GALLAHANS have been farming in southern Prince George's County for more than a century, but recently they've had to make some adjustments. Five years ago, the family began replacing 60 acres of tobacco with apple and peach orchards, and added almost two dozen more acres of strawberry plants to the three planted in 1956. They also began cultivating other popular pick-your-own crops such as blueberries, spring onions, eggplant, tomatoes, peppers, beans and corn.

"We really relied on that tobacco," says Pat Gallahan, 53, who operates Cherry Hill Farm with her husband, William Alton Gallahan III, and their sons Danny and Mike. The 180-acre spread borders both sides of the winding country lane named after them in Clinton.

The Gallahans say that with tobacco consumption down nationally, coupled with increased European tobacco production, local farmers have had to turn to other, less labor-intensive crops. (The Maryland Farm Bureau reports a pound of average-grade tobacco that sold for $1.75 in 1981 is now down to $1.18. At one point this year, low-quality tobacco dropped to five cents a pound.)

The farmers hope the new crops will keep them in the farming business, but if not, they are encouraged by the ongoing land rush as developers send property values skyrocketing. But until then, say the Gallahans, they'll continue planting berries, beans and beets and keep their fishpond stocked with largemouth bass, bluegills and catfish.

Other area farmers also have added acres of pick-your-own crops, a trend that has created more supply than demand in some cases. But some farmers have resorted to including exotic crops along with snap beans, cucumbers and the like.

At Fauquier County's Lakota Gardens, pickers have their choice of such Asian vegetables as bok choy, Chinese cabbage, Thai eggplants, yard-long beans and a dozen types of peppers. The farm also encourages pickers to cut their own snapdragons, gladiolus, carnations, shasta and gerbera daisies. In Fredericksburg, the asparagus lover's dream comes true with cut-your-own asparagus at only 80 cents a pound. Take that, Giant and Safeway.

In Vienna, the 26-acre Potomac Vegetable Farms offers the standard vegetables plus scoop-your-own compost for a dollar a bushel. If you want to tell them to take their job and shovel it, the farm offers free delivery of truckload purchases of the horse manure-and-bedding mixture to gardeners within an eight-mile radius.

After a spring of rain and prime growing conditions, farmers are reporting one of the best strawberry crops in memory. Mike Gallahan, who oversees the orchards at Cherry Hill, predicts that late July will bring a bumper crop of peaches.

Here's a roundup of most area farms that open their fields to the public. Many also have stands selling produce, smoked hams, jams and jellies. Remember, crop availability changes daily, so call ahead. MARYLAND

ALLEGANY COUNTY STEGMAIER ORCHARDS 301/722-5266. Md. Rte. 8, Cumberland. Strawberries, peaches, apples, bramble plants, lettuce, squash, broccoli.

ANNE ARUNDEL COUNTY MT. AIRY U-PICK FARM 301/798-1862. 832 Mt. Airy Road, Davidsonville. Strawberries, red raspberries, corn, green peas, pumpkins. PUMPHREYS HOME GROWN 301/987-0669. 10 Maryland Rte. 3 South, Millersville. Large selection of vegetables; topsoil. ROBEY FARM 301/674-7285, 674-6168. 1392 Waugh Chapel Rd., Odenton. Strawberries. WATTS FARM 301/858-1393. 948 St. Stephens Church Road, Gambrills. Strawberries.

BALTIMORE COUNTY ARMACOST FARMS 301/239-3440. 16926 Gorsuch Mill Rd., Upperco. Strawberries by appointment, peaches, apples. BERRY PATCH FARM 301/429-2922. 5500 Rte. 91 Fowblesburg. Thornless blackerries, red raspberries and blueberries. HUBER'S FARM 301/679-1948, 679-1941. 12010 Old Philadelphia Rd., Bradshaw. Strawberries, complete selection of vegetables. MOORE'S ORCHARD 301/256-5982. 5242 E. Joppa Rd., Perry Hall. Apples, peaches. STOECKER FARMS 301/329-2140. 2501 Gibson Rd., White Hall. Strawberries, thornless blackberries, black raspberries, red raspberries, complete selection of vegetables.

CALVERT COUNTY SEIDEL FARM -- 301/535-2128. Md. Rte. 263, Plum Point Road, Huntingtown. Thornless blackberries, wine and table grapes. TROTTS FRESH FARM PRODUCE -- 301/741-5393. Md. Rte. 4, Dunkirk. Tomatoes, turnips.

