Coming to Grips with Thrips

Happily, several years have passed since I’ve noticed thrips in my garden, but they are back!

Thrips are flying insects that are less than 1/20 of an inch long and eager to suck juices from plants. There are over 500 genera and 5,000 species of thrips, some of which are beneficial predators of mites, scales and even other thrips. Others feed on fungal spores or pollen, and some feed only on favored plants, such as avocados, citrus, blueberries, or various others.

My garden’s visitors almost certainly are Western Garden Thrips (Franklinia occidentalis), which attack a wide range of herbaceous ornamentals, vegetables, fruits and some shrubs and trees, including roses and stone fruits.

Thrips are poor flyers but can find juicy plants and infest all of a large shrub or a good-sized bed of a plant that they find tasty.

In my garden, they found a 200 sq. ft. bed of Bergenias (an unidentified cultivar of the popular species, Bergenia crassifolia). This plant from central Asia has numerous common names: Elephant Ears, Pig Squeak, Large Rockfoil, etc. It is an attractive groundcover plant, growing one-to-two feet high, with clusters of small, bell-shaped pink flowers on stems that rise above the foliage in early spring.

Bergenias are generally free of pests, at least until thrips discover them.

Thrips might make only limited damage to ornamental plants and could be ignored. When conditions are favorable, however, they can cause great cosmetic damage.

When thrips attacked my Bergenia bed, nearly all the leaves, each about four by six inches, became discolored and distorted, and their upper surfaces turned silvery gray, the result of losing their waxy surface. The leaves also were stippled with black specks of the thrips’ fecal matter.

The normal growth of Bergenias has new leaves emerging above older leaves that turn brown without dropping off. New leaves cover the old leaves, but removing the dead leaves improves the plant’s appearance. In this instance, the action of the thrips revealed the dead leaves, and the entire bed was devastated.

The damage was limited to the leaves. The stems and roots of the plants were alive and well, and able to produce new leaves. Our treatment was to remove all living and dead leaves, using garden clippers, and then spray the plants with Spinosad, an organic pesticide that kills insects both by direct contact and by ingestion.

I used a Spinosad-based product, Captain Jack’s Deadbug Brew, in a container designed conveniently as a hose-end sprayer. Despite its promising name, the spray doesn’t guarantee success: a follow-up spray might be needed to control the newest of up to eight generations of thrips for the year.


Thrips are members of the order Thysanoptera. The earliest fossils of thrips date to the Permian Period, 250 to 299 million years ago. This period was the last part of the Paleozoic Era, which ended with the largest mass extinction in Earth’s history, when nearly 90% of marine species and 70% of terrestrial species died out.

That thrips were among the survivors of that period suggests the difficulty of eliminating them from our gardens.

When the gardener suspects that thrips are present, they can be detected either by close examination of the underside of affected leaves, or by beating or shaking a branch or flower onto a sheet of paper. Yellow sticky traps also can be used to monitor their presence.

The most important way to encourage biological control of pest thrips is to conserve naturally occurring populations of beneficial insects, e.g., predatory thrips, minute pirate bugs, predaceous mites, and green lacewings . This can be done by controlling dust and avoiding the use of persistent pesticides in the garden.

For other control, see the University of California’s Integrated Pest Management information page on How to Manage Pests—Thrips.

Gopher Overview

Most of these columns are on current priorities in my own garden. The premise of course is that many gardeners could be dealing with the same issues at any given time.

Recently, when we listened to those people who always seem to know what’s going at, we learned that 2012 is a big year for gophers.

In the past, I have relied on two visiting cats to keep my garden gopher-free. (I don’t know where else they get their food, so they probably are feral felines.) The result of their advancing age plus a peak in the local gopher birth rate is a record number of gopher mounds.

It might be possible to avoid gopher problems entirely by limiting the garden to plants that gophers do not enjoy. This approach limits the gardening experience severely and still might not succeed: hungry gophers don’t always follow the rules.

The first step toward “control” (the polite term) is to confirm that you have gophers, rather than relatively harmless moles. Gophers are herbivores; moles are omnivores, but mostly eat earthworms and insects. Gopher mounds are crescent or horseshoe shaped when viewed from above. The hole, off to one side of the mound, usually is plugged. Mole mounds are more circular and volcano-shaped when viewed from the side.

One gopher can create and abandon several mounds in day, so the gardener’s challenge is to find the gopher’s main burrow, which will be six-to-twelve inches deep and connected to a mound. When you see a fresh-looking mound, poke around with a stick or metal probe until you feel a drop of about two inches, indicating that the probe has entered the burrow. This might be the main burrow, where you should set your traps.

Use a shovel to expose the burrow enough to set your traps. The popular Cinch Trap, available at most garden centers, comes in small, medium and large sizes. Use the size that fits snugly in the burrow you found. Set two traps according to instructions (watch your fingers!) and place them in opposite directions in the burrow, to trap the gopher coming from either direction.

Baiting the traps is optional. Fruits, vegetables or peanut butter are good choices. You could also use toxic baits, but that is personal choice and probably not ecologically wise.

Connect the traps to stakes with baling wire or light chain, for easy removal. Cover the excavation with dirt clods, wood, cardboard or anything else to exclude light.

Check the traps regularly and reset them as needed. If you haven’t caught a gopher in three days, try a different location.

Benjamin Franklin probably wasn’t thinking about garden pests when he said, “Energy and persistence conquer all things,” but that’s good advice for gopher hunters.

Growing Gorgeous Geophytes

The fall season invites gardeners to plant bulbs to blossom in the spring and create bright swaths of color for the new gardening year.

Right now is an excellent time to design bulb bed(s) and select spring bulbs for the garden. There is a lot to consider.

