What do ice cream, potato chips, Scotch and Spanish Broom, and Tree of Heaven have in common? While they're all tempting to indulge in, less is more. In fact, plants such as Scotch, Spanish Broom, Tree of Heaven, Pampas Grass, Green Fountain Grass, and dozens of plants are all considered invasive plants in California. Simply put, they should not be planted. There are some great alternative plants that are better choices listed at the end of this blog.
Truth be told, I admit to falling madly in love with the Spanish Broom (Spartium junceum) shrubs adorning Highway 18 on my drive from San Bernardino to Lake Arrowhead in early spring 1984 right after my job interview for my current position. Being a “wet behind the ears” recently hatched graduate student from the Midwest I was truly in awe of their lovely yellow blooms and vowed to plant one if I got the chance to move to California. Fortunately, I found out very soon that, while the plantings were made on purpose, they were a mistake and needed to be removed due to their invasive nature.
While they were ‘recruited' from Europe and had what seemed like a perfect resumé (fast growth, lovely yellow flowers, adaptability to poor infertile soil and disease and insect-resistance), they didn't play well with others, a fatal flaw. In California, they were aggressive and crowded out native plantings. Fires only exacerbated the situation. After the 2003 burns, the Spanish Broom populations exploded, obliterating any remaining natives and taking an even larger area hostage. In summer 2010, the San Bernardino National Forest removed the plants in a costly but necessary $500,000 project under a partnership with the American Recovery and Reinvestment Act. Constant monitoring continues in the San Bernardino Mountains and other areas of the state to prevent its reestablishment which is challenging due to its ability to quickly resprout, seed longevity, and effective dispersal. It has definitely earned its ‘noxious weed' label!
This is just one example of the problems posed by invasive plants. In effect, they grow too well! They outcompete desirable plants in our gardens, lawns, and other urban and natural areas for water, nutrients, and space. They also shade sun-requiring plants. Threatened and endangered plant species and other California native plants are particularly vulnerable to their encroachment. (In most cases, invasive plants are non-native species.) Interestingly, our beloved state flower, the California poppy, is an invasive plant in New Zealand, Hawaii and other locations outside of California.
As urban gardeners, we can all greatly reduce the impact of the encroachment of invasive plants in our urban environments. Please don't plant invasive sane remove plantings on your property to stop their spread. Below are some great resources to learn more about invasive plants and find viable replacements:
California Invasive Plant Council: https://www.cal-ipc.org/
Don't Plant a Pest: https://www.cal-ipc.org/solutions/prevention/landscaping/dpp/
Invasive Plants of Southern California:https://www.cal-ipc.org/solutions/prevention/landscaping/dpp/?region=socal
Watch out for these insects! Invasive shot hole borers (ISHB) represent two related species of beetles (polyphagous and Kuroshio) in the genus Euwallacea. Both spread fusarium dieback, a disease that restricts the flow of water and nutrients in the tree, resulting in dead branches, dropped limbs, and even death. Over 60 species of native and non-native ornamental trees and avocados in Southern California are susceptible the ISHB/fusarium dieback complex.Examples of known hosts of the ISHB/fusarium dieback complex include: Box Elder (Acer negundo), Avocado (Persea americana), English Oak (Quercus robur), Valley Oak (Quercus lobata), California coast live oak (Quercus agrifolia), Big leaf maple (Acer macrophyhllum) silk tree (Albizia julibrissin) Liquidambar (Liquidambar styraciflua), Coral tree (Erythrina coralladendron), California sycamore (Platanus racemose), Blue Palo Verde (Cercidium floridum), Purple orchid tree (Bauhinia variegate), Kurrajong (Brachychiton populneus); and many species of Acacia.
The beetles are native to Southeast Asia and were likely introduced into California in shipped goods, wood products, or packaging. While tiny (about the size of a sesame seed), they are prolific, tunneling into host trees and living and reproducing in galleries while feasting on the disease-causing fungus they spread from tree to tree. Females are black and much more common than the small wingless brown males, which are rarely found.
While the initial infestation occurred in Los Angeles County in 2003, beetles travel about 12 miles a year and have now spread fusarium dieback into Orange, San Diego, Riverside, San Bernardino, Ventura, and Santa Barbara Counties. Orange County alone has spent millions of dollars removing infested trees and managing the spread of the ISHB/fusarium dieback complex.
Identifying ISHB in a timely way is essential to reduce damage and slow its spread. Look for round borer entry holes about the size of the tip of a medium ballpoint pen. Staining, gumming, discoloration of wood beneath the bark, and/or frass (sawdust-like material) are other common signs and symptoms. There may also be white powdery exudate around beetle entry holes. Keep in mind that there are many other disorders that have similar symptoms. For example, other fungal diseases produce exudate and staining and other types of borers leave entry holes of various shapes and sizes.