CARROLL COUNTY BACHMAN VALLEY FARM 301/876-1776, 848-7719. 460 Old Bachman Valley Rd., Westminster. Strawberries, sweet and sour cherries, peas. BAUGHER'S FARM 301/857-0111, 848-7490, 848-5541. 1236 Baugher Rd., Westminster. Strawberries, black raspberries, sweet and sour cherries, sweet peas, green beans. BLUE RIDGE FARMS 301/848-3431. 2307 Uniontown Rd., Westminster. Strawberries, thornless blackberries. BUPPERT'S FARM 301/795-6484. 795-6815. 6914 Ridge Rd., Marriottsville. Thornless blackberries, strawberries. PHILLIPS BERRY FARM 301/781-6339. 1801 Bennett Rd., Eldersburg. Strawberries, red raspberries. SEWELL'S FARM 301/756-4397. 3400 Harney Rd., Taneytown. Strawberries, green peas, string beans, lima beans. UNGER'S FRUIT FARM 301/374-9243. 2020 Albert Rill Rd., Hampstead. Strawberries, peaches, apples, sweet and sour cherries.

CHARLES COUNTY STANLEY FARM 301/753-6229. Md. Rte. 484, Marbury. Strawberries, black and red raspberries, thornless blackberries, apples, large selection of vegetables.

FREDERICK COUNTY CATOCTIN MT. ORCHARD 301/271-2737. U.S. 15 near Franklinville Road, Thurmont. Strawberries, black raspberries, blueberries, sweet cherries. GLADE-LINK FARMS 301/898-7131. 9332 Links Bridge Rd., Walkersville. Strawberries, blueberries, broccoli, pumpkins, spinach, turnips, kale, cauliflower. HIGH HOPES FARM 301/371-6228. 2704 Bennies Hill Rd., Md. Rte. 17 south of Middletown. Strawberries, red raspberries. PRYOR'S ORCHARD 301/271-2693. 13841 B Pryor Rd., Thurmont. Sweet and sour cherries. THANKSGIVING FARMS 301/662-1291. 1619 Buckeystown Pike, Adamstown. Strawberries, snap beans, broccoli, green peas, baby limas, fava beans.

HOWARD COUNTY

DUNTEACHIN FARM 301/465-8310, 465-0503. 5377 Kerger Rd., Ellicot City. Blackberries, pumpkins. KOANDAH GARDENS 301/531-5444. Md. Rtes. 108 and 216, Highland. Strawberries. LARRILAND FARM 301/442-2605, 854-6110. Three miles south of I-70 on Md. Rte. 94 at 2525 Florence Rd., Woodbine. Strawberries, thornless blackberries, red raspberries, peaches, apples, sweet cherries, blueberries, complete selection of vegetables. SHARP FARM 301/489-4630. 3779 Sharp Rd., Glenwood. Strawberries, complete selection of vegetables.

MONTGOMERY COUNTY BECRAFT'S FARM 301/236-4545. 14722 New Hampshire Ave., Silver Spring. Sour cherries, snap beans, okra, turnips, kale. BUTLER'S ORCHARD 301/972-3299. 22200 Davis Mill Rd., Germantown. Strawberries, red raspberries, thornless blackberries, large selection of vegetables. DAYSPRING CHURCH FARM 301/428-3454. 11301 Neelsville Church Rd., Germantown. Strawberries, thornless blackberries, red raspberries, snap beans, tomatoes, pumpkins. HOMESTEAD FARM 301/977-3761, 977-3898. 15600 Sugarland Rd., Poolesville. Thornless blackberries, red raspberries, table grapes, strawberries, large selection of vegetables.

INNSTEAD FARM 301/972-7247. 18020 Edwards Ferry Rd., Poolesville. Strawberries, thornless blackberries. LEWIS ORCHARD 301/349-4101. 18900 Peach Tree Rd., Dickerson. Apples. PAUL'S PATCH 301/774-6779. 805 Greenbridge Rd., Brookville. Strawberries, black and red raspberries, wine grapes, Italian paste tomatoes, flowers. RASPBERRY HILLS 301/774-2543. 4600 Sundown Rd., Laytonsville. Strawberries, red raspberries (fall variety). ROCK HILL ORCHARD 301/831-7427. 28600 Ridge Rd., Mt. Airy. Large selection of fruits, snap beans, peas, pumpkins, flowers..

PRINCE GEORGE'S COUNTY CHERRY HILL FARM 301/292-4642, 292-1928. 12300 Gallahan Road, Clinton. Strawberries, apples, blueberries, peaches, large selection of vegetables. DARROW BERRY FARM 301/390-6611, 390-6191. Bell Station and Glenn Dale roads, Glenn Dale. Strawberries, blackberries, red raspberries, snap beans, lima beans, pumpkins. HARE'S BERRY FARM 301/868-4755, 868-7704. 7806 Colonial Lane near Andrews Air Force Base, Clinton. Strawberries, fishing. JOHNSON'S BERRY FARM 301/627-8316. 17000 Swanson Rd., Upper Marlboro. Strawberries, thornless blackberries, black and red raspberries, blueberries. KERBY'S MARKET 301/567-4375. 8407 Indian Head Highway, Oxon Hill. Complete selection of vegetables. MAPLE HILL FARM 301/292-6393. 4203 Floral Park Rd., Brandywine. Strawberries. MILLER FARMS 301/297-5878, 297-9370. 10140 Piscataway Rd., Clinton. Strawberries, large selection of vegetables. PARKER FARMS 301/292-3940, 12720 Parker Lane, Clinton. Thornless blackberries, strawberries, table grapes, large selection of vegetables, canning supplies, organic grains. QUEEN ANNE FARM 301/249-6475. 18000 Central Ave., Mitchellville. Herbs, pumpkins. ROBIN HILL FARM 301/579-6844. 15800 Croom Rd., Brandywine. Apples, pumpkins. WALZEL FARM 301/292-1647. 11108 Fort Washington Rd., Fort Washington. Strawberries. WYVILL'S STRAWBERRIES 301/627-3975. 14700 Mt. Calvert Rd., Upper Marlboro. Strawberries.

