A sparse and weedy lawn is an indication that you have a turf problem, not a weed problem. If the turf was thick and vigorous, weeds could not find a place to establish themselves. Applying chemical herbicides will only leave more holes in the lawn for new weeds to fill in. Therefore your best approach to weed control is to either 1) over-seed an existing lawn, or 2) completely remove the old lawn and install a new one. Before choosing either approach you must first correct any underlying problems that contributed to the lawn's decline.
Solving Underlying Problems
The big mistake most gardeners make is attempting to reseed without first taking a critical assessment of the environmental and cultural factors contributing to lawn decline. Failure to address these issues prior to re-seeding will seriously compromise the success of the effort.
Low soil fertility or improper pH: This can be determined with an inexpensive soil test kit. Many people omit this simple procedure, but without it you're shooting in the dark. You need a pH around 6.0 to 6.5 for most turf species. If you haven't applied lime in the last three years, assume the pH is low and apply pelletized lime at the rate of 40-50 lbs. per 1000 sq. feet.
Improper mowing height and frequency: Probably the single most common factor of turf decline is mowing too close and too infrequently. In general, for cool season grasses, 2 ½" to 3 ½" is the best cutting height for a healthy lawn. You should mow often enough to remove no more than one third of the grass blade at any one cutting.
Soil compaction: This is a common problem in old lawns where moss is present. Rent a core- type aerator that pulls plugs of soil out of the ground. Rake Pine Bark Soil Conditioner or Leafgro into the holes to keep them open. You can omit this step if you are doing a complete renovation and using a tiller. (See below)
Deep shade: It's a losing battle to install turf in deep shade. Wherever you can, convert undesirable areas for turf into gardens and reduce your labor. Plant evergreen ground covers like Periwinkle, English Ivy, or Pachysandra in heavily shaded areas and don't try to seed.
1. Over-seeding an Existing Lawn
Criterion: If the lawn is at least 50% desirable turf it is easier to over-seed.
Timing: The best time to over-seed is late August to early October for most cool season grasses. Eighty percent of all root growth occurs during this period. In addition there is less competition from weeds and less demand for water. In shaded areas, or areas with tall perimeter trees, seeding in Oct. means falling leaves will interfere with proper development of new grass plants. The second best time is late April to early May, or whenever you get daytime temperatures consistently in the high 60's. You will have more problems with watering and weeds in Spring than in Fall. If you must install a new lawn in the heat of summer, sod is more durable than seed, although it is more expensive.
Turf Selection: Whether you are over-seeding or completely renovating your lawn, the best choice for most of the Washington metro area is turf type tall fescue. The newer cultivars are highly attractive, largely disease resistant, heat tolerant, don't build up thatch, and are wear-resistant once established. Turf type tall fescue will tolerate a wide range of conditions, but to do well should have 4 hours or more of sun. Many cultivars are available. A blend of several gives you more diversification and a greater chance of success. Let us recommend the best cultivars for this area.
Fine Fescue is more shade tolerant than Tall Fescue, but not as durable under traffic and you will probably have to reseed yearly to keep a thick stand.
Mow the grass to about an inch.
With a garden or special de-thatching rake, vigorously remove as much old thatch and weeds as possible. If the area is large you may want to rent a vertical mower called a verticutter to dethatch and loosen the soil surface. Adjust the verticutter to cut through the turf into the soil 1/4 to 1/2".
Apply new lawn starter fertilizer and pelletized lime, if you need it, with a drop spreader at the recommended setting.
To get even seed coverage, divide the total seed in half, set the spreader at half the recommended setting and seed half across, and half up and down the lawn. Rake the seed in lightly.
From the time you seed until germination, the lawn must be kept moist. Try to water in the morning, not at night, which promotes disease in both lawn and garden.
Do not mow the grass until it has reached its normal mowing height of about 3".
You may need to over-seed for several years to build up a thick, weed-proof turf.
Fertilize two (or three) times in the Fall, once in September and once in October or early November, with a good organic fertilizer like the ‘Bradfield Organics Luscious Lawn & Garden’ or the 'Ringer Lawn Restore'. The third application is helpful if grass clippings are removed, the lawn is heavily used, there is a severe crabgrass problem, or there has been pest or stress damage.
2. Complete Lawn Renovation
Criterion: Consider this option if the lawn is clearly less than 50% desirable turf species.
Complete Renovation Procedures
First, eliminate all existing lawn vegetation. This involves spraying with a non-selective herbicide like ‘Natura Weed-A-Tak’ or ‘Burn Out’ on a day when the lawn is actively growing, it will not rain for at least 24 hours, and the wind is down. Be careful. The herbicide will damage any leaf surface they contact, so keep the sprayer low to the ground and use a course spray to minimize drift.
It takes about one week for the grass and weeds to die completely. Mow the dead grass as close as you can, an inch or less.
At this point you have a number of choices: If you have no underlying problems, and all you want to do is add new lawn starter fertilizer, lime and seed; then vigorous de-thatching with a hand rake or verticutter set at the proper depth (see above) will prepare an adequate seed bed. But if you have compacted soil with poor drainage you can do one of two things: Rent a core- type aerator as described above, or rent a tiller and incorporate organic material into the top 4" to 6" of the soil. Organic material would include Leaf Gro, Pine Bark Soil Conditioner, or top soil. Rake the area smooth to grade. Apply the starter fertilizer and lime with a drop spreader. If you buy or rent a tiller, try to get one with the rotary blades in the back, not the front. They are much easier to use.
When seeding use a drop spreader set at half the recommended rate and put down half the seed across the lawn and half up and down to ensure uniform coverage.
Good contact between the seed and soil is essential. Lightly rake the seed into the soil or carefully sprinkle a 1/4" of top soil or Leaf Gro over the seed.
Apply a thin layer of straw, seed mat or a crop cover over the lawn to prevent erosion and loss of seeds. This is more important with complete renovation because there is no vegetation to prevent wash out.
Keep the lawn moist by lightly watering once or twice during the day, but stop before puddling or run off occurs. Do not water at night.
Do not mow the grass until it's reached its normal mowing height of about 3".
You may need to over-seed for several years following renovation to build up a thick, weed- proof turf.
Fertilize two (or three) times in the Fall, once in September and once in October or early November, with a good organic fertilizer like the ‘Bradfield Organics Luscious Lawn & Garden’ or 'Ringer Lawn Restore'. A third application may be necessary (see Over-seeding Procedures above).