To date, no effective control measures have been found once trees are infested, although entomologists and plant pathologists from the University of California, The USDA Forest Service and other agencies continue to research viable integrated pest management options including biological control.
To prevent and/or reduce spread of ISHB/fusarium dieback:
- Bring only ISHB-free greenwaste (used for mulch and soil amendments) and firewood onsite. Both products can harbor ISHB which can persist for months.
- Treat infected wood onsite whenever possible. Wrap or completely cover wood that cannot be immediately treated and wood that is moved offsite for treatment elsewhere.
- Chipping infested wood to 1 inch or less can kill 95% of the beetles, while solarizing it using a clear tarp eliminates both ISHB and spores produced by the fungus. Logs can also be solarized. Composting is another option that, when done correctly, can kill both beetles and fungal spores.
- Untreated logs, branches, or woodchips infested with ISHB should not be used for firewood or mulch.
- Remove stumps as well as dead trees.
- Follow International Society of Arboriculture pruning standards and never top or flush cut trees which leave open wounds.
- Spray a 70% ethanol solution on equipment and tools since fusarium fungi can adhere to both.
- Keep trees healthy (water mature trees deeply and infrequently, avoid over or under-fertilization, etc.).
- Monitor susceptible tree species often to identify damage as early as possible. Useful detection and reporting tools and more detailed information on the ISHB/fusarium dieback complex can be found here: ucanr.edu/sites/pshb
For more detailed information on the ISHB/fusarium dieback complex, please visit these websites: https://ucanr.edu/sites/pshb/ https://www2.ipm.ucanr.edu/agriculture/avocado/polyphagous-shot-hole-borer-and-kuroshio-shot-hole-borer/ https://cisr.ucr.edu/invasive-species/polyphagous-shot-hole-borer https://californiareleaf.org/pests/
Most trees in California need supplemental irrigation above and beyond what Mother Nature supplies naturally. Even drought-resistant species need regular watering through their first growing season due to their shallow roots. Once trees become established, it's important to water less often but more deeply to encourage deep rooting and structural balance above and below ground. Both under and overwatering can lead to unhealthy trees and even death if the situation is not corrected. Trees receiving too little or too much water exhibit similar symptoms since, in both cases, water is not available to the plant. Trees initially wilt, grow slowly, and develop yellow leaves. Over time, growth stops and leaves become brown and drop. Overwatered trees often develop lower crown and root rot from one or more disease-forming pathogens.
Knowing what type of soil you have (soil texture) is as important as knowing the water needs of your trees. Use the ‘feel test' (pictured below) to find out how much water your soil holds and how often to water. Heavier clay-based soils hold water longer and drain more slowly than sandier soils that need to be watered more often for shorter periods of time.
Trees should not be watered on the same irrigation system used for lawns and groundcovers. Soaker hoses and drip systems allow trees to be watered less often but for longer periods of time than your lawn or groundcover. Avoid applying water too close to the trunk. Instead water half-way between the trunk and the dripline of the tree and outward. If you use a garden hose, apply the water on the lowest volume possible slowly, moving the hose every few hours to each of four quadrants around the tree.
Applying a 3-4 inch layer of mulch around the tree can reduce soil evaporation. Use only non-flammable mulches in fire-prone areas within five feet from the house and non-contiguous for the first 30' away from the house. In all cases keep mulch a few inches away from tree trunks to keep the trunks dry.
Tip: Before planting a tree, make sure there is adequate drainage. Dig a hole where you want to plant it (the same depth of the pot, which is about one foot) and fill the hole with water. Let it completely drain and refill it. Measure the time it takes to drain one inch using a ruler. If it does not drain more than one inch an hour it is not a good location for your tree. Avoid adding compost or soil amendments to try to correct the problem since tree roots will likely grow in circles, staying within the confines of the amended hole rather than growing outward the confines of the amended hole rather than growing outward.
Unlock the Door to One of the Best Kept Secrets in Southern California!
Did you know that University of California Cooperative Extension has trained Master Gardener volunteers ready to answer your home gardening and landscaping questions throughout the greater Los Angeles area?
Email the Master Gardener helpline in the county in which you reside for the most accurate information since climates and conditions vary across the southland:
Los Angeles County: Email: firstname.lastname@example.org
San Bernardino County: Email: email@example.com
Riverside County: Email: firstname.lastname@example.org
Orange County: Email: email@example.com
Visit http://mg.ucanr.edu for links to other Master Gardener helplines in over 50 California counties!