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What you should know about indicator plants in your garden

June 17, 2023

Just the other day, at the base of the trunk of my neighbor’s eucalyptus tree, I spotted a laurel sumac (Malosma laurina) seedling that had only recently sprouted. I have frequently seen volunteer seedlings of Mexican fan palm, Shamel ash, mulberry, Brazilian pepper, and fig trees in my neighborhood, but never a laurel sumac.

Such seedlings, known as volunteers, generally sprout from seeds that have been consumed and then passed through the gut of birds or other animals before being excreted. The stomach acid of these animals assists in dissolving some of a seed’s protective covering, known as the seed coat, creating an aperture wide enough to allow a radicle or baby root from the plant’s embryo to poke through, followed by the emergence of the first leaf, after the excreted seed has been hydrated as the result or rain or irrigation. It’s also possible, of course, that a volunteer seedling results when the wind blows a seed from a nearby tree into the garden.

Years ago, I learned that laurel sumac, a California native, is an indicator plant where avocado trees are concerned. In other words, wherever you see laurel sumac growing, you can plant avocado trees with confidence that they will thrive in that environment. Since laurel sumac is frost sensitive, you can assume that frost is not an issue where it grows, an important factor in deciding where to plant avocado trees, since they are frost sensitive, too. Early growers of orange trees in Southern California would also select sites for planting orange trees, which are also frost sensitive, based on laurel sumac’s presence. You can view laurel sumac growing all along Sepulveda Boulevard between Sherman Oaks and West Los Angeles.

Interestingly enough, laurel sumac, like avocado trees, grows especially well on slopes. There are two reasons for this. First, both plants require excellent soil drainage. Avocado trees are plagued by Phytophthora root rot where soil drainage is impaired which explains why large acreages of them, whether in Santa Paula or in Temecula and Fallbrook, are grown on steep slopes. Second, even in areas with occasional frosts, planting on slopes is still an option where frost-sensitive plants are concerned since frost, like water, rolls downhill and settles there. After a frosty night, plants at the base of a slope may show cold damage while plants growing upon that same slope remain healthy. Even in a garden, following a freeze, plants in low spots may display frostbitten, blackened leaves while plants only a foot or two away, but at a slightly higher elevation, may be fine.

Indicator plants serve a variety of purposes. Dandelions and spotted spurge (Euphorbia maculata) thrive in compacted soil so where you see them you know that soil improvement is necessary, while wild mustard grows in sandy soil and is an indicator of a good area for planting California natives. Clover (Trifolium repens) is a sign of a nitrogen deficient soil (or lawn) while two weed species – chickweed (Stellaria media) and lamb’s quarters (Chenopodium album) – indicate a soil that is nitrogen-rich.

Through trial and error, we can identify our own indicator plants as far as watering is concerned. Especially now that hot weather is just around the corner, we want to minimize irrigation frequency so that our plants acclimate to our soil as it becomes increasingly dry. The longer we can go without watering, the deeper the roots of our plants will grow. I have a variety of flowering annuals and perennials in a bed that receives half-day sun; slightly wilted petunias guide me as to when watering is necessary.

When flowers on apple trees drop off it’s time to plant corn and we are rapidly approaching the deadline for planting it unless we are willing to accept unsightly gray smut fungus bulging out of our ripening ears. When jacarandas are in full bloom, as they are now, it is a sign that spring is ending and summer has arrived. all the more reason to make sure mulch is in place before the assault of sizzling heat.

“Tough Plants for Tough Places: Invincible Plants for Every Situation” (Firefly Books, 2023), by Sharon Amos, provides us with the security of knowing which plants we can rely upon to grow, regardless of sun or shade exposure, soil type, or climate conditions. One of my favorite plants is sea holly (Eryngium bourgatii) and I was delighted to learn that it thrives in extreme conditions of drought, high wind, and sun-baked soil. This species is a study in purple flowers, purple thorns, and purple stems, growing two feet tall and two feet wide.

This book was my portal to the discovery of ivy leaf or hardy cyclamen (Cyclamen hederifolium), a tough ground cover suitable not only for dry shade and every type of soil, but for the coldest winters you can imagine, including those in North Dakota. Of Mediterranean origin, all cyclamen species are spring and summer dormant, with flowers and foliage appearing in late summer or fall. In the manner of the familiar cyclamen planted for winter color, hardy cyclamen’s soil must be kept dry during its dormancy period to protect its tubers from rotting. However, in the case of hardy cyclamen, if you can keep it alive you will be rewarded with new plants sprouting from its seeds.

Select from 12 hardy cyclamen varieties, including one with silver leaves, at plantdelights.com, an online nursery that delivers a cornucopia of uncommon plants to your door. Incidentally, cyclamen is derived from “kuklos,” the Greek word for circle and refers either to the shape of the plant’s tubers or to the earthward, circular movement of its flower stems once petals have wilted. Russian sage (Perovskia atriplicifolia) is another extremely tough species. It thrives in any well-drained soil and is unrivaled in the quantity of lavender blue flowers that it displays and in its status as an insect-attracting pollinator plant. Maintenance is easy; just cut it down to the ground when it finishes flowering in the fall.

California native of the week: Laurel sumac (Malosma laurina) is an evergreen that exhibits vigorous growth throughout the year. After a wildfire, it is the first plant to recover as it sends up new shoots from the remains of its woody base. Laurel sumac is a robust shrub that reaches a height and girth of 20 feet. It makes an outstanding informal hedge. New foliage is strikingly red in color before it turns dark green, but stems, leaf edges and leaf veins remain red. Laurel sumac is sometimes referred to as taco plant because its leaves curve upward from the center like taco shells. This tendency of its leaves to fold up is shared by sugar bush (Rhus ovata), a botanical cousin, and is a strategy employed to prevent water loss. Some people experience an allergic reaction to this plant. While not as notoriously dermatitic as poison oak – a California native in the same plant family – contact with laurel sumac may create a skin rash on some people. Laurel sumac foliage is famous for its fragrance, which has been likened to that of both citrus and apples.

Do you have any plants you use as indicators for performing garden tasks or evaluating soil conditions? If so, please let me know about them. Your questions, comments, and photos are always welcome and should be sent to [email protected].

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What you should know about indicator plants in your garden

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