From our perspective as Lupinus species, our relationship with summer heat is complex and deeply tied to our genetic origins. We are a diverse genus, with many of our most popular garden varieties originating from the cool, well-drained slopes of North American western regions and the Mediterranean. This heritage means we are fundamentally cool-season perennials. While we possess certain survival mechanisms, prolonged and intense heat presents a significant physiological challenge that you, our gardeners, can help us manage.
When temperatures soar consistently above 85°F (29°C), our systems begin to struggle. Our primary response is to enter a state of dormancy or die back. This is a survival strategy, not a sign of death. We sacrifice our above-ground foliage—which loses water too rapidly through transpiration in the hot, dry air—to conserve energy and resources in our taproots deep in the soil. This allows us to survive the stressful period and potentially return when cooler, wetter conditions arrive in the fall. Furthermore, extreme heat can hinder our ability to set buds and often causes existing flowers to wilt and drop prematurely, cutting our prized blooming season short.
Our most vulnerable point during a heatwave is not our leaves, but our roots. We develop deep taproots to access cooler, moister soil layers, but the surface soil temperature is critical for our overall health. If the sun bakes the soil around our crown, it can literally cook our root system, causing irreversible damage. Therefore, the single most important strategy is to keep our root zone cool and consistently moist (but never waterlogged). This is more crucial than simply watering the leaves.
Your choice of where to plant us greatly influences our ability to cope. We perform best in locations that receive full sun in the spring but are afforded light afternoon shade during the fiercest summer heat. This could be the dappled shade from a deciduous tree or the shadow cast by a fence or building in the late afternoon. This partial shade during the hottest part of the day significantly reduces heat stress and slows soil moisture evaporation.
Applying a generous 2-3 inch layer of organic mulch, such as shredded bark, straw, or compost, around our base is akin to giving us a protective blanket—but one that keeps us cool. This mulch layer is vital because it acts as an insulator, shielding the soil from direct sun and dramatically reducing the root zone temperature. It also conserves soil moisture by drastically reducing evaporation, meaning the water you provide lasts longer and benefits us more directly.
Deep, infrequent watering is far superior to frequent, shallow sprinkling. Please water us slowly and deeply at the base of the plant, early in the morning. This practice encourages our taproots to grow deeper into the cooler earth and ensures we have ample water reserves to draw upon during the day. Morning watering also allows any accidental moisture on our foliage to dry quickly, helping to prevent fungal diseases like powdery mildew, to which we are susceptible, especially when stressed by heat and humidity.