One strategy is to favor the most familiar bulbs, choosing either old favorites or recent introductions. The most popular bulbs include Daffodils (Narcissus), Tulips (Tulipa), Hyacinths (Hyacinthus) and Dutch Crocuses (Crocus vernus). Garden centers offer many varieties of these plants: the long popularity of daffodils and tulips in particular has motivated hybridizers to develop a range of colors and interesting forms.

The next most familiar bulbous plants include Lilies of the Valley (Convallaria, from rhizomes), Spanish Bluebells (Scillas), Grecian Windflowers (Anemones, from tubers), Snowdrops (Galanthus), Dwarf Irises (Iris reticulata, from bulbs), and Grape Hyacinths (Muscari). There are many appealing options within these genera, as well.

Adventuresome gardeners can explore a long list of less familiar bulbs, each of which brings unique characteristics. Visit my website,, for links to additional options.

A different group of geophytes are summer-bloomers. This group includes Gladioli, Calla Lilies, Dahlias, Tuberous Begonias, and Crocosmias. They are planted in the early spring about the same time we plant tomato seedlings.

Other geophytes we enjoy are fall-bloomers, which are planted in the late summer: Autumn Crocus, Winter Daffodil, Guernsey Lily, Saffron Crocus, and some species of Snowdrops.

With planning, you could enjoy glamorous geophytes during much of the gardening year.

Some spring-blooming bulbs need a chilling period to bloom their best. Winter in the Monterey Bay area rarely provides a chill that is long enough and cold enough for these plants, so schedule six weeks of cold storage. The kitchen refrigerator will suffice except for larger projects, when gardeners will appreciate the luxury of a second refrigerator. Consider organizing a chilling co-op with gardening friends.

Many mail order bulb sellers offer pre-chilled bulbs to be shipped at the right time for local planting. A welcome service!

Here are the basics of planting bulbs. Choose a site that receives all-day sun, and drains well. Select larger bulbs of the preferred genus. Plant the bulbs at a depth that is about three times the bulb’s diameter, and take care to position them with the pointed end up.

Bulbs can be planted very close together and may be arranged in either formal or informal patterns. Fertilizers are not required, but a small amount of bone meal in the planting hole could help. For clay soil, add compost to improve drainage. Water to settle the soil then let the seasonal rains take over.

Prepare now for a spectacular spring.


Information About Uncommon Geophytes

North Carolina State University

The Plant Expert

Pacific Bulb Society Wiki

Mail-order Suppliers of Uncommon Geophytes

Brent and Beck’s Bulbs

Odyssey Bulbs

Telos Rare Bulbs

African Bulbs

The Bulb Man

Mulch for Much Easier Gardening

Mulching represents a gardener’s not-so-secret strategy for achieving the universal objective: a low-maintenance garden.

If your garden is time-consuming, frustrating and disappointing, it probably needs mulching.

Several other possible causes should be considered: nutrient-poor soil, lack of irrigation, inappropriate plant selection, etc., but let’s focus for the moment on mulching.

A three-inch deep layer of an organic mulch between plants discourages weed growth, conserves water and, in time, breaks down to add nutrients and texture to garden soil.

We should not ignore the downsides of mulching. For example, a larger garden will require several cubic yards of organic mulch, leading to significant charges from a landscape supply company for the material and its delivery. In addition, distributing the mulch within the garden, while not difficult, consumes time and energy.

One more downside: organic mulches decompose in time, so the time and expense will have to be incurred again. Finer mulch materials break down faster than coarse materials.

Some organic mulch is free, e.g., disease-free wood chips that tree services will dump on your property, upon request (to avoid landfill charges), or seaweed harvested during low tide. Wood chips might be regarded as less attractive than finer mulches, and seaweed, although excellent in nutrient content, often is troublesome to gather, buddy and smelly.

A landscape supply company will offer a variety of organic mulches in bulk, e.g., redwood sawdust, shredded cedar, fir bark and gorilla hair (shredded redwood bark, really), in a range of sizes and prices.

Another option is cocoa shell mulch, which is available in bags of two cubic feet for about $5.00. This material looks good, emits a nice fragrance and breaks down slowly, but contains theobromine, a natural compound that if eaten is highly toxic to dogs and cats.

The gardener could consider each of the available options, and perhaps try a sample bag in the garden.

Home supply stores offer organic mulches in bags of two cubic feet of material. These can be appropriate for smaller beds, but for larger projects will be more expensive than bulk materials. To move one or two yards of mulch, it’s good to have a friend with a pickup truck.

Gravel mulch might be preferred for a rock garden or succulent garden. For a rock garden in particular, gravel mulch simulates scree, the debris of broken rock seen on mountain slopes.

Like organic mulches, gravel mulch discourages weeds, conserves moisture and looks better than bare soil.  Gravel is much more expensive per cubic yard than organic mulches, but a one-inch layer should be enough and gravel doesn’t decompose. Gravel mulch is difficult to remove, so for practical purposes treat the installation as permanent.

Enjoy your mulched garden.


Estimating Mulch Needs

To decide how much material you will need, estimate the surface area of the garden bed(s) to be mulched: length times width (in feet). Multiple by the intended depth of the mulch layer: for a three-inch layer, multiply by .25. Finally, convert cubic feet to cubic yards by dividing by 27.

Example: a 12 feet x 18 feet garden bed has a surface area of 216 square feet. For a three-inch layer of mulch, .25 x 216 yields 54 cubic feet. Divide by 27 to determine a need for two yards of mulch.

Seaweed Mulch

An interesting article, How to Use Seaweed to Mulch Your Garden, is available on the website for Eartheasy: Solutions for Sustainable Living. Gardeners who live close to the beach might wish to try this mulch material, but it seems too smelly and buggy for my taste. It also decomposes into slime rather quickly.