Efforts to recover endangered and threatened plants frequently involve using nursery-grown plugs to establish new populations. These species often have limited quantities of high-value seed, making out-planting of plugs more common than direct seeding. However, out-planting also involves the considerable expense and effort of producing the plugs as well as physically installing them on a site. Our experience restoring golden paintbrush (Castilleja levisecta), a threatened forb found in grasslands of the maritime Pacific Northwest, led us to question the effectiveness of out-planting plugs as a recovery strategy at most sites. Since the early 2000s, out-planting plugs was the primary method used to establish golden paintbrush at new and extant sites until resources allowed for the production of sufficient seed. After using seeding, plugging or both techniques at many sites over multiple years, we conducted a retrospective analysis to examine the relative effectiveness of these two approaches for restoring populations. For each out-planting, all flowering plants were counted annually, including both out-planted individuals as well as new recruits.
Results/Conclusions
Of all out-plantings using plugs, less than ten percent increased in size over the long-term. Out-planting success was no higher when plugging was used to augment existing wild populations, where conditions were expected to be more optimal and success more likely. In contrast, seeding golden paintbrush established new populations at multiple sites including two with greater than 40,000 flowering individuals. The difficulty in knowing and finding suitable microsites to plant individual plugs likely leads to the high frequency of out-planting failures. We conclude seeding golden paintbrush is more successful because it distributes far greater numbers of propagules (~2,200 seeds per gram) across larger areas and into many more potential microsites increasing the likelihood of establishment. For reintroductions of similar species, resources focusing on increasing available seed for direct sowing may provide better recovery gains than out-planting plugs.