Forbs: What They Are and Their Ecological Importance

Forbs are herbaceous plants that are not grasses, sedges, or rushes. Think wildflowers, clovers, sunflowers, milkweeds, and dandelions. They lack the woody stems of shrubs and trees, and their soft, broad leaves distinguish them from the narrow blades of graminoids (the grass family). Despite often playing a subordinate role in terms of sheer abundance in grasslands, forbs are largely responsible for the taxonomic, phylogenetic, and functional diversity of those ecosystems.¹ That gap between their modest ground cover and their outsized ecological influence is one of the more interesting stories in plant ecology.

What Makes a Plant a Forb

The category “forb” is defined more by what a plant is not than by shared ancestry. A forb is any herbaceous (non-woody) flowering plant that does not belong to the grass, sedge, or rush families. This means forbs span an enormous range of plant families: from legumes like clover and lupine to composites like asters and goldenrod, to entirely unrelated lineages like violets, buttercups, and wild carrots. There is no single evolutionary origin for forbs. The term is a practical grouping ecologists use to separate the broad-leaved herbaceous plants from the narrow-leaved graminoids when studying plant communities.

Within that broad umbrella, forbs vary widely in form. Research on Patagonian steppe forbs identified four distinct functional types based on traits like rooting depth, seasonal growth patterns, and whether the plant was annual or perennial. These ranged from shallow-rooted annuals that complete their life cycle quickly to deep-rooted evergreen species built for long-term persistence.² That diversity of strategies is the point: forbs are not one kind of plant doing one kind of thing. They occupy niches that grasses cannot, and that versatility ripples through the rest of the ecosystem.

Why Pollinators Depend on Forbs

Grasses are overwhelmingly wind-pollinated. Their flowers are small, drab, and produce no nectar. Forbs, by contrast, are the primary source of pollen and nectar for bees, butterflies, hoverflies, and other insect pollinators in grassland systems. Without forbs, a grassland is essentially a pollinator desert.

The relationship goes deeper than just providing flowers. Different forb species bloom at different times throughout the growing season, and that staggered timing is critical. Pollinators need continuous floral resources from early spring through late fall. Research on grassland floral resources found that this seasonal succession of blooming plants is an essential consideration for pollinator conservation in agricultural landscapes, where uncultivated grasslands often serve as the last remaining habitat.³ A grassland with only a few forb species might support pollinators in July but leave them without food in May or September.

Not all forbs contribute equally. Flower color and plant family both shape which pollinators visit and how abundant they become. Research on nutrient-enriched grasslands found that pollinator abundance varied strongly by floral family, with purple and yellow Asteraceae species (the daisy and sunflower family) acting as core hubs in plant-pollinator networks.⁴ When nitrogen enrichment caused those species to increase, the pollinator community shifted in response. This means that changes in forb composition, not just forb presence, can reshape an entire pollinator network.

Food for Wildlife

Forbs punch above their weight as wildlife food. Their leaves tend to be more nutritious than grass blades, with higher protein concentrations and more digestible energy. This matters most during critical life stages.

A study of bison migration and diet found that during the neonatal period, when mothers are nursing calves, bison shifted from diets dominated by graminoids and shrubs in their winter range to diets where shrubs and forbs were predominant. The neonatal range had roughly 3.8 times more forb biomass than the core winter range in spring, and the crude protein content of those forbs was about 3.2 times greater.⁵ The bison were not wandering randomly. They were migrating to areas where the most nutrient-rich forbs were available, precisely when their calves needed it most.

The benefits extend to birds and insects as well. Forb seeds and the invertebrates that live on forb leaves are major food sources for grassland birds. Research on agricultural grasslands in the UK found that bird species feeding on foliar invertebrates or forb seeds have been hit hardest by modern grassland management practices.⁶ A related study found that the ideal grassland for supporting farmland bird food resources is one with a mixture of grasses and broad-leaved plants with a range of vegetation heights and structures.⁷ Uniform grass monocultures are structurally simple, and that simplicity starves the food web.

How Forbs Improve Soil

Forb root systems differ structurally from those of grasses. Many grasses have dense, fibrous root mats concentrated near the soil surface, while forbs often produce taproots and branching roots that penetrate deeper and create larger root channels. This matters for how water moves through the ground.

A study on trampled areas and trails found that as forb species with greater root channel area established during revegetation, soil infiltration rates increased.⁸ In practical terms, this means water soaks in rather than running off the surface, which reduces erosion and recharges groundwater. A grassland with a healthy proportion of forbs handles heavy rain better than one dominated entirely by grass.

Below the surface, forbs also influence nutrient cycling. Forb leaves tend to have higher nitrogen concentrations and lower carbon-to-nitrogen ratios than typical grass litter, which means they decompose faster. A study in a semi-arid Chinese grassland demonstrated substantial differences in decomposition rates across species, with those having higher nitrogen content and lower carbon-to-nitrogen ratios breaking down more quickly.⁹ Faster decomposition returns nutrients to the soil more quickly, making them available for the next generation of plants.

