"HERE IT COMES!" yells nine-year-old Katie as a tractor-pulled haywagon lumbers down the hill and parks near the stand at Potomac Vegetable Farms, a bucolic family farm in the shadow of Tysons Corner. This is the vehicle that will transport us -- four adults, six children and one Cabbage Patch Kid -- to the farm's far-flung pumpkin patch.

There are quicker and easier ways to buy a pumpkin -- the supermarkets are full of them. And you won't save any money for your time and labor. At pick-your-own places, pumpkins weigh in at 20 to 25 cents a pound, which is not much less than you'd pay at the supermarket, particularly if you figure in your gasoline cost. But when you pick your own pumpkins you get a bonus: a fall adventure in the country.

At Potomac Vegetable Farms, the haywagon shuttle leaves for the pumpkin patch weekend afternoons at 1:30, 2:30 and 3:30. At other times you're welcome to hike it, but if you pick a weighty pumpkin, it's a heavy walk back.

"Want to see the tractor, Michael?" prompts a father who wants to see the tractor while the other children scramble aboard, roll in the hay, throw it at each other and dive into it, yelling, "GI Joe, American Hero!"

The "hay," confesses our genial driver, is really straw and its main function seems to be to cushion the shock as we bounce up the hill past the raspberry patch and the cider shed (pressings Saturday at 2) and the goose pen and across a field where a cow grazes among the rusted farm implements. Then we lurch downhill and into the fall-smelling woods.

"We're following the trail of pumpkins," exclaims my three-year-old, noting the pumpkins sitting on up-ended baskets on either side of the dirt path. The trail ends at the pumpkin patch, where bright orange pumpkins peek from under green leaves.

"The pumpkins got ripe early this year and the weeds just grew up over them," explains our driver, and the kids scramble to find perfect potential jack-o-lanterns. Perfect, however, is in the eye of the beholder.

"Here's one with a scar," says four-year-old Andrew, gleefully lugging an oversized pumpkin with a lurid brown gash down the middle of what would be its face. "I'm going to call him Scarface."

At Butler's Orchard in Germantown, some pumpkins are already in Halloween dress.

"Mom, come here," orders my three-year- old. "I just saw a man with a pumpkin head and that one's a lady -- it has make-up on. And Mickey Mouse is in the train. And there's a witch and a house and a horsey!"

Pumpkins growing quietly in a field don't generate this kind of excitement, and Butler's has artfully created "Pumpkinland," a sort of "storyland" between its pumpkin patch and its picnic area. The pumpkins, fleshed out with large zucchini limbs, perform in tableaux. A pumpkin version of Lucy dispenses psychiatric advice while a pumpkin Snoopy lounges on his doghouse and a pumpkin Charlie Brown looks suitably downcast.

A pumpkin-faced pirate opens an old chest filled with a treasure of golden gourds. The big, bad wolf, menacing in spite of his pumpkin face, preys on the three little pigs. There are also the Old Woman in the Shoe, Spiderman and a cast of hundreds.

For the raw material you need to create your own Snoopy or Spiderman, you have to go to the pumpkin patch, where pumpkins and gourds are growing among the cornstalks.

"This one's like a girl because it has curls," says a little girl, choosing a pumpkin with some sort of extraneous growth on both sides.

For a smaller boy, the choice is dictated by size.

"This is the only one he can carry, so it's going to be his," says his mother.

At Robin Hill Farm Nursery, a 152-acre spread in rural Prince George's County, there's a big, bright orange fiberglass pumpkin shell to play in, but the main attractions are the live animals.

"Hi, piggy. Hi, pig pig," says my three-year- old, running from the baby pigs to the rabbits to the ducks to the calf, whose name is Champ and who was the Prince George's County Fair poster animal. For pumpkin-picking season, farmers Russell and Shirley Watson have taken the animals out of the fields and ponds and barns and placed them in pens where visiting children can see and pet them. There are also picnic tables on the grounds and, on weekend afternoons, hayrides around the farm.

The pumpkins are strewn in a field that stretches as far as the eye can see. After toying with a bulbous green-and-orange number, my daughter chooses a conventional orange pumpkin, medium size.