ST. MARY'S COUNTY OWEN'S BERRY FARM 301/862-3427. Md. Rte. 235, Lexington Park. Strawberries, green peas, pumpkins. VIRGINIA

CULPEPER COUNTY DOUBLE NICKLE FARM 703/825-3087. Va. Rte. 734, Brandy Station. Complete selection of vegetables. Stawberries. RAPIDAN BERRY GARDENS 703/672-4235; in state, 800/552-2379. Va. Rte. 615, Rapidan. Strawberries, thornless blackberries, watermelon, blueberries, complete selection of vegetables. MOORMONT ORCHARDS 703/672-2730; toll free (after June 15): 800/572-2262 in Virginia, 522-0463 in D.C. and Maryland. Va. Rte. 697, Rapidan. Peaches, apples, nectarines, damson plums, pears, grapes.

FAIRFAX COUNTY COX FARMS 703/830-4121, 281-0165. 15621 Braddock Rd., Centreville. Strawberries. POTOMAC VEGETABLE FARMS 759-2119, 9627 Leesburg Pike, Vienna. Red raspberries, large selection of vegetables, scoop-your-own compost.

FAUQUIER COUNTY LAKOTA GARDENS 703/439-8878. Va. Rte. 660 off Rte. 651, Remington. Large selection of vegetables, Asian vegetables, herbs and flowers. MANOR LANE BERRY FARM 703/347-4883. Va. Rte. 681, five miles west of Warrenton. Strawberries, thornless blackberries. WINDER MERE FARM 703/347-2239. Va. Rte. 674, Broad Run. Peas, green beans, canning tomatoes, kidney beans, onions, peppers, squash, cucumbers, cabbage, broccoli.

LOUDOUN COUNTY BROWNDALE PRODUCE 703/338-7445. Va. Business Rte. 7, Purcellville. Strawberries, full selection of vegetables. CHANTILLY FARM MARKET 703/378-6623, 378-7113. U.S. 15 south, Leesburg. Strawberries. HILL HIGH ORCHARD 703/471-1448, 478-8032, Va. Rte. 7, Round Hill. Black raspberries. JEFFERSON FARM 703/662-8705. Va. Rte. 670 north, Clearbrook. Strawberries, red raspberries, thornless blackberries, small mulberries, rhubarb, table grapes, apples. SUNSHINE FARMS 703/338-2499. Va. Rte. 287, Purcellville. Red raspberries. WHEATLAND VEGETABLE FARMS 703/882-3568. Va. Rte. 287 near Rte. 9, Purcellville. Strawberries, black and red raspberries, beans, leaf lettuce, spinach, compost.

ORANGE COUNTY DOUBLE B FARMS In Virginia, 800/572-2261; elsewhere, 703/854-4277. Va. Rte. 20 off Rte. 3, Rhoadesville. Strawberries, thornless blackberries, red raspberries, blueberries.

RAPPAHANNOCK COUNTY JORDAN RIVER FARM 703/636-9388, Va. Rte. 637, Huntly. Strawberries organically grown, peas, lettuce.

SPOTSYLVANIA COUNTY RICK & VAN'S 703/775-7890. Va. Rte. 3, Fredericksburg. Strawberries, complete selection of vegetables. BELVEDERE PLANTATION 800/641-1212, 703/371-8494. Va. Rte. 17, Fredericksburg. Strawberries, thornless blackberries, red raspberries, broccoli, peas, sweet corn. SNEAD'S ASPARAGUS FARM 703/371-9328. Va. Rte. 17, Fredericksburg. Strawberries, asparagus, English and snow peas, sugar snap peas. GREAT OAK FARM 703/775-7668. Va. Rte. 623, Jersey. Blueberries.

WARREN COUNTY HARMONY HOLLOW ORCHARD 703/636-2009. Va. Rte. 604, Front Royal. Blueberries.

WESTMORELAND COUNTY WESTMORELAND BERRY FARM 804/224-9171. Va. Rte. 637, Oak Grove. Scottish tayberries, strawberries, thornless blackberries and black, red, heritage and purple raspberries, blueberries. PERRY'S BLUEBERRIES 804/224-0440. Va. Rte. 3 in Montross and Rte. 638 in Oak Grove. Blueberries.