Holocarpha macradenia, commonly known as the Santa Cruz tarplant, is an endangered plant endemic to Northern California.

[2] Specifically Santa Cruz tarplant likes to inhabit terraced locations of coastal or valley prairie grasslands with underlying sandy clay soils.

The Santa Cruz tarplant has an extremely long tap root, allowing it to thrive longer into the season than most coastal wildflowers.

Further protection from the rainless summer is a resinous coating on leaves and stems, allowing retention of water until late in the season.

Ex-situ conservation of that colony was used to allow construction of a new shopping center, and a limited number of seeds from that population were moved immediately across and east of Interstate 80 onto the CalTrans right-of-way.

In 2002, the US Fish and Wildlife Service published the final designation of Critical Habitat for the species, as follows: UNIT A - Mezue (Contra Costa County, Wildcat Canyon Regional Park) - County/City ownership.

The members have been a researcher from UC Berkeley, a biologist from California Fish and Wildlife, a biologist from US Fish and Wildlife Service, an ecologist from the California Coastal Commission, three private consultants, a UC Cooperative Extension Livestock advisor, and Santa Cruz city staff from the Planning and Parks Departments.

[11] Twin Lakes State Park (Unit D0 population management between 1998 and 2003 consisted of mowing in 1998, 1999, and 2001 once in spring, and once in fall using a string trimmer.

The nursery plants were more robust and produced over ten times the number of seed heads than the naturally occurring individuals.

[14] As a comparison to the decades of work being done at Arana Gulch, only five miles south at 300 Byers Lane in La Selva Beach on 70 acres of private property—the same area and habitat as the Arana Gulch property—the native grassland habitat at Byers Lane, started as 99% weed-covered, and was restored back to 95% native cover in only ten years.

[17] [18] A 2020 review of the East Bay populations, outlined the history and current status of the tarplants in that area.

These populations only continue to grow currently in three locations, all of which are within Wildcat Canyon, and the largest stand is along the Mezue trail.

January, 2021, the City of Santa Cruz Parks & Recreation Department's Open Space & Greenways team, in collaboration with the Arana Gulch Adaptive Management Working Group, the UCSC Greenhouse, and UCSC volunteers, did an experimental outplanting of 400 Santa Cruz tar plant, to try and ensure its persistence as a key goal of the Arana Gulch management plan.

The long-term goal is to increase the seedbank of the tarplant at Arana to a level that will ensure a high probability of persistence for 100 years, or in perpetuity.

This planting is something of a landmark—if successful could potentially exceed the targeted population goal for the Santa Cruz tarplant management program.

These studies were conducted, to determine the health of the Coastal Prairie habitat which the tarplant is a member, and what improvements could be made to produce future self-sustaining populations.

[21] Study 2: Has cattle grazing over time, remove soil nutrients that tarplant seedlings need for survival?

[22] Study 3: Is there a connection between where wild tarplants grew in the past, with the percentage of Coastal Prairie cover in Area-A?

Overlaying the location of wild tarplant on top of the vegetation cover survey, found that the tarplants at Arana Gulch seem to need a high quality Coastal Prairie matrix to maintain a self-sustaining population, that is approximately 80-95% native grass cover, consisting of a 60% Danthonia-30% Stipa-10% native Bromus species mix.

Spring 2021, UCSC-greenhouse grown seedlings, sprouted from originally wild-harvested Arana Gulch seeds, were planted out in Area-A.

Photographed in June, 2021, showed that there were at least twelve different forms of the tarplant seedlings—from plants with widespread branches, to round compact shapes, and each flowering at slightly different times and in different manners.