California coastal prairies are dominated by non-native Eurasian grasses and forbs. These species create an annual grassland that without disturbance results in a build-up of thatch (litter) that suppresses many native species germination and growth. Fire removes the thatch layer, creating suitable gaps for other species to germinate. However, many species that invade these grasslands are from Mediterranean climates and are also fire-adapted, so they may quickly recover if seeds are available in the seed bank or able to disperse from nearby areas. Using fire to control for these invasive weeds requires burning during the correct window of time when seeds of target species are still ripe and have not dropped to the soil surface (DiTomaso et al., 2006).

Can fire control invasive grasses?

Using prescribed fire to control invasive grasses in California grasslands has shown some success under specific conditions, but it is not a universal solution. Fire tends to work best on species with late phenology—those that mature after native species have completed their life cycles. For example, medusahead (Taeniatherum caput-medusae) and barb goatgrass (Aegilops triuncialis) have been significantly reduced through carefully timed late-spring burns that destroy viable seeds (Bartolome et al., 2019; DiTomaso et al., 2001). The success of this strategy depends on the absence of nearby seed sources and the prevention of dispersal by livestock or wildlife (Berleman et al., 2016).

Despite these isolated successes, most research suggests that fire alone is rarely sufficient for lasting invasive grass control. Repeated burns, integration with seeding or herbicides, and grazing may be necessary to suppress persistent grasses and encourage native recruitment (Keeley et al., 2001; Keeley et al., 2023). For example, burning perennial grass like harding grass (Phalaris aquatica) before herbicide application can improve efficacy by removing thatch and exposing new leaves (DiTomaso et al., 2006).

A recent study by Musto (2025) evaluated the seedbank pre- and post-burn at four coastal prairie sites. He found that while non-native annual grasses decreased in abundance post-burn, there were plenty of seeds still viable to recover in the following year. This suggests that effective control could only be achieved with repeated burns, and following-up with seeding native plants.

Can fire control for invasive flowering plants and shrubs?

Like grasses, non-native forbs and woody species show mixed responses to prescribed fire. For instance, prescribed burning can reduce the seed bank of French broom (Genista monspessulana), a nitrogen-fixing shrub, but may require up to five consecutive burns to deplete it fully (Alexander and D’Antonio, 2003). Additionally, its alteration of soil nitrogen can create favorable conditions for other invasive plants to spread.

Many forbs escape fire impacts entirely if they complete their life cycle before enough fuel has accumulated to carry a burn (DiTomaso et al., 2006). Others, like biennials in the rosette stage, are protected by their short stature. And even when fire reduces one problematic species, it often opens space for another. For example, stork’s bill (Erodium spp.) have been observed to increase following burns that reduced annual grasses (Berleman et al., 2016; Keeley et al., 2023). Wind-dispersed species, particularly from the Asteraceae family, are also quick to colonize post-burn landscapes (DiTomaso et al., 2006).

Fire can perpetuate many early detection, rapid response species

According to the Open Space Authority, Santa Clara Valley, several early detection, rapid response invasive plant species (those that are newly invading the Central Coast region) are of primary concern for controlling infestations. These species include Arctotheca calendula (capeweed), Brachypodium sylvaticum (false-brome), Carex pendula (Pendulous sedge), Carthamus lanatus (woolly distaff thistle), Centaurea calcitrapa (purple starthistle), Centaurea diffusa (diffuse knapweed), Centaurea iberica (Iberian knapweed), Centaurea stoebe ssp. Micranthos (spotted knapweed), Chondrilla juncea (rush skeletonweed), Dittrichia viscosa (false yellowhead), Elymus caput-medusae (medusahead), Euphorbia heliscopia (sun spurge), Fallopia japonica (Japanese knotweed), Lepidium draba (hoary cress), Nassella tenuissima (Mexican feathergrass), Oncosiphon pilulifer (globe chamomile), Onoprdum Illyricum (Illyrian thistle), and Rhaponticum repens (Russian knapweed).

Limited research exists on how fire affects these species, but most are from fire-adapted ecosystems. For perennial grasses, such as Brachypodium sylvaticum, that form dense mats, fire can open the canopy and stimulate regrowth from the crowns, potentially worsening infestations; a study in Pacific Northwest forests found that low-severity burns increased cover by 60%, with seedling germination also increasing by 32% (Poulos and Roy, 2015). Thistles are usually only sensitive to fire after bolting, and burning can stimulate both thistle and knapweed’s extensive seedbanks. The Fire Effects Information System (FEIS) noted that since all Centaurea species regenerate almost entirely from seeds, prescribed burning can deplete the soil seed bank (Innes, 2022). However, this is only possible with several follow-up burns, as well as reseeding with natives or applying other control methods like herbicide or hand pulling. Woody resprouting herbs, like Chondrilla juncea, can increase in germination following fire (Lynn Kinter et al., 2007). Fast-colonizing forbs, such as Dittrichia viscosa, favor disturbance and particularly after fire (CRC for Australian Weed Management Guide, 2003). Overall, these studies suggest that burning is likely to increase the persistence of these invaders of concern and should be used cautiously to control for them.