As fall approaches, there are many opportunities to enhance your knowledge of horticulture.

Washington

National Arboretum, Administration Building, 24th and R streets, NE. Telephone 202-245-5898; Web site: www.usna.usda.gov

U.S. Botanic Garden Conservatory, southwestern corner of the Capitol grounds, 245 First St., SW. Call 202-226-4082 to register for programs; for information, call 202-225-8333. Web site: www.usbg.gov

* Sept. 4, 12, 19: Bonsai demonstrations: Bonsai experts transform a nursery plant into a bonsai. Learn the basics of pruning, wiring, and repotting. This is an excellent opportunity for beginners who want a basic introduction to the art of bonsai. 1 to 2:30 p.m. Free. National Arboretum.

* Sept. 7: Meander through a Mayan jungle: Spend an hour with plant expert Rosita Arvigo in the Garden Court. Discover which plants play a role in the everyday life of the people in Central America. The event is free and preregistration required. 3 to 4 p.m. U.S. Botanic Garden.

* Sept. 10: Pollination of native and introduced plants: Can pollinators tell the difference between native and introduced plants? Some do not care if their food comes from native or imported plants; others haven't readily adapted to the changes humans have made in the landscape. Discover more about pollinators' preferences in flowers and find out what types of plants you should grow to attract pollinators to your garden. Noon to 1 p.m. Free. Preregistration required. U.S. Botanic Garden.

* Sept. 11: Landscape as spirit, creating a contemplative garden: Through a slide presentation, landscape architect Martin Mosko and writer and photographer Alxe Noden will show how to create a beautiful outer environment using our inner resources. Explore a world of sensual delights --

cascades of water, gently swaying trees, and mountains of boulders -- to create a place of peace and contentment. The lecture will be followed by a book sale and signing. 10:30 a.m. to noon. Fee $12. U. S. Botanic Garden.

* Sept. 11-19: Visiting bonsai exhibit: Members of the Potomac Bonsai Association Lancaster Bonsai Club exhibit bonsai from their private collections. The member-led demonstration offers practical information for beginners on the secrets for pruning, training and repotting bonsai. Exhibit daily 10 a.m. to 3:30 p.m. Demonstration Sept. 19, 1 to 2:30 p.m. National Arboretum, Bonsai and Penjing Museum.

* Sept. 15-Oct. 15: Herbs used in Hispanic cultures: In recognition of Hispanic Heritage Month, this self-guided tour is like a treasure hunt with your own personal curator. Learn facts about selected medicinal, industrial and culinary herbs in Hispanic cultures. Discover symbols throughout the collection, and mark the location of plants described in the educational brochure available at the garden's entrance. Free. 8 a.m. to 5 p.m. National Arboretum, National Herb Garden.

* Sept. 17: Farming with the wild: Join Dan Imhoff as he guides us through a visual journey of the U.S. conservation-based agriculture movement. Imhoff spent 21/2 years researching farmers, ranchers, government agencies, land trust organizations and others who are working to integrate nature into agricultural landscapes. His stories are documented in the award-winning book "Farming with the Wild: Enhancing Biodiversity on Farms and Ranches," which will be available for sale and signing after the program. Noon to 1 p.m. Free. Preregistration is required. U. S. Botanic Garden.

* Sept. 18: Pollination celebration: This one is great for the kids. Learn about plants and their pollinators. Find the queen bee in a live, observational hive; take a tour of the terrace exhibit or get creative with children's activities. Talks and hands-on activities will be available for all ages. 10 a.m. to 5 p.m. Free. U. S. Botanic Garden.

* Sept. 22: Promoting pollinators in public places: A habitat-conservation workshop targeting resource managers, public gardeners, educators and others interested in enhancing habitat for butterflies, bees and pollinators in public spaces. Participants will receive resources on pollinators and their habitat to promote conservation efforts in gardens, parks, golf courses and utility properties. 8:30 a.m. to 4:30 p.m. Free. Registration deadline is Sept. 15. National Arboretum in co-sponsorship with National Park Service and University of Maryland.

Maryland

Brookside Gardens, 1800 Glenallan Ave., Wheaton. Registration and information, call 301-962-1451 to register, or call 301-962-1400 for information. Web site: www.brooksidegardens.org.

* Through Sept. 19: Wings of Fancy live butterfly show: Be surrounded by butterflies flying freely in a colorful garden of tropical flowering nectar plants. Learn about the butterfly life cycle. Observe exotic tropical species from Costa Rica in addition to Maryland natives. Fee $4 adults, $3 children. 10 a.m. to 4 p.m. daily. Brookside Gardens South Conservatory.

