Grass seed in pounds by species, not by guesswork

Between a quarter of a pound and 10 lb on the working bands this page uses, and which end you are at is decided by the species. Centipede goes down at a quarter to half a pound, Kentucky bluegrass at 2 to 