us for the Succulent Sale! The Marin Master Gardeners are hosting their annual succulent sale on Saturday, June 14, from 9 a.m. to 12 p.m. at the Falkirk Cultural Center’s rear parking lot on Mission Street in San Rafael. There will be individual plants, potted arrangements, and bromeliads for sale. Experienced Master Gardeners will be on hand to answer your questions. https://www.youtube.com/watch?v=c5uRa4Shya0&t=2s

By incorporating succulents into your garden, you conserve water and create a beautiful, resilient landscape. Whether you're a novice or an experienced gardener, the sale offers a chance to find unique varieties and gain expert advice. We hope to see you there!

By Dot Zanotti Ingels, May 24, 2025

Image

Marin no longer has a “fire season.” Like many other areas, our fire season is now year-round due to climate change and other factors. 

Southern California’s recent horrific fires underscore the urgent need to make our gardens ready for wildfire. “Create passive protection by careful garden design, and actively maintain your landscape. These strategies will make your property fire adapted, helping your home survive a wildfire,” recommends Kathleen Cutter, UC Marin Master Gardener co-lead of the Fire-smart Landscaping Team and Marin Wildfire’s Senior Wildfire Mitigation Specialist.

Passive protection means using thoughtful design. Start with the three defensible space zones and work out from your house (and other structures, including attached decks): 

• Zone 0, 0’-5’ from your house: Your goal is no combustibles, which is best practice because “60-90% of homes ignite from embers,” says Ms. Cutter. Removing organic materials, or combustibles, on the ground in the first 5’ perimeter around your home “eliminates the receptive ember bed, increasing the chance your house will survive a wildfire.” (Your roof and gutter must also be free of leaves and other debris.) Use inorganic mulch (rock, gravel, pebbles); eliminate all plants; remove all other combustibles (e.g., firewood, garbage bins, wicker furniture, cushions, propane tanks). Trim tree limbs 10’ from your chimney and 5’ from your roof.
• Zone 1, 5’-30’ from your house: Your goal is to keep fire from spreading from plant to plant. Group plants in islands separated by fuel breaks such as hardscape, dirt, rock swales or seating areas. Trees should be 18’ apart. Create space for firefighters to defend your home.
• Zone 2, 30’ and beyond: Again, group plants in islands. Keep native grasses well hydrated and no higher than 4”. Limb up trees (6’-10’ from the ground, or 1/3 the tree's height). Keep tree canopies 12’ apart to prevent fire from jumping from tree to tree.

Plant choice is also critical. Look for plants with an open structure so embers fall through the canopy to the ground instead of igniting the plant. Densely structured plants (e.g., juniper, Italian cypress) trap embers and contain dead material inside, making them easier to ignite. Choose plants that are low growing and slow spreading for easier maintenance. Good choices are plants with minimal shedding to reduce debris (avoid bamboo and eucalyptus) and those with needles or feather-like leaves. Plants with high moisture content, such as succulents or with wide leaves, are good choices. Marin fire departments recommend avoiding plants with high resin or volatile oils because they may be more likely to ignite. Don’t overcrowd plants; consider a plant’s size at maturity before purchasing.

Active protection means consistently maintaining your plants and landscape. Visualize how a wildfire would travel through your garden, and then take steps to help prevent or slow it:

• Check the condition of your plants: Be sure they are well hydrated, remove all dead plants, and prune out dead materials. Larger plants should be further away from your house; Ms. Cutter recommends that “the closer to the house, the lower the plant.”
• Avoid tightly structured plants which hold debris and dead materials.
• Remove ladder fuels to prevent wildfires from spreading up from the ground into vegetation and then into tree canopies.
• If you have hedges, prune them to open them up and remove dead material inside.

Slopes present unique challenges. Flames travel uphill more quickly, preheating the vegetation above, making it more likely to ignite. More separation is needed between plants on hillsides. You can also install a low wall (stucco, rock) to act as a fuel break and deflect heat.

