I recently wrote about Cotoneaster lacteus. It’s a gorgeous woody perennial reaching up to 12 feet in height with an arching, fountainesque habit of growth. It is covered with white flowers in the spring that give way to generous clusters of red fruit that attract birds and other wildlife. I have never seen it visited by pests or show any signs of disease. It needs no fertilizer and, once established, no water other than winter rain. It even received the prestigious Award of Garden Merit by Great Britain’s Royal Horticultural Society.

Unfortunately, Cotoneaster lacteus has been recognized as an invasive species that threatens California’s native plant communities. Dr. J. Mark Sugars from Irvine wrote me as follows: “Cotoneaster species are extremely invasive in California. One whole chapter of ‘Invasive Plants of California Wildlands’ (University of California Press, 2000), by Carla C. Bossard (et al.), is devoted to Cotoneaster.” And Mona Erhardt, who gardens in Oak View and in Ojai, cautioned me with these words: “Cotoneaster is a horribly invasive plant that has the power to destroy native plants and everything in its path. It should never be recommended for anything but eradication, right up there with arundo and tree of heaven. I’m locked in mortal combat with the stuff, as I am with mallow and filaree.”

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I thought it would be illuminating to examine the plants mentioned by Ms. Erhardt since, whether or not you know their names, you have probably encountered them. They are botanical nuisances, yet it is easy to be seduced by their charms even while, before you know it, they have gotten out of control and taken over your yard and garden.

The arundo to which she refers is Arundo donax. It is commonly known as giant reed, giant cane, or elephant grass. It was imported from its Mediterranean habitat to Los Angeles in the 1820s as a roofing material and for erosion control. It also has ornamental qualities. The first time I saw it I thought, “What an attractive leafy bamboo.” There are even variegated types with green and white or creamy yellow and white stripes. Like bamboo, arundo is a member of the grass family and an extremely rapid grower. It may put on four inches of new growth in a single day and quickly reaches its mature height of 20-30 feet. It seldom produces seeds and, when it does, they are nearly always sterile. The problem with arundo is its rhizomes, since they can burrow to a depth of three feet, leading to explosive vegetative growth. Flooding is its friend since rhizome pieces that break off are carried away in the water and then sprout stems and leaves in locations distant from the mother plant. Despite its invasiveness, it is raised as a cheap biofuel in some countries. It has also achieved notoriety due to its stems, which are used for making reeds for woodwind instruments.

Some gardeners express surprise at tree of heaven’s name, cited by Ms. Erhardt as one of her chief combatants, since it possesses unmistakably hellish qualities. It is a highly invasive plant, flourishing in wet and dry climates alike. You chop it down to its stump and it promptly suckers up again. These suckers will grow up to ten feet tall in a single season. Unless you pull a tree of heaven seedling before it reaches three months of age, it must be utterly deracinated to keep it from coming back since the smallest piece of taproot will sprout a shoot that will develop into a full-fledged tree. Still, if you assiduously chop away at your tree of heaven and dig up all adventitious shoots for a year or two, it will eventually give up the fight and cease its struggle for life. Tree of heaven (Ailanthus altissima) is dioecious, meaning that it has separate male and female trees. The males emit a foul odor.

Especially when young, tree of heaven (Ailanthus altissima) is alluring and a group of its young, robust seedlings take on the appearance of a fern forest. Do not be deceived. Within a few months, these seedlings will become trees which, when mature, will each produce up to 350,000 seeds per year, nearly all of them viable. Tree of heaven seeds are dispersed primarily by the wind, landing as far as a hundred miles away from their mother tree.

Tree of heaven is the arboreal species that provided inspiration for “A Tree Grows in Brooklyn,” a 1943 novel by Betty Smith that became a successful film. It tells the story of a child of immigrant parents who overcomes adversity. “There’s a tree that grows in Brooklyn,” Smith wrote. “Some people call it the tree of heaven. No matter where its seed falls, it makes a tree which struggles to reach the sky…It grows lushly…without sun, water, and seemingly earth.”

