"Identifying Historic Plant Material" is one of the lectures in a morning devoted to gardening at Gunston Hall Saturday. Register between 9:30 and 10. The $10 fee includes a picnic lunch. (Call 703/550-92209)

Visitors to Woodlawn Plantation Saturday and Sunday will be given tours of the rose gardens, where most of the varieties date before 1850. Charles Bell, an authority on old roses and member of the Heritage Rose Society, will talk about the care and culture of old roses, Saturday at 10:30 and 2, and Sunday at 2. On Sunday at 10:30, George Smith, administrator of Woodlawn, will lead the group. Admission is $2 for adults and $1 for students. (Call 780-3118.)

LANDSCAPE IT YOURSELF - A two-part program on homelandscaping will be held Monday and Tuesday from 7:30 to 9 at the New Carrolton Branch Library, 7414 Riverdale Road, New Carrolton. Horticulture agents from Prince George's County Cooperative Extension Service will discuss principles of landscaping and selection of just the right plants. (Call 699-3500, extension 291.)

FOR MUM-LOVERS - Old Dominion Chrysanthemum Society is holding its 19th annual sale Saturday, starting at 9, in Giant Food Stores' parking lot at Steven Corners, U.S. 50 and Hillwood Avenue in Falls Church. About 150 cultivated varieties - 8,000 plants in all - will be looking for homes.

Potamac Chrysanthemum Society has its plant sale Saturday, too, from 8 to noon in four locations: at Bethesda United Church of Christ, Fernwood Road and Democracy Boulevard; and at Suburban Trust Company parking lots, in Gaithersburg Square; on U.S. 29 north of the White Oak Shopping Center; and on University Boulevard between Georgia Avenue and Viers Mill Road, Wheaton.