* Sept. 8: Favorite plants for the metro area: Holly Shimizu, executive director of the U. S. Botanic Garden, will discuss some of the plants she favors for gardens in this area based on their beauty and vigor. They will include trees, herbs, perennials and annuals. Hosted by Stadler Nurseries and the regional Landscape Contractors Association. Preregistration required. Fee $30. 5:30 p.m. at Stadler Nurseries, 6815 Olney-Laytonsville Rd. (Route 108), Laytonsville. For information and to register, call 301-948-0810.

* Sept. 11: Plant sale: Friends of Brookside Gardens will hold its annual fall plant sale, rain or shine. You can buy rare specimen plants that you generally can't get anywhere else and plants that have never before been available. Miniatures and collectibles from growers in the Mid-Atlantic states, and uncommon plants grown on-site for months in preparation for this sale, are offered on this date only. Members only from 8 to 10 a.m. The public is welcome from 10 a.m. to 4 p.m. Brookside Gardens.

* Sept. 18: Children's day: This day run by the Friends of Brookside Gardens has become a family tradition. The 25-acre botanical garden will be stocked with an array of nature activities designed for children. Free. 11 a.m. to 4 p.m. Brookside Gardens.

* Sept. 25, 26: National Capital Dahlia Society: The 69th annual Dahlia Show features spectacular flowers of award-winning growers, which will be on display Sept. 25 from 2 to 5 p.m. and Sept. 26 from 10 a.m. to 3 p.m. at Brookside Gardens.

Virginia

Northern Virginia Regional Park Authority, www.nvrpa.org

Potomac Overlook Regional Park, 2854 N. Marcey Rd., Arlington, 703-528-5406.

Pohick Bay Regional Park, 6501 Pohick Bay Dr., Lorton/Mason Neck, 703-339-6104.

Meadowlark Botanical Gardens, 9750 Meadowlark Gardens Ct., Vienna, 703-255-3631.

Green Spring Gardens Park, 4603 Green Spring Rd., Alexandria, 703-642-5173, www.greenspring.org

* Sept. 4: Northern Virginia Regional Park volunteer cleanup day: Help park staff members with indoor and outdoor chores at Potomac Overlook Regional Park. 10 a.m. to noon.

* Sept. 10: Figs; propagating, growing and cooking: Easy-to-grow figs add a tropical flair to your garden with their large, odd-shaped leaves and delectable fruit. Ed Raduazo, a passionate fig grower who cultivates at least half a dozen varieties in his Virginia garden, will share his experiences, tips, tricks and fig recipes. Enjoy a fig tasting and take home a little fig tree for your garden. Fee $18. 1:30 to 3 p.m. Reservations required. Green Spring Gardens Park.

* Sept. 11: Home lawn care made easy: Take care of your lawn like a professional. John Delsignore, lawn specialist from Pinecrest Golf Course, will lecture and answer questions on environmentally sound procedures for spring lawn care. Participants receive a full-year step-by-step maintenance calendar with tips on procedures and timing. Learn the tools that will keep you from sharing weeds with your neighbors. 9 to 11 a.m. Fee $9. Reservations required. Green Spring Gardens Park.

* Sept. 18: Naturalist-guided canoe trip: Explore one of the area's finest wildlife habitats on a canoe trip at the Northern Virginia Regional Park Authority's 1,000 acre Pohick Bay Regional Park on Mason Neck peninsula. See bald eagles, osprey and migrating birds. Bring binoculars and a field guide. Fee $25. 8 a.m. to noon. Reservations required. Pohick Bay Regional Park.

* Sept. 18: Gardening with butterflies: How can you increase the numbers of butterfly visitors to your garden? Horticulturist Laurie Short will answer this, offer tips on butterfly plants' upkeep and more. Handouts. Fee $5. 10 a.m. Reservations required. Meadowlark Botanical Gardens.

* Sept. 19: Equinox hike: Mark the change of seasons and explore the changes in flora and fauna on a two-mile hike of moderate difficulty. Free. 2 p.m. Reservations required. Potomac Overlook Regional Park.

* Sept. 25: Plant sale: The Potowmack hapter of the Virginia Native Plant Society will hold a plant sale at its Fall Fling from noon to 4 p.m. Native perennials, ferns and woody trees and shrubs will be on display and sold from Green Spring Garden's propagation beds, located behind the horticulture center. Free. Noon to 4 p.m. Green Spring Gardens Park.

* Sept. 25: Mushrooms of Potomac Overlook Regional Park: Get acquainted with the variety of fungi at this nature preserve and learn about their role in the forest on a guided hike. Free. 2 p.m. Reservations required. Potomac Overlook Regional Park.

* Sept. 26: Tree identification hike: Learn to identify trees in their fall grandeur at this indoor and outdoor program. Free. 2 p.m. Reservations required. Potomac Overlook Regional Park.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.