California Clovers: Unsung Heroes of the Meadow
Featured Image: The rare Pacific Grove clover (Trifolium polyodon). Photo by Alix Soliman.
By Andrew Evans, Conservation Grazing Associate
Clovers are the underdogs of wildflower world. Standing shorter than your hiking boots, often with small flowers, many wildflower lovers miss the stellar diversity of native clovers in California. The Preserve has 13 closely related native species, nine of which are detailed in this post. Most of our species are annuals, meaning that when they sprout, they must succeed in flowering and laying seed in order for the population to survive in future years.
Native clovers are an excellent indicator of grassland health, as they help other plants grow by “fertilizing” the soil with nitrogen, coexist without competing with each other, and are only abundant in locations unburdened by invasive species.
Nitrogen is a vital nutrient for plants. It makes up approximately 78% of our atmosphere, but is not accessible to plant roots in this form. Clovers (and other legumes like lupines, deer vetches, and Pacific peas) fix nitrogen in the soil through a symbiotic relationship with Rhizobium bacteria, which attaches to the clover’s roots. The clover produces sugar that gives the bacteria energy, and the bacteria converts atmospheric nitrogen into a form that plants can take advantage of: ammonia (NH3). Many farmers and gardeners use either industrially produced nitrogen fertilizers, which can cause degradation of our waterways in high quantities, or cover crops of invasive legume species. By adding native clovers to your garden as ground cover, you can naturally fertilize your plants without risking unintended consequences.
To find clovers on The Preserve, walk up the trails of Black Mountain, Vasquez, Penon Peak, or the Flats and LOOK DOWN. You’ll notice that all of the species listed here begin with the genus Trifolium. Tri means “three” and folium means “leaf,” so you’re always looking for a plant with three leaves.
1. Pinpoint Clover (Trifolium gracilentum)
Pinpoint clovers (Trifolium gracilentum) are identifiable by their lack of an “involucre,” meaning they do not have a leafy membranous cup holding the small flowers within their flower clusters. Their flowers are smaller than a dime, and often reddish/pinkish and white. Before producing seeds, the flower heads “reflex,” meaning they fold to point downward. They are known as “pinpoint” clovers because once their flowers reflex and shrivel, small modified leaves around each flower called “calyces” or “sepals” become much more obvious and look like sharp pins.
2. Notch Leaf Clover (Trifolium bifidum)
Another clover that lacks an “involucre” cup and reflexes is the notch leaf clover (Trifolium bifidum). To tell them apart from pinpoint clovers, notch leaf clovers are mostly smaller (about the size of a pencil eraser), lighter and rosier in color, and often have slimmer leaflets with a heart-like notch at the tip.
3. Tomcat Clover (Trifolium willdenovii)
Tomcat clovers (Trifolium willdenovii) are quite striking. The flowers are about the size of a quarter, can be bright magenta to soft pink in color, often have long and slender leaflets, and distinctly appear to be hollow in the center of the flower cluster. These features, and their reaching about shin-height, make them easily recognizable. Tomcat clovers begin our group of clovers with “involucres,” or small leafy cups holding the flower cluster.
4. Cowbag Clover (Trifolium depauperatum)
Cowbag clovers (Trifolium depauperatum) are favorites of the Conservancy’s grazing program. Once pollinated, their flowers swell to look like purple cow udders! Before reaching this state, they may look quite similar to other clover species. Make sure to look close to the ground, cowbag clovers often appear on trails and can be just one inch tall.
5. Small-headed Clover (Trifolium microcephalum)
Small-headed clovers (Trifolium microcephalum) are quite fuzzy, have soft pink flowers, and have a distinct rounded and fuzzy cup. Their fuzziness, color, and tiny size (comparable to Lincoln’s head on a penny) help to identify this species.
6. Bearded Clover (Trifolium barbigerum)
Bearded clovers (Trifolium barbigerum) are endemic to the Central Coast, and their presence helps indicate good site quality for other native grassland species. Although they may be mistaken with small-headed clovers, as they are both fuzzy, bearded clovers are larger (about the size of FDR’s head on a dime) and noticeably purple.
7. Thimble Clover (Trifolium microdon)
Thimble clovers (Trifolium microdon) are also quite special. They are immediately identifiable by their white, flat-bottomed “involucre” (cup). The Conservancy’s collaborating botanist Dylan Neubauer describes this flower shape as a “cupcake.” Although the stem may be slightly fuzzy, their leaves and flowers are mostly hairless. Always an exciting find!
