Invasive Plants of California's Wildland

Yellow bush lupine (Lupinus arboreus) is a bushy shrub to six feet tall, usually with bright yellow (sometimes blue) sweet-smelling flowers and green, sparsely pubescent (appearing glabrous), palmately compound leaves. It occurs as an invasive species in northern California coastal dunes. It appears to be native to California from Sonoma County south, where plants often have a hairy upper leaf surface. Yellow bush lupine is predominantly a dune species, but it can be found along roadsides and in disturbed areas. Flowers appear in May to July in northern California (earlier in the southern part of its range). Yellow bush lupine hybridizes with the native L. littoralis, which is smaller (less than two feet) and more prostrate-decumbent, has purple and white flowers, and always has a hairy upper leaf surface. Intermediates usually are smaller and more prostrate than L. arboreus, with blended yellow, purple, and white flowers (Wear 1998). In the southern portion of its range yellow bush lupine can be easily distinguished from blue bush lupine (L. chamissonis), which can achieve the same height but has silver, densely hairy leaves that appear gray-blue and light-violet to blue flowers.

Yellow bush lupine occurs from the mouth of the Ventura River in California northward to at least Vancouver Island, British Columbia. It is primarily a coastal plant, but has been found inland east of Berkeley, California (Sholars, pers. comm.). Yellow bush lupine is native in the southern part of its current range, although the demarcation between native and naturalized populations is still disputed. Davy (1902) reported its range as Point Reyes south. Hickman (1993) suggests that populations as far north as Bodega Bay are native.

Naturalization of yellow bush lupine and subsequent gene flow within the genus has resulted in taxonomic and range confusion. Under the current classification (Hickman 1993), there are both purple and yellow forms of Lupinus arboreus. For the purposes of identifying yellow bush lupine as an invasive weed in Humboldt County and north, the problem is simplified by the fact that the invasive form has yellow flowers. However, there are intermediate hybrids between the yellow-flowered form and the purple-flowered dune native L. littoralis (found from Sonoma County to British Columbia). Where L. arboreus occurs off dunes (e.g., along roadsides), it also hybridizes with the purple-flowered L. rivularis (found from Mendocino County to British Columbia). Yellow bush lupine is not yet present on the Ten Mile Dunes at MacKerricher State Park in Mendocino County. However, a blue-flowered, woody species identified by by Teresa Sholars as L. arboreus is common at Manchester Dunes in Mendocino County. These dunes have already been extensively invaded by Ammophila arenaria, and it is not clear whether yellow bush lupine is increasing at this site.

Yellow bush lupine is native to southern and central California. It was introduced repeatedly to many dune systems as a sand stabilizer during the early to mid-1900s. The introduction of yellow bush lupine to the Humboldt Bay dune system was traced by Miller (1988). In 1908 the operator of a fog signal station on the north spit of Humboldt Bay gathered seeds of yellow bush lupine from the Presidio (where it had previously been introduced) and planted them around the station. In 1917 seeds from the new signal station population were collected and scattered beside railroad tracks along the spit. From these and later plantings, the extent of yellow bush lupine has increased from 244 acres (98 ha) in 1939 to over 1,000 acres (400 ha) (Pickart and Sawyer 1998). Yellow bush lupine now dominates 28 percent of the total vegetation cover on Humboldt Bay dunes (Pickart and Sawyer 1998).

As do other members of the genus, Lupinus arboreus has relatively large seeds with corresponding high seedling survival. Once a population becomes established, it spreads short distances by rodents or by seeds rolling from parent plants down dune slopes.

Yellow bush lupine invades coastal dunes in northern California, where no other large, shrubby, native lupines are found (although Lupinus littoralis is sometimes classified as a subshrub). The seeds of yellow bush lupine are long-lived and form a persistent seedbank, creating the need for repeated removal. However, more serious problems are caused by yellow bush lupine’s ability to cause ecosystem-level changes. As a nitrogen-fixer, bush lupine readily colonizes the open, mat-like vegetation of northern California dunes (known as dune mat, or the Sand-Verbena/Beach Bursage series). Once the lupine has been present for more than a few years, it causes elevated nitrogen levels that facilitate invasion by non-native weedy grasses (Pickart et al. 1998).

