Common Weeds & Invasives in Bay Area Gardens

"Invasive plants damage ecosystems around the world. They displace native species, change plant community structure, and reduce the value of habitat for wildlife."
- California Invasive Plant Council

As fine-gardeners one of our main services has always been hand-weeding in favor of using dangerous & toxic herbicides. Every once in a while we find weeds & invasive plants which are hard to identify, uncommon or just plain otherworldly. Nature itself doesn't know a weed or invasive from a redwood and she (mother nature) is perfectly happy to allow for unlimited growth potential for all plants. Lucky us! While some weeds & invasives are easy to spot, some smaller, more attractive or ornamental selections can be a bit harder to identify. To assist curious minds interested in weed & invasive identification in the Northern California Bay Area, we here present 'Identifying Common Weeds & Invasives in Bay Area Gardens'. While this article is offered as a general guide, it is by no means an exhaustive list. Additions, comments, corrections, questions & suggestions always welcome.

All photography (c) 2008-09 Taproot Garden Design & Fine Gardening


Annual bluegrass
Scientific name: Poa annua (Family Poaceae) DESCRIPTION: Annual bluegrass is a low-growing, cool-season grass which dies early in the summer when the top layer of soil dries out. It can easily be distinguished from other grasses by its typical leaf tip which is shaped like the bow of a boat. The leaf blade is often crinkled at the midsection. Annual bluegrass has a membranous ligule that is rounded with a slightly pointed tip. The mature plant grows as dense, low-spreading tufts, 3 to 12 inches (8 - 30 cm) tall, and often roots at the lower nodes.
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Annual sowthistle
Scientific name: Sonchus oleraceus (Family Asteraceae)
DESCRIPTION: Annual sowthistle is a widespread annual weed. It is commonly found in California's Central Valley and coastal areas where it grows year-round. Its seeds germinate only in the top 1/2 inch (1.2 cm) of soil, so preplant cultivations and deep plowing are effective controls. Seed leaves are stalked and covered with a powdery, gray bloom. They have smooth-edges and are spoon-shaped. True leaves have wavy edges and prickles. Upper leaf bases clasp the stems with clawlike lobes. Mature plant may reach a height of 3 to 6 feet (0.9 - 1.8 m). Yellow flowers mature into white, fluffy seed heads, similar to common groundsel. Hollow stems secrete milky juice when cut or crushed.
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California burclover
Scientific name: Medicago polymorpha (Family Fabaceae)
DESCRIPTION: California burclover is a summer annual weed. Stems are up to 2 feet (60 cm) long and tend to trail along the ground, but may grow upright. The trifoliate leaves resemble those of clover and usually have reddish-tinged midveins. Small, bright yellow flowers form in clusters at the end of stems. The seed pod is a bur that contains several yellowish or tan, kidney-shaped seeds. Seed leaves are oblong. The first true leaf is rounded with a single leaflet. Later leaves have the characteristic cloverlike shape.
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Common groundsel
Scientific name: Senecio vulgaris (Family Asteraceae) DESCRIPTION: Common groundsel is an early season weed in most areas, but can grow all year in coastal areas of California. Seed leaves of groundsel are elongate, with a blunt, rounded tip. The first true leaves have shallow teeth; the third and fourth leaves are more deeply lobed. Groundsel grows up to 60 cm (2 feet) tall. Leaves are alternate on the stem and deeply indented. Upper leaves are attached directly to the stem, but lower leaves have a short petiole. The green bracts surrounding the flower cluster have conspicuous black tips that distinguish groundsel from other weeds in the thistle family.
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Cutleaf geranium
Scientific name: Geranium dissectum (Family Geraniaceae) DESCRIPTION: Cutleaf geranium is a freely branching annual that remains prostrate in turf. Leaves are dissected into narrow leaf divisions. Small, purple flowers are conspicuous. Seed pods are thin and may be up to 2 inches (5 cm) in length.
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Dandelion
Scientific name: Taraxacum officinale (Family Asteraceae)
DESCRIPTION: Dandelion is a widely distributed perennial weed. The mature plant arises from a strong, deep taproot that exudes a milky substance when cut. There is no visible stem. Leaves are sparsely hairy or without hairs, have deeply serrated margins, and are clustered in a rosette at the base of the plant. Dandelion can reproduce from seed almost year-round or it can regrow from its taproot. Bright yellow flower heads, 1 to 2 inches (2.5 - 5 cm) across, consist of petal-like ray flowers and are borne singly on the tip of a hollow stalk, 3 to 12 inches (7.5 - 30 cm) long. Seeds are enclosed singly within fruiting bodies and are attached to a long slender stalk that terminates in a parachute-like structure called a pappus. Seeds are transported in the wind. The bristles on the seeds can clog cultivation equipment.
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Lesser-seeded bittercress
Scientific name/Other Common Names: Little bitter-cress, Few-seeded bitter-cress
DESCRIPTION: Lesser-seeded bittercress is an annual or biennial plant 3 to 12 inches (7.5 - 30 cm) tall with several branched, smooth stems emerging at the soil line. Mature plant leaves, divided into 5 to 11 leaflets each, radiate from the base of the stems in a rosette. Leaflets are rounded, bright green, and have short stalks and several lobes. Upper stem leaflets are narrower than those near the base. Two to ten white flowers are borne from the stem on stalks of unequal length. The narrow, 0.5 to 0.75 inch (1.3 - 1.9 cm) long pods split open into two curling valves when mature, explosively projecting the flattened and finely pebbled seed up to several yards (meters) from the plant. Hairy bittercress (C. hirsuta) is similar but has fewer, lobed or kidney-shaped leaflets.
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Sweet Alyssum (Lobularia maritima) A low, spreading perennial 2" to 3" tall. The small, white, (sometimes light purple) fragrant flowers can be seen blooming for most of the year.




Arum italicum (Lords and Ladies) Clump-forming Bulb, Height: 10-12 inches. Width: 12-16 inches. Spring flowers are light pale green spathe folds over yellow spadix. Leaves are a glossy, deep green arrow-shaped foliage with white veining. Part shade to full sun. Humus-rich, moist, well-drained soil. ( http://www.rainyside.com/features/plant_gallery/bulbs/Arum_italicumPictum.html ) This vigorous spreader needs containment or a large area to fill, but in the right spot it is an easy ground cover that has both summer and winter interest. During the fall, large, arrow-shaped deep green leaves with white veins appear and persist through the winter. As the leaves fade in summer, they are replaced by greenish-white spathes and quickly followed by dense spikes of orange-red berries which persist until the leaves return in fall. If grown in a partially shady site, it will produce large leaves with fewer berries, in an open, sunny site, smaller leaves with many berries.
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Additional Resources:
California Invasive Plant Council