Big-leaf lupine: Our native invasive

Two words are bandied about in horticulture to describe such a range of plants that one wonders whether they still have meaning. "Native" and "invasive" are both so widely used when speaking of big-leaf lupine (Lupinus polyphyllus) that it's easy to be confused about appropriate sitings and other plants in the genus.

I was introduced to this West Coast native by Kirsten Rohde at Sahale during the 2021 Northwest Permaculture Convergence. She showed me a pollinator garden she had planted with a packet of wildflower seeds, which included big-leaf lupine.

The garden thrived, particularly the perennial big-leaf lupine. It had grown 3 feet tall in two years and, even in October, was vivid green and sporting seed pods and flowers. Other lupines in the planting had long since dried up.

Kirsten was concerned that the big-leaf was invasive, and so vigorous that she called it evergreen because it grows through winter. She thanked me for thinning it from her garden. It had been flopping over and crowding out other wildflowers.

I potted four of the largest plants and took them home, noting that they had been planted in some well-amended soil frequented by earthworms. Lupine needn't be coddled, because it's a nitrogen-fixing pioneer plant that thrives in, and improves, poor soil.

The genus was named after wolves because it was once believed that it devoured the nutrients of the soil. Today we understand that wolves enrich their ecologies; how fitting that we similarly reevaluate the contribution of their namesake.

Big-leaf lupine is native from Santa Cruz County to British Columbia, and east to Idaho, Montana and Nevada. The nursery trade has carried it all over the world, seduced by the brilliant flower spikes of different hybridized cultivars.

In most ecologies outside of its indigenous range, it is overly opportunistic. It spreads via seeds and rhizomatous expansion, which can eventually overwhelm companions in a planting.

The best way to manage that and benefit from its stored nitrogen is to mow or prune it and leave the cuttings to decay and add biomass. Nitrogen stores are greatest at peak flowering, but begin to recede with the formation of seed pods. The roots will die back proportionate to the pruning and release the nitrogen the lupine stored to adjacent plants.

In native wild stands, lupines are heavily preyed on by an aphid that has speciated to feed on lupines. Several species of butterfly also feed their larvae on its foliage. Bears dig up Nootka lupines to eat their roots, and birds enjoy lupine seed pods. It's easy to see how, in the context of their evolution, lupines fill niches without exceeding them.

A couple of years ago a friend brought me lupines she had dug up from a clearcut. They were unfortunately already in decline, having flowered and started to form seed pods. Fall is a good time to divide and replant big-leaf lupines, when they can root deeply in the fall and winter rains - though they may be flowering and seeding at the same time!