Maron and Connors (1996) also observed this phenomenon on coastal prairies at Bodega Bay, where yellow bush lupine is believed by some botanists to be native (there is a relict native dune scrub community containing Lupinus chamissonis at Bodega Bay). Eventually, desirable native species in invaded areas are almost entirely displaced by a combination of lupine shrubs, weedy grasses, and/or adventive natives such as Scrophularia californica. This assemblage of species has been labeled the Yellow Bush Lupine series by Sawyer and Keeler-Wolf (1995). Eventually, the Yellow Bush Lupine series may give way to the Coyote Brush series, which otherwise would not occur on northern California dunes. In both of these situations, plant cover is much higher than that of the native dune community, so the dune system becomes overstabilized. As a result, sand is not able to move from the foredunes to the backdunes, and physical processes are disrupted. This can result in elongation of the deflation plain behind the foredunes and/or stabilization of the backdunes. Although dunes are naturally subject to cyclic stabilization and rejuvenation in response to major tectonic events, exotic species such as yellow bush lupine and European beachgrass (Ammophila arenaria) can greatly accelerate stabilization and could conceivably replace dune mat altogether in the post-disturbance stage of the cycle.

Yellow bush lupine reproduces solely by seed. It is self-compatible and is pollinated by bumble bees (Wear, pers. comm.). Flowering takes place from May to July in northern California, and seed dispersal occurs in late summer and fall. Pods open explosively, propelling seeds for short distances. Seeds are characterized by a hard seed coat, typical of the genus. However, seed coat dormancy is overcome in the dune environment, probably by the abrasion caused by sand and wind, and seedlings emerge from November to March (Pickart and Sawyer 1998).

Plants grow rapidly, reaching reproductive maturity in one to two years. Yellow bush lupine plants are short-lived for their size, generally living seven years or less (Davidson and Barbour 1977).

(click on photos to view larger image)

Control of yellow bush lupine has so far been limited to manual and mechanical means. The cyclic die-offs observed in both Humboldt Bay and Bodega Bay populations suggest that biological control may be effective, but more research is needed. Chemical control has not been attempted. The observed effects of wildfires in the dunes indicate that yellow bush lupine can resprout after burning, although a very hot fire might kill these plants. Regardless of the method of removal, additional steps may be needed to restore soils to a condition suitable for native vegetation.

Manual methods: If yellow bush lupine has been present only a short time, and native vegetation is still intact beneath the shrubs, restoration of dune mat can be accomplished by cutting mature lupines at the base of the trunk and splitting the trunk to discourage resprouting. Alternatively, a weed wrench can be used to remove the root intact, but this may prove awkward on steep, sandy slopes. Small plants can be pulled by hand. Plants should be removed prior to seed set. It may be easier to detect plants during flowering, but in a large population this could have detrimental impacts on pollinators. Plants are usually piled on a nearby bare area and burned after a few weeks of drying. Treatment will need to be repeated for as many years as plants emerge from the seedbank. To ensure that all plants have been removed, recheck the area during the flowering period, when smaller plants are readily seen.

If yellow bush lupine has been present long enough to alter soils, non-native grasses or other plants not normally found in dune mat will be present, and a duff layer will have accumulated. In this case lupine can be removed using the method described above, but this action will not be sufficient to restore dunes. In addition to removing lupine, all associated non-native or adventive native plants must be removed, and the duff layer should be scraped off to reveal the mineral soil (Pickart et al. 1998a). This treatment will need to be repeated for up to four years. The disturbance caused by this treatment stimulates germination from the seedbank, so bush lupine and other weeds in the seedbank will be depleted sooner than if bush lupine alone were being removed. Depending on the amount of remnant native vegetation, native species may need to be reintroduced.

Mechanical methods: Dune areas invaded by yellow bush lupine have been restored using heavy equipment in small-scale experiments at Humboldt Bay dunes (Pickart et al. 1998b). Areas in which few native species remain, and that are relatively flat and accessible, are suited to this treatment. In the experiments, all vegetation was removed using a brush rake, followed by scraping off the duff layer with a plough blade. Weed mat was then placed over the soil surface, fastened with staples, and left for two years. One year after weed mat removal, recolonization by lupine and other non-natives was low. The weed mat apparently killed seeds in the seedbank. Revegetation with native species is essential for this treatment.

No USDA approved biological agents are known for control of yellow bush lupine. However, at Bodega Bay, where the species is believed by some botanists to be native, some coastal prairie populations of Lupinus arboreus have been found to be cyclic in nature, with cover increasing from zero to as much as 60 percent, then plunging back to zero within three to ten years (Strong et al. 1995a). Population fluctuations have been linked to herbivory by subterranean ghost moths (Hepialus californicus), whose populations are in turn controlled by entomopathogenic nematodes (Strong et al. 1995b). In naturalized populations at Humboldt Bay, similar cyclic die-offs have been observed. No ghost moths have been detected, but many other herbivores, as well as fungal pathogens, are present. In 1996-97, during a major dieback, the “VGC-2” strain of the fungus Colletotrichum gloeosporoides, was collected from affected plants. This fungus has caused significant mortality in cultivated yellow bush lupine in New Zealand (Dick 1994).

No chemical control techniques have been investigated for this species.