Leguminous forbs, like clovers and vetches, go a step further. They fix atmospheric nitrogen through symbiotic bacteria in their roots, essentially manufacturing fertilizer. Research on 33 grassland species found that nitrogen fixation by legumes led to greater aboveground biomass production and accelerated ecosystem nitrogen cycling compared to non-legumes.¹⁰ A separate grassland experiment found that soil carbon and nitrogen pools were enhanced not by the raw number of species present, but by the presence and biomass of specific nitrogen-fixing legumes and forbs.¹¹ Diversity alone was not the mechanism; the identity of the species mattered.

Partnerships with Soil Fungi

Most land plants form underground partnerships with arbuscular mycorrhizal (AM) fungi, thread-like organisms that extend the plant’s effective root network in exchange for sugars. Forbs, as a group, are particularly reliant on these relationships and respond more strongly to them than grasses do.

Research in semi-arid grasslands found that forb species benefit more than grasses from a diverse AM fungal community.¹² The fungi help forbs access phosphorus and other soil nutrients that their roots alone cannot reach efficiently. And the relationship is not one-directional: the composition of the fungal community can determine competitive outcomes between plant species. A study found that AM fungi balanced competition between forbs and grasses by enhancing the competitive ability of the forbs, suggesting that natural fungal communities help maintain plant diversity by preventing grasses from completely dominating.¹³

These responses vary with the type of plant and its growth season. Greenhouse studies of tallgrass prairie species showed that perennial warm-season grasses and forbs generally benefited from mycorrhizal colonization, while cool-season grasses showed little response and had lower fungal colonization rates.¹⁴ This means the timing of disturbance, grazing, or land management can selectively help or hurt the plants most dependent on fungal partners.

Surviving Fire and Drought

Grasslands are disturbance-driven ecosystems. Fire, drought, and grazing are not catastrophes for these landscapes; they are the forces that maintain them. Forbs have evolved a variety of strategies to persist through these events.

After fire, many forbs regenerate vegetatively from belowground organs rather than relying on seed. A study of Brazilian Campos grasslands found that vegetative regeneration from roots, rhizomes, and other buried structures was the most important recovery strategy for most studied forb species.¹⁵ These buried bud banks allow forbs to resprout quickly once a fire passes, sometimes outpacing grasses in their recovery.

Drought tolerance in forbs works differently from grasses. Research comparing temperate forest understory forbs and graminoids found that the two groups use contrasting strategies: grasses emphasize stress tolerance, while forbs tend to maintain higher total biomass and leaf area, using a competitive strategy aimed at maximizing resource acquisition. Forbs also showed lower residual water loss after their stomata closed, meaning they conserved water more effectively once they sealed up.¹⁶ Despite these different approaches, both groups experienced similar overall biomass reductions under drought. The paths were different, but the outcomes converged.

A separate study found that among herbaceous seedlings, drought survival was best predicted by plant size: smaller plants with lower water-loss rates survived longer, regardless of their broader functional strategy.¹⁷ This matters for restoration and management, because it suggests that small-statured forb seedlings may be surprisingly resilient during establishment, even in dry years.

Getting Forbs Back into Restored Prairies

One of the persistent frustrations in grassland restoration is that forbs are hard to establish. Native grass seeds are cheap, available in bulk, and the plants grow aggressively. Forb seeds are expensive, less commercially available, and the seedlings are easily outcompeted.

Experimental work in northeast Kansas showed exactly how this plays out. Four years after seeding, higher grass seed densities decreased forb cover, biomass, and species richness. Higher forb seed densities, meanwhile, increased forb richness, confirming that seed availability limits some forb populations.¹⁸ The researchers recommended keeping dominant grass seeding rates low and spatially separating forb seeds from grasses, concentrating forbs in areas with little or no grass seed. Because forb seed is expensive, spreading it evenly across an entire restoration site is often prohibitively costly. Strategic placement, essentially giving forbs a head start away from grass competition, produced better results for less money.

This is a lesson that many restoration practitioners have learned the hard way. A prairie planted with a heavy grass seed mix may look green and full within two years, but it will be functionally impoverished: low pollinator activity, limited wildlife value, and simplified soil processes. Investing in forb diversity up front, and managing grass competition during establishment, pays off in ecosystem function over the long term.

The Mammoth Steppe and the Paradox of Forb Abundance

Today’s grasslands are visually dominated by grasses, with forbs contributing most of the species diversity but relatively modest ground cover. This may not always have been the case. Evidence is mounting that forbs were far more abundant in the extensive, highly productive grasslands of the Pleistocene, the so-called “mammoth steppe” that stretched across northern Eurasia and into North America.¹

The mammoth steppe supported enormous herds of megaherbivores: mammoths, woolly rhinos, giant bison, and horses. These animals grazed and trampled intensively, which likely suppressed grass dominance and created conditions favorable for forbs. When the megafauna went extinct at the end of the Pleistocene, the heavy grazing pressure disappeared, and grasses gained a competitive advantage they have held ever since. Modern grasslands, in this view, are not the natural baseline but a post-extinction artifact. The forb-rich steppe was the older, richer condition.

