Horticulturist and author Thomas Ogren started delving into the topic of plant pollen because his wife suffered from allergies. Not finding much information, he eventually developed a scale he dubbed OPALS (Ogren Plant-Allergy Scale), which categorizes plants numerically from 1 to 10. The higher the number, the more likely that the plant will cause allergy symptoms. Find his OPALS numbers for over 5000 plants in Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping (Ten Speed Press, 2000).
Favorite or New Plant
Butterflies crowd the fuzzy lavender-blue flowers of Conoclinium (syn. Eupatorium) greggii 'Boothill' from late spring through fall. Plant this small herbaceous perennial where it receives afternoon shade. If planted in full sun, it will need extra water in summer to thrive. Mature size is about 2 feet tall by 2 to 3 feet wide. Plant in groups beneath tree canopies in filtered light as ground cover. (Too much shade reduces blooming and causes leggy foliage growth.) It will survive to zero degrees F, although suffers foliage damage at 20 degrees F. Cut back at the end of winter to rejuvenate and promote bushiness as needed. It recovers quickly from underground stems.