8. White-tipped Clover (Trifolium variegatum)
White-tipped clovers (Trifolium variegatum) grow in wet meadows or moist drainages. They are completely hairless, often have serrated leaflets, and have reddish flowers with white or pink tips. They may be confused with pin point clovers (T. gracilentum), but white-tipped clovers are distinguishable for having a spiky cup or “involucre” at the flowers’ base. White-tipped clovers are the closest relatives to The Preserve’s rare species of clover: the Pacific Grove clover (Trifolium polyodon).
9. Pacific Grove Clover (Trifolium polyodon)
Pacific Grove clovers (Trifolium polyodon) have been listed by the State of California as a rare species. As such, they are protected by the California Endangered Species Act and California’s Native Plant Protection Act. The Preserve is home to what is likely the largest population of PG clovers within their small range around Monterey Bay. Their preferred habitat is wet meadows with low competition–their Preserve locations are often on mowed trails. However, due to their annual life cycle, it is essential to prevent disturbance, mowing, or other forms of “take” while they are growing and attempting to reproduce. They look similar to their close relatives, white-tipped clovers, but appear pinker, may have larger flower heads, and–you’ll need a hand lens or magnifying glass for this one–spiky lobes on their “calyces” (those modified leaves that clasp onto individual flowers within the flower head), which the white-tipped clovers lack.
Threats to Native Clover Diversity
On The Preserve, the two major threats to our unique native clover diversity are 1) disturbance during growth and 2) invasive species. Given all of The Preserve’s native clovers are annual species, any time they grow, they need to successfully flower and seed in order to maintain their population. Therefore, any interruption during reproduction, whether it be mowing, trampling, or mis-timed grazing, can seriously impact our clovers.
Native clovers also struggle to compete and reproduce under a mat of invasive species. Though invasive annual grasses surely smother out native clovers, there are also few invasive clover species that also pose legitimate threats including T. hirtum, T. subterraneum, and T. dubium (details below).
Common Non-native Clovers
10. Rose Clover (Trifolium hirtum)
Rose clovers (Trifolium hirtum) rely on mowed roadsides to proliferate and feed into grassland interiors. Erosion control seed mixes first brought them to our area. They are identifiable because they are fuzzy, larger than our native species (with flowers the size of a quarter), and the flowers are cupped by veiny leaves rather than the traditional cup of our native species. Rose clovers are listed as invasive by the California Invasive Plant Council.
11. Subterranean Clover (Trifolium subterraneum)
An issue around the San Francisquito Flats and other wet basins, subterranean clovers (Trifolium subterraneum) form thick bunches that exclude all other species. They are immediately identifiable by their large, fuzzy leaves. When their flowers are visible, they appear as limited clusters of white flowers, potentially with a red band near the base of the cluster. They are gnarly invasives because of their thick clumps, but also because they inject their seeds directly into the soil (hence the name “subterranean”). If you become confident in identifying clover species, this is the one you’ll want to pull out.
12. Little Hop Clover (Trifolium dubium)
Also forming thick, smothering colonies are little hop clovers (Trifolium dubium). They may appear similar to many natives, as their leaves are not fuzzy. However, these non-natives are immediately distinguishable by their yellow flowers, while their leaves appear less serrated than lookalike natives. Their flowers also reflex, or bend downward after being pollinated, similar to our native pinpoint clovers and notch leaf clovers. Because their leaves can be deceptive, it would take a trained eye to adequately treat this non-native species before it flowers. However, often near troublesome subterranean clovers, they still pose a real threat to wetland-loving native clovers.
Rules of Thumb
It’s an invasive clover if: it’s large (the size of a quarter) and fuzzy, the flower is yellow. If it has neither characteristic, upload pictures of it to iNaturalist so experts can weigh in.
When hiking The Preserve’s trails or walking around your property, keep an eye down and start observing which clovers you see!
Other Clover Species on The Preserve
- Trifolium oliganthum (looks similar to white-tipped clovers)
- Trifolium ciliolatum (leaves large, no cup or “involucre”, flowers pink)
- Trifolium albopurpureum (oblong, fuzzy, purple flower heads)
- Trifolium macraei (fuzzy flower heads, often in “double-head” pairs)
- Trifolium campestre (yellow)
- Trifolium fragiferum (leaflets large and oblong, fruits obviously “cottony”)
- Trifolium glomeratum (not fuzzy, flowers white and somewhat spiky)
- Trifolium angustifloium (long, fuzzy, pink flower head, leaflets long and skinny)
- Trifolium striatum (flowers oblong and white with false cup)