The American Horticultural Society will have an open house and bull roast at George Washington's River Farm, Mount Vernon, Sunday, May 7, noon to 4 p.m. The public is invited. There will be an admission charge of $2 for adults, 50 cents for children 6 to 12, to benefit the society's educational programs.

Dr. Henry M. Cathey, president of the society, has put together an outstanding program for gardeners, with top scientists participating, including Dr. Howard Heggestad, USDA Beltsville research plant pathologist, Dr. Frank Santamour, National Arboretum research geneticist, Dr. Robert A. Wearne, USDA Extension Service horticulturist, and Dr. Francis Gouin, University of Maryland horticulturist.

Cathey is also chief of the florist and nursery crops laboratory, USDA Agricultural Research Service, Beltsville, and is considered one of the top authorities in the country on floriculture.He is probably one of the few who could have assembled such as an array of talent for such an occasion.

Heggestad will deal with the impact of air pollution on vegetation. Wearne will cover gardening for food and fun. Santamour will discuss his research on how to save urban trees from air pollution and which trees are best for city planting. Gouin will provide guides for use of sewage sludge in growing plants. Recent developments, he says, may soon permit us to provide screened composted sewage sludge, manufactured in 60 days or less, with texture and odor similar to garden compost, to nurserymen and gardeners for growing plants. It will save a lot of money and solve a serious environmental problem.

Giant pumpkin seeds will be given to all attendees with written instructions on how to grow them. Those not wishing to purchase food can bring a blanket and their own picnic lunch.

To get to River Farm, go south on George Washington Memorial Parkway, past the stone bridge overpass 3 1/2 miles south in Alexandria, and turn left at the next exit to enter grounds.