DEPENDING ON the size of your lawn and garden you may want to consider buying or renting a mulching machine this fall.

The machine chops up dead leaves, tree limbs, weeds and twigs to make mulch for your garden.

"Mulches," explains "The Complete Book of Gardening," "smother annual weeds and can be used to cover the soil between shrubs, roses, fruit bushes, herbs and vegetables." The mulch helps hold the moisture in the soil around these plants.

Most gardeners prefer to apply mulch once in the fall and sometimes in the spring.

The mulching machines come in two sizes: one for leaves and small twigs (a shredder) and one for larger limbs (a chipper). Both look like a small chest with an engine in back, about 3-by-5-by-2 cubic feet. The blades on the inside cut the leaves or branches. Then the pieces are forced through a screen to refine them further.

The decision whether to rent or buy depends on how large an area you have and how tidy a gardener you are.

Ralph Lemieux at Lane's Gardening Center in McLean says, "It depends on the yard. Yards with more trees will get more leaves and sticks. The need for a mulching increases. Having your own machine may pay for itself. I recommend buying the machines if the homeowner has a garden to use the compost in."

Don Whitehead, manager of Bethesda Pools and Hardware, thinks that your decision to use any mulching machine at all depends on whether or not you have a garden -- somewhere to put all the mulch you've grounded up. "The only reason to use a mulcher is to provide mulch for your garden. If you don't have one I would suggest just raking up the leaves and twigs and putting them at the curb for the garbage collector," says Whitehead.

Bruce Kibler of W.H. Rentals, where they sell and rent mulching machines agrees: "It depends on the kind of use the customer will get from it. For five acres we suggest buying a machine. Farmers, for instance, should buy. Homeowners should buy only if they have that much land."

Chris Leatherwood, manager of Rental Tools in Silver Spring, not surprisingly thinks renting is economically more feasible: "Most people only use a mulching machine once or twice a year. The price of the rental pays for itself."

Don Brown, manager of Oxon Hill Rentals, adds: "When you rent you don't have to worry about maintenance. Letting the machine sit in the garage all year round is a waste of storage space. And when a machine goes unattended for that long, the gas as well as parts such as the spark plugs go bad and have to be replaced."

Rick Wise, manager of Beltsville Rental Center, says, "It's cheaper to rent the chipper the large mulching machine than to buy it. But it may prove more economical to buy the leaf mulcher than to rent it, since you may use it more often."

A group of neighbors in northwest Washington chipped in together for a leaf mulcher. "It's worked out well," says one of the neighbors, "but since our family uses it the most, we end up paying for most of the repairs and maintenance. However, it's still been cheaper than renting a machine twice a year." BUYING

Hechinger's carries a 5-horse-power mulching machine for leaves by Yardman for $449.99. The larger 10-horse-power machine, or "Chipper mulcher," grinds up branches and costs $499.99.

At Lane's Gardening Center in McLean, the leaf mulchers cost between $489 and $589 while the larger chippers cost between $840 and $940.

W.H. Rentals, despite its name, also sells mulching equipment. The medium-size shredder runs about $590, while the chipper is $1,300. They don't sell the leaf mulcher -- only rent it. RENTING

At W.H. Rentals, a mulcher for leaves only costs $30 a day; an in-between size for twigs costs $40 a day, and the chipper for branches costs $50 a day.

Beltsville Rental Center rents a machine that mulches leaves and small branches up to a half inch in diameter for $27 a day and $79 per week. The larger machine cuts up branches and costs $44 a day and $120 per week. The machines rented out by Beltsville are made by MacKissic, Dia-dem and Didier.

At Bethesda Pools and Hardware Equipment, a leaf mulching machine costs $13.50 for a minimum of three hours and $27 for the day.

Oxon Hill Rentals charge $5 an hour (minimum of three hours) for their leaf mulcher. They charge $45 for a minimum of four hours (or $65 per day) for their larger mulching machine.

Rental Tools in Silver Spring rents a leaf mulcher for $18 a day and a tree limb or brush mulcher for $48 a day.

The above are only a sampling of area mulcher sales and rentals. Check your yellow pages for others.