This evolutionary context reshapes how ecologists think about grassland management. If forbs were historically more abundant than they are today, then management strategies that increase forb cover, through targeted grazing, controlled burns timed to favor forbs, or reduced fertilization, are not creating something artificial. They are nudging ecosystems back toward a state they occupied for millions of years.

Forbs in Livestock Agriculture

Farmers have traditionally viewed forbs as weeds competing with productive grasses. But a growing body of research suggests that certain forage forbs can improve livestock nutrition and reduce environmental impacts.

A dairy cattle trial comparing chicory and plantain (both forbs) against perennial ryegrass found that after an initial adjustment period, cows offered forbs produced roughly 19% more milk and 17% more milk fat and protein yield than cows on ryegrass alone. The forbs also degraded faster in the rumen than ryegrass did. Perhaps more striking, rumen ammonia and urinary nitrogen concentrations dropped by up to 37% and 38% respectively when forbs were included in the diet.¹⁹ High urinary nitrogen is a major source of nitrous oxide emissions and nitrate leaching from pastoral systems, so this reduction carries real environmental significance beyond milk yields.

The catch is that forage forbs are harder to manage than ryegrass. They require different grazing rotations, are less persistent under heavy stocking, and may need reseeding more frequently. Still, for farms looking to reduce nitrogen losses while maintaining productivity, integrating forbs like chicory and plantain into pasture mixes is an increasingly practical option.

Chemical Warfare and Defense

Forbs produce an extraordinary array of secondary metabolites, chemical compounds that serve no direct role in growth or reproduction but function as defenses against herbivores, pathogens, and competing plants. These include alkaloids, terpenes, phenolics, and volatile organic compounds that can deter insect feeding, attract predators of herbivores, or signal neighboring plants to ramp up their own defenses.²⁰

This chemical repertoire has ecological consequences beyond the individual plant. When invasive forb species enter a new ecosystem, their root exudates and litter chemistry can alter soil nutrient pools and disrupt existing plant-microbe relationships. A review of invasive plant impacts found that invasive species can modify soil biogeochemistry and that secondary metabolites released by these plants play important roles in soil chemistry and in interactions between plants and between plants and microbes.²¹ Some invasive forbs are effective precisely because their chemical arsenal is unfamiliar to the native soil community.

Parasitic Forbs and Community Structure

Not all forbs play nicely with their neighbors. Some are parasitic, tapping into the roots of nearby plants to steal water and nutrients. The most ecologically studied example is yellow rattle (Rhinanthus minor), a hemiparasite, meaning it photosynthesize on its own but supplements its nutrition by parasitizing other plants, especially grasses.

Research on Rhinanthus found that parasitic plants affect the growth, reproduction, and metabolism of their hosts and can influence the outcome of competitive interactions between host species, reshaping entire plant communities.²² By weakening dominant grasses, parasitic forbs create openings for other forb species to establish, increasing overall plant diversity. This is why conservation managers in Europe sometimes deliberately sow yellow rattle into species-poor grasslands: it is one of the few tools that can break grass dominance without herbicides or heavy machinery.

The dynamics resemble a rock-paper-scissors game. Grasses outcompete forbs. Parasitic forbs weaken grasses. Non-parasitic forbs benefit from the weakened grasses. The parasite keeps any one group from monopolizing the community, maintaining a more diverse and dynamic system than would exist without it.

Forbs in Cities

Urban environments are surprisingly rich in forbs, though not always the ones ecologists want to see. Surveys of spontaneous vegetation across five Chinese cities recorded 633 species, of which about three-quarters were herbaceous plants. Native species accounted for roughly two-thirds, but about a third of the non-native species were classified as invasive.²³

Urban forbs grow in sidewalk cracks, vacant lots, road margins, and neglected corners. Many are dismissed as weeds, but they provide real ecosystem services: pollinator habitat in otherwise barren cityscapes, stormwater absorption, soil stabilization on bare ground, and microhabitat for insects and small animals. The growing interest in “rewilding” urban green spaces often comes down to allowing native forbs to establish rather than mowing everything to a uniform grass lawn. A patch of clover, dandelions, and self-heal in a park is doing more ecological work than the manicured turf around it, even if it looks less tidy.

Communities in rural parts of the world have long recognized the direct value of wild forbs. Surveys of wild edible plant use around forest reserves in Uganda documented widespread consumption of wild herbaceous plants, driven by food scarcity, their use as spices, perceived nutritional and medicinal value, cultural practice, and simple preference for their taste.²⁴ Many plants that Western horticulture treats as weeds, like amaranth, purslane, and lamb’s quarters, are valued food plants in other traditions. The line between forb and food is largely cultural.