Ambrosia deltoidea aka Triangle Bur Ragweed

Triangle bursage is a low-growing shrub reaching 30–80+ cm (roughly 30–80 centimetres) in height. Its branches are erect, dark brown, ridged, and strongly resinous. Individual plants can live approximately 50 years. Spread data are not well documented in primary sources, but the plant forms part of dense desert scrub communities.

Ambrosia deltoidea is native to the Sonoran Desert of North America. Its range spans southern Arizona in the United States and northern Mexico, including Baja California, Baja California Sur, and Sonora. It typically occurs in sandy washes, alluvial benches, and rocky slopes at elevations of 200–1,000 metres.

As a drought-deciduous species, Ambrosia deltoidea may drop leaves during extended dry periods and re-foliate after rainfall events. Flowering occurs from February through May. No other seasonal care requirements are documented in primary sources.

Flowers appear from February through May. Staminate (male) flower heads are cup-shaped, 4–8 mm in diameter, each containing 12–30 or more florets, arranged in spike inflorescences. Pistillate (female) heads cluster near the staminate heads and contain 2–3 florets. Fruits mature into spine-covered burs that cling to animal fur and clothing.

No named cultivars or horticultural varieties of Ambrosia deltoidea are documented. The primary synonym recognised in the literature is Franseria deltoidea Torr.

Triangle bursage is a desert shrub suited only to outdoor growing in arid to semi-arid climates matching the Sonoran Desert — hot, dry summers with mild winters. It grows on steep rocky slopes, sandy washes, and alluvial benches, preferring moister microsites such as gully margins. It is highly drought-tolerant and requires full sun. The species readily colonises overgrazed grasslands; areas protected from grazing tend to have reduced populations.

Triangle bursage produces a persistent seed bank and propagates naturally by seed. The spine-covered burs adhere to animal fur and clothing, facilitating dispersal. No horticultural propagation methods are documented in primary sources.

No significant pest or disease issues are documented in primary sources for this species. It is not grazed by livestock. The plant's pollen is a known contact allergen for humans and is one of the major allergenic pollen sources in the Sonoran Desert region. The species is not listed in the IUCN Global Invasive Species Database.

Ambrosia deltoidea is wind-pollinated, as is typical for the genus Ambrosia. It produces abundant airborne pollen that is a recognised allergen in Sonoran Desert regions and can cause contact dermatitis in sensitive individuals.

Triangle bursage functions as a significant nurse plant in the Sonoran Desert. Its canopy provides shade and its presence increases soil nitrogen, benefiting seedlings growing beneath it. It is the primary nurse plant for saguaro cacti at Organ Pipe National Monument and also shelters yellow paloverde, ocotillo, and certain prickly pear species from herbivory and excessive heat.

Triangle Bur Ragweed should be watered regularly, allowing the soil to dry out between waterings.

Triangle Bur Ragweed loves a well-draining soil. Perlite and vermiculite help with drainage, while coco coir adds organic matter, so a good potting soil mix will have all three. You can improve store-bought soil by adding some perlite to it.

Triangle Bur Ragweed requires abundant, bright, and direct light to thrive. The best placement for this plant is within one foot of a window to ensure it receives enough light. If you live in an area with more extreme weather, such as hot summers or cold winters, you may need to adjust the placement of the plant accordingly. During hot summer months, it is important to place the plant away from full sun, as this can cause the leaves to yellow or burn. During cold winter months, it is best to move the plant closer to the window to take advantage of the extra light that can help it survive. By keeping an eye on the current weather in your area, you can make sure your Triangle Burr Ragweed is always in the right place to thrive.

There is no verified data on the toxicity of this plant in the records of Ploi. If any person, including yourself, a family member, or a pet, consume plant material with an uncertain toxicity level, it is advisable to seek the advice of a healthcare expert.