Tarweeds are delightful plants that provide fields of yellow color on the Plateau long after most plants have succumbed to heat and dryness. To our knowledge, they are the only flowers that form true rings around the vernal pools of the Plateau, at least in the last five years.
The Name: Tarweed, tarplant, gumweed, gum plant and rosinweed...all these names refer to the sticky substance on the surface of the leaves and stems of these plants. One reference says it "...coats the faces and legs of livestock with a tarry resin"2 and another says "tarweed mascaras the horses' muzzles"1 meaning the tarry substance sticks to the animals faces and the dirt sticks to that. This reference goes on further to say that the "...viscid exudation is particularly ruinous to wool and clothing, but alcohol is a solvent for it, and will generally remove it."1
Adaptations to California's climate: Tarweeds are "late bloomers", flowering after the grasslands and vernal pools have become dry. Because they bloom during summer they have less competition from other spring-blooming herbs and grasses for pollinators and for water, soil nutrients and light.
They are coated with resinous material that helps to retard water loss during the heat of the day. The strong smell and sticky substance also help to repel things that might eat them earlier in the spring when they are young and tender.
Tarweed relies on stored soil moisture for summer growth:By the end of winter, the tarweed plant has developed a deep taproot and about a dozen broad leaves in a rosette. Roots of tarweed go deeper than most of the winter annual grasses, reducing competition with them for soil nutrients and moisture. The shallow-rooted, short statured, early maturing alien annual grasses use less light and water than the late successional perennial grasses or taller annual grasses. This results in a surplus of moisture that tarweed is able to utilize.2
We have, so far, observed two different tarweeds at the SRP:
The following table gives the characteristics of each species as we have observed them on the Plateau. A few bracketed items [...] give information from others that have not yet been verified by us.
Characteristics Slender Tarweed Sticky Tarweed Common names slender tarweed
Scientific names Hemizonia fasciculata
Holocarpha virgata ssp. elongata Bloom period (seasonal) late May to ? [May-Nov] June to ? [Nov] Flower yellow yellow # of rays (3-lobed petal-like outer flowers) 5 3-7 # of disks (center flowers with black anthers) 6 9-25 Leaf-like structures beneath flower head (phyllaries) keeled, barely overlapping many, little, gland-tipped projections Flower position many, clustered at tips of branches few, at tip of stalk or along branch with a short stem Shape of plants rounded, multi-branched single upright stem, branching above half way up Size 1-2' [up to 40"] 6"-2' [up to 4'] Location ring the vernal pools open, dry slopes, swales, edges of trails and roads Soil volcanic, primarily hard-baked or heavy clay soils Leaf shape linear, wider at base, tapering to a slender tip (1) linear one lower on the stem, getting smaller as they go up the stem
(2) reduced, short, stubby leaflets in the axil beginning higher on the stem
Leaf margin long pointed, indented teeth (1) linear: widely separated, short, sharp teeth
(2) short: tip truncated (cut-off) with a pit at end, difficult to see without a magnifying glass
Leaf texture sandpapery sandpapery Leaf habit (1) falls off at base when flowering begins linear leaf falls off at base when flowering begins, small leaf remains on stem Leaf habit (2) leaf pressed against stem linear leaf at right angles to stem; smaller, reduced cleft leaf hugging stem; crowded on upper stem Leaf habit (3) [may be rosette in spring] [may be rosette in spring] Sticky part stem and leaf leaf tip Scented part flower leaf Seeds not yet observed by us not yet observed by us Plant associations downingia, button celery, grasses, sticky tarweed vinegarweed, turkey mullein aka dove weed, grasses Plant uses pollen for honeybees, forage for livestock, seeds for Indians, ground squirrels, songbirds, gamebirds Native yes yes Book references Dale, Flowering Plants, p. 67
Niehaus and Ripper, A Field Guide to Pacific States Wildflowers, p. 192
Munz, A California Flora
Hickman, The Jepson Manual
Schoenherr, A Natural History of California, p. 522
Other web pictures 1, 2, 3 1, 2 Web references (1) The Scent of Summer
(2) Ecology and Management of Tarweed
(3) Graceful Tarplant from Craig H. Reiser's Rare Plants of San Diego County
(4) Graceful Tarplant from theWestern Riverside County Multiple Species Habitat Conservation Plan
Diagrams Slender Tarweed Sticky Tarweed
If anyone can positively identify any other tarweeds at the SRP or has additional information that would help to clarify the distinctions, we would be deeply appreciative if you would send us an email.
Copyright © 2000 by Jane Strong and Tom Chester.
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Updated 14 September 2000.