It is often said that a weed is a plant for which no useful purpose has yet been found. By this definition, there is probably no such thing as a weed since every plant, once its history is investigated, is revealed to have beneficial qualities. Tree of heaven roots, for example, are said to be used therapeutically in some folk remedies. Tree of heaven is also host to a silkworm. The silk it makes is not as fine as that of the renowned mulberry silkworm, but more durable.

The mallow garden invader to which Ms. Erhardt refers is cheeseweed (Malva parviflora). I can vouch for its speedy spread and was convinced it was all bad until reading “Foraging Southern California” (Adventure Publications, 2020) by Douglas Kent.  Cheeseweed appears on Kent’s list of “top 10 edibles.” The plants on this list were chosen for their widespread availability, from urban to wilderness areas, and for the fact that “nearly every part of these plants is consumable,” including roots, leaves, flowers, and seeds. Cheeseweed (Malva parviflora) gets its name from its fruits which — tasty and thirst-quenching when eaten raw — resemble cheese wheels. Kent also considers cheeseweed a “superfood” due to its high vitamin, mineral, and antioxidant content. Remember care, safety and research are a must for potential foragers: He shows photos of geraniums, which are inedible but are often confused with cheeseweed. For the record, members of the mallow family include hibiscus, hollyhock (Althaea rosea), marshmallow (Althaea officinalis) — from whose roots the eponymous confection was once manufactured, and okra (Abelmoschus esculentus). The mucilaginous sap of these species has long been used for medicinal purposes.

Ms. Erhardt’s last-mentioned weedy foe is filaree, several species of which are locally seen. Filaree is one of the most beautiful weeds, with small, finely cut lacy leaves and delicate pink flowers. Filaree belongs to the geranium family, whose plants are recognizable by their elongated beak-like seed capsules. Geranium is derived from geranos, the Greek word for crane and all the plants in the geranium family are collectively known as cranesbills. The most commonly seen members of the geranium family are of the genus Pelargonium, derived from pelargos, the Greek word for stork. Finally, filaree belongs to the genus Erodium, derived from erodios, the Greek word for heron. Collecting seeds from any geranium species requires placement of a muslin bag over a flower since once the seed capsule forms, its contents are dispersed almost instantly. Having the bag in place just when a flower fades assures that you will catch its seeds.

Upon perusing the list of plants labeled invasive by the Invasive Plant Council (cal-ipc.org), two species stand out for their attractiveness to specific butterfly populations. One of them is blue gum eucalyptus (Eucalyptus globulus). Up and down the California coast, blue gums have long been a primary overwintering site for monarch butterflies. Although these eucalypts pose a threat to California native plants, there is concern that their elimination would leave the monarchs without a secure winter home. The other invasive plant with unique appeal to a certain butterfly is sweet fennel (Foeniculum vulgare), also known as anise fennel on account of its fragrance. In this case, the butterflies involved are black swallowtails, which reliably appear wherever sweet fennel is found, even as the butterflies seem nonplussed by any other flora in the vicinity. Ultimately, a choice between the presence of certain plants and the presence of certain butterflies may have to be made.

Tip of the week: Sometimes there’s a plant whose rampant growth can be a nuisance, but you put up with it because you enjoy its presence just the same. Peruvian lily (Alstroemeria spp.) is an example of such a plant. If you water your garden frequently, Peruvian lily foliage will almost always be visible and some flowers too. Because of its underground rhizomes, it can withstand periods of drought and, once watered, will return to life. Still, if you withhold water long enough, it will eventually die. Peruvian lily flowers most abundantly in fall and spring. It will tend to crowd out neighboring plants but is easily controlled due to its shallow roots. Peruvian lily’s greatest virtue is the longevity of its blooms as cut flowers; they consistently last for two weeks in a vase. Their typically pink flowers combine well with yellow daylilies and blue lilies of the Nile (Agapanthus spp.), although Alsroemerias in yellow, orange, red, and purple are also available. Tiny thread-like insects known as thrips, which may nibble flower petals, are often a problem so make sure you shake them out of flowers you wish to bring inside for vase arrangements.

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