Finally, consider your privacy without increasing risk. Avoid a continuous line of plants that would act as a fire conveyor. Stagger plants and install noncombustible fence panels (metal, stone, stucco) between plants. Install non-wood fencing in the last 5’ from structures.

Now is the time to create passive and active protection to make your property fire adapted and help your home survive wildfires throughout the entire year. To learn more about Fire-smart Landscaping, please visit https://marinmg.ucanr.edu/BASICS/FIRESMARTLANDSCAPING/.

By Julie McMillan, May 3, 2025

As Eric Carle once wrote, there was a “very hungry caterpillar.” It hatched from an egg and began its search for food. After about two weeks, it became a big, fat caterpillar—two thousand times its original size! Then, it built a small cocoon, stayed inside for another two weeks, and eventually emerged as a beautiful butterfly. Finding caterpillars in your garden is a wonderful way to observe the butterfly lifecycle up close and appreciate this critical stage of metamorphosis—the transformation between egg and chrysalis.

Caterpillars are often mistaken for pests in the garden. While some, like the tomato hornworm, can cause damage, their overall impact on plants is usually minimal, especially in a healthy, balanced ecosystem that has evolved to tolerate them. In fact, caterpillars play a crucial role as an essential food source for birds and a vital part of the food web. This is especially true during the breeding season when adult birds need to meet the demands of hungry chicks.

Caterpillars are picky eaters, so it is important to identify the host plants for specific species. My favorite caterpillars are Monarchs. Their host plant, milkweed, is the only plant where they lay their eggs and the only food their caterpillars will eat.

While eating milkweed, Monarch caterpillars grow 3,000 times their original weight. They go through several instars (growth stages), shedding their skin multiple times. Some caterpillars hide on the undersides of leaves, and others use camouflage to avoid predators. Monarchs and Pipevine Swallowtails absorb toxic compounds called cardenolides from their host plants and store these poisons in their bodies. This makes them distasteful to birds and other predators. 

The U.S. Fish and Wildlife Service (USFWS) has determined that Monarchs are threatened under the Endangered Species Act. That means they are likely to become endangered soon. In California, they are protected by the California Department of Fish and Wildlife, which prohibits raising or handling without a scientific permit. Participate in Community Science by reporting caterpillars to the  Monarch Larva Monitoring Project or https://www.inaturalist.org/.

Use sustainable gardening practices and strategies to support caterpillars and the butterflies and moths they become. Plant California native plants that have evolved alongside local wildlife and provide essential resources caterpillars need. The more native plants in your garden, the more caterpillars you’ll attract.

Provide both native host plants and nectar plants. Different species of butterflies and moths have specific host plant preferences. Here are some common examples:

• Milkweed for Monarchs (Asclepias fascicularis)
• Oak trees for a wide variety of caterpillar species, including the California Sister.
• Herbs, dill, parsley, and fennel for swallowtails
• California pipevine (Aristolochia californica) for the Pipevine Swallowtail

Avoid using chemical pesticides and herbicides, especially neonicotinoids, which can harm not only caterpillars but all insects in your garden. Instead, natural pest control methods, such as Integrated Pest Management (IPM), should be used.

Predators like yellow jackets and paper wasps are carnivorous and prey on caterpillars. To discourage yellow jackets:

• Remove pet food from outside.
• Use traps, and when you find a nest, contact the Mosquito and Vector Control District of Marin and Sonoma County for safe nest removal (a free service).

A significant parasite affecting Monarchs is Ophryocystis elektroscirrha (OE). Infection occurs when a caterpillar ingests OE spores from an infected butterfly on a milkweed leaf. The spores lodge in the caterpillar’s gut, causing weakness, deformities, and death. Non-migrating Monarchs that rely on non-native tropical milkweed (Asclepias curassavica) are particularly vulnerable to OE. Unlike native milkweed species, tropical milkweed does not die back seasonally, allowing OE spores to accumulate over time. Please cut back the foliage on the ground or replace it with the native species. 

By creating a garden that supports caterpillars, you are nurturing the next generation of butterflies and moths and contributing to a healthier, balanced ecosystem for all wildlife.

By Alice Cason, April 19, 2025