Looking back over the past three to four weeks, it's close to miraculous what you have achieved on the lawn. Where the lawn was in very poor shape a month ago, not one weed lives today. On imperfect lawns of a month ago, the weeds have succumbed to repeat applications of herbicides. As for the old grass, it has paled considerably from neglect, but don't worry. In the next two weeks, the old grass will become green, thanks to the turf management program you soon will inaugurate.

Meanwhile, this last weekend of August marks a critical point in your crusade to create a spectacular lawn over the next month. Major chores, including de-thatching, are on tap this weekend and early next week, some of which will be tackled manually on small lawns and mechanically on sprawling suburban and rural lawns. The thatch has got to come up, so let us explore your options and procedures.

Regardless of the lawn being renovated, disaster or 60-40 lawn (the lawn with at least 40 percent good grass), de-thatching is a prerequisite of seeding or over-seeding.

Thatch is any organic matter that contains "lignin," an enzyme. Lignin is found in all weeds and wild grasses (such as crab grass or goose grass) and in the stems and stalks of cultivated grasses, too. For example, if you had a tall fescue lawn that died this summer for one reason or another, the stalks and roots of the dead grass contain lignin. Of itself, lignin isn't harmful. It's just that any tissue containing lignin resists decay. Leaf blades of grass plants do not contain lignin, so they decay quickly on the lawn. However, dead weeds and other organic matter containing lignin will not decay, so this trash must be lifted from the lawn surface before seed can be applied.

Seed must come in contact with the soil if it is to germinate and produce a grass plant. If seed came to rest on a layer of thatch, the seed would probably germinate, but the seedling plant would die because the thatch would retain all moisture and deny the needed water to the plant to stay alive.

Any thatch is a liability on the lawn, but sometimes it is worse than others. Golf course superintendents will tolerate a quarter-inch of thatch on fairways, while the tolerance level for home lawns is usually a half-inch. Anything above this is cause for concern.

If you have killed your disaster lawn and will be seeding over the Labor Day weekend, the thatch must be removed. Even on 60-40 lawns where weeds have been destroyed, thatch must come up so seed has a chance to "bite" into the soil.

On lawns not being renovated, thatch is not a problem now because no over-seeding is contemplated. However, you should strongly consider attacking the thatch here in late August so there is some decay between now and mid-October. An application of hydrated lime at two pounds per 1,000 square feet of lawn area now will drive soil bacteria from the soil into the thatch. By late October, some of the lignified thatch will be gone. Another treatment early in April should decay leftover thatch.

How you de-thatch is a matter of preference.

On small lawns of 1,000 square feet or less, such as those found at condominiums and town houses, hand raking is equal to the task. For this, you need a bamboo or flexible tine steel rake that bends at the base when you apply pressure on the handle. Wear gloves, too. De-thatch this weekend or any day next week. Don't work when the lawn is wet with dew or after a rain.

Rake for short distances up to six feet, then make a pile. Bear down hard on the handle so the canes or tines dig into the top layer of soil -- the idea is to raise thatch and organic matter from the lawn surface. If you are de-thatching a 60-40 lawn with existing grass plants, the combing action of the rake will not dig plants from the soil as long as the soil is dry.

After the initial raking, stuff the piles of thatch and debris into plastic trash can liners, then rake at a 90-degree angle to pick up plant residue missed on the first try. This second raking will be easy since most of the debris will already have been lifted from the soil.

If your lawn is greater than 1,000 square feet, hand-raking is out of the question. You need a powered machine to do the job effectively. If you have not already reserved a machine, call a rental store pronto and make arrangements. De-thatching can be done any time up to and including next weekend, even an evening next week.

Whether you rent a power rake or a "verticut" machine, make certain you know how to operate the unit before leaving the rental store. The equipment basically functions the same way: hundreds of stationary tines on the rotating shaft of the power rake, or nine to 21 stationary, double-edged steel blades on the rotating shaft of the verticut machine. Tines or blades are supposed to comb into the soil to lift the thatch and debris from the surface so it can then be raked or vacuumed from the lawn.

Have the rental store clerk demonstrate how to fire up the machine, even making a dry run yourself so you have it down pat. Pay particular attention to the front control for adjusting the depth to which the tines or blades comb the soil. A wing-nut usually controls this depth gauge, so be sure to identify it.

Back home, fill the fuel tank, then fire up the machine and make a trial pass on a three-foot strip of the lawn. Shut down the engine while moving the unit to the side. Then, get down on all fours and measure how deeply the machine is combing the surface of the lawn. The slits in the soil should be a quarter to a half-inch deep. If you're lucky, the machine will be combing to this depth when you run your test; if not, adjust the wing-nut to raise or lower the tines or blades, then make another pass on a different strip of the lawn. Measure again. Keep adjusting until the unit combs to the precise depth, then de-thatch the lawn.

Some practical hints will simplify the job.

Wear sturdy work pants that cover both legs. Wear high-top boots, too. Having used these machines for over a generation, we've had our share of bruised ankles and legs from stones hurled back by the rotating tines and blades.

These machines never have self-propelled wheels, but they behave as if they do. The tines or blades seem to propel the machine forward, but keep a firm grip on the handle so the machine grips the soil at all times, not gliding over it. Keeping the machine under control isn't easy, as you'll soon discover. Sturdy, gloved hands are essential.

De-thatch the border first, then the rest of the lawn. Work in a north-south direction first, de-thatching the entire lawn this way, then come back and proceed in an east-west direction. By crossing the lawn at right angles, all thatch should be lifted from the lawn surface.

Some time estimates: a 2,500-square-foot lawn can be de-thatched in an hour, a 5,000-square-foot lawn in two hours. Sprawling lawns could well have you de-thatching for most of the day.

If you think de-thatching is difficult, wait until you return the machine to the rental store and are confronted with the massive raking job you left behind.

Hand-raking the thatch (lifted off the lawn surface, of course) from a 1,000-square-foot lawn is a 90-minute job, but four to five hours for 2,500 square feet, and 10 hours for 5,000 square feet. Faced with this challenge, you might rather do anything else, but there is an escape. It's the lawn vacuum, specifically the Billy Goat vac, available from the same store where you rented the de-thatching machine.

Some pointers:

You need not de-thatch and vacuum the same day. Even with rain, the thatch won't be pounded into the soil to escape the vacuum action of the Billy Goat. A few days between de-thatching and vacuuming won't matter.

Some Billy Goats come with self-propelled wheels, making the job much easier for those with massive lawns undergoing renovation. If you rely on a self-propelled mower for cutting the lawn, insist on the self-propelled Billy Goat.

Billy Goats work so well that you'll have to be emptying the vacuum every three to five minutes. After the first few passes on the lawn, shut down the machine and check the level of debris in the bag. Before it fills to the top, empty the bag on the driveway or atop a plastic sheet on the sidewalk. A few dozen heavy-duty plastic trash can liners will be needed to bag the thatch, after which you can shower and collapse. We didn't say it was going to be easy.

Jack Eden is the host of "Over the Garden Fence" Sundays from 10 a.m. to 1 p.m. on WTOP Radio (1500 AM).