"It's not a daddy one. It's not a mommy one. It's not a baby one," she explains. "It's a sister one."

BUTLER'S ORCHARD -- 22200 Davis Mill Rd., Germantown. 301/972-3299. Hours: 10 a.m. to 5:30 p.m. From the Beltway, take I-270 north, then Rte. 118 east toward Damascus. At deadend on Rte. 355, turn left for 1/2 mile, then bear right on Rte. 27, then right on Brink Road and follow signs to farm.

POTOMAC VEGETABLE FARMS -- 9627 Leesburg Pike, Vienna. 703/759-2119. Hours: 9 a.m. to 7 p.m. From the Beltway, take Rte. 7 west about five miles past Tysons Corner; farm is on left.

ROBIN HILL FARM NURSERY -- 15800 Croom Rd., Brandywine. 301/579-6844. Hours: 9 a.m. to dark. From the Beltway, take Rte. 4 south to U.S. 301, then four miles south on 301, then left onto Rte. 382 (Croom Road) for about eight miles to farm.

OTHER FERTILE FIELDS

Here are some other Washington-area patches to check out if you'd like to pick your own Great Pumpkin this weekend.

MARYLAND

ANNE ARUNDEL COUNTY

MOUNT AIRY U-PICK UP FARM -- 832 Mount Airy Rd., Davidsonville. 301/798-1862. Call for reservations.

PUMPHREY'S HOME GROWN -- Rte. 3 south, Millersville. 301/987-0669. Hours: 9 a.m. to 6 p.m.

MONTGOMERY COUNTY

BECRAFT'S FARM PRODUCE -- 14722 New Hampshire Ave., Silver Spring. 301/236-4545. Hours: 10 a.m. to 6 p.m.

HOMESTEAD FARM -- 15600 Sugarland Rd., Poolesville. 301/926-6999. Hours: 7 a.m. to 6 p.m.

DAYSPRING CHURCH FARM -- 11302 Neelsville Church Rd., Germantown. 301/428-3454. Hours: Saturday 9 a.m. to 6 p.m.; Sunday noon to 5 p.m.

ROCK HILL ORCHARD -- 28600 Ridge Rd., Mount Airy. 301/831-7427. Hours: 9 a.m. to 6 p.m.

PRINCE GEORGE'S COUNTY

DARROW BERRY FARM -- 5900 Bell Station Rd. (field 1), Glenn Dale. 301/390-6611, 390-6191. Hours: 10 a.m to 6 p.m.

E.A. PARKER AND SONS -- 12720 Parker Lane., Clinton. 301/292-3940. Hours: Saturday 9 a.m. to 7 p.m. and Sunday 9 a.m. to 6 p.m.

VIRGINIA

CHESTERFIELD COUNTY

KELMARBI FARM -- 4400 Kelmarbi Rd., Colonial Heights. 804/526-4369, 520-4368. Hours: 8 a.m. to dusk.

FAIRFAX COUNTY

COX FARMS -- 15621 Braddock Rd., Centreville. 703/830-4121. Hours: 9 a.m. to 5 p.m.

LAKOTA GARDENS -- Rte. 1,(take U.S. 29 south to Remington, turn right at caution light to 651, three miles to right on 660 and follow signs), Remington. 703/439- 8878. Hours: 10 a.m. to 6 p.m.

LOUDOUN COUNTY

CHANTILLY FARM -- Route 2 (21/2 miles south of Leesburg on Route 15), Warrenton. 703/378-6623. Hours: 9 a.m. to 6 p.m.

WHEATLAND VEGETABLE FARMS -- Rt. 1, Purcellville. 703/882-3568. Hours: 8 a.m. to 8 p.m.

SPOTSYLVANIA COUNTY

BELVEDERE PLANTATION -- Star Rte. (7 miles southeast of Fredericksburg on Rte. 17. From I-95 take Massaponas exit (south-17 bypass). Go east 6 miles on south-17 bypass. Continue straight ahead 1 mile on Rte. 17. Fredericksburg. 703/371-8494, 1/800/641-1212. Hours: Friday 4 p.m. to 7 p.m.; Saturday 9 a.m. to 6 p.m., closed on Sunday.