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Cardaria draba (L.) Desv.
NOTE: This species is actionable only when found in field or vegetable seed for planting; tolerance applies.
Common names: hoary cress, whitetop
Fruit a 2-chambered silicle with 1-2 seeds per chamber, cordate, compressed in cross-section, 2-4 mm tall, 3-5 mm broad, with conspicuous reticulation on surface.
Seeds elliptic to obovate with a notched, bluntly pointed base, 1.2-2.2(3) mm long, 1.0-1.5(1.7) mm wide, (0.5)1.0-1.25 mm thick, cotyledon edge thicker than radicle edge, cross sectional outline ovate. Testa dull, dark reddish-brown or deep purplish-red, sometimes with yellowish tinge. A faint groove or line runs from the base, gradually veering away from the margin and ending at about the middle or upper one-fourth of the seed. Surface minutely granular. Hilum in basal notch, inconspicuous, often hidden beneath funiculus or funicular material. Embryo bent, cotyledons incumbent, radicle same length as cotyledons; endosperm nearly absent.
The seeds of the three species of Cardaria, C. chalepensis, C. draba, and C. pubescens, may be distinguishable if they are well developed. If immature, they may be indistinguishable. For example, the degree of redness may vary with conditions of development. The silicles of the three species, however, are quite distinct.
Cardaria chalepensis (L.) Hand.-Mazz.
Cardaria pubescens (C. Meyer) Jarmol.
Mature seed characteristics:
Cardaria draba: darker reddish brown than C. pubescens and C. chalepensis.
Cardaria draba and Cardaria chalepensis: cotyledon edge thicker than radicle edge
Cardaria pubescens: more broadly elliptic, flatter, smoother than C. draba and C. chalepensis; thickness uniform throughout.
Widespread in temperate regions of Africa, Europe, Australia, North and South America.
Native to the Mediterranean.
Temperate, from subalpine to arid environments; tolerates a wide range of soil types and moisture conditions; a weed of wildlands, disturbed open sites, field crops, orchards, pastures, roadsides, neglected areas.
Cardaria draba is a deep-rooted perennial herb up to 0.5 m tall. Seed production is prolific, but this species is an aggressive weed primarily because of its extensive horizontal and vertical root network. New shoots arise from buds on lateral roots or root fragments. Both seeds and root fragments in mud or soil can be carried by livestock, farm equipment, and streams, and cultivation can facilitate spread by creating root fragments. Cardaria draba rapidly establishes dense stands that crowd out forage plants and native vegetation. It has become a major weed of crops in Europe and the Middle East, and of cereal crops in the U.S., particularly in the western states. Cardaria draba is one of ten species covered under the Federal Seed Act for which tolerance is applicable to its introduction (7 CFR 361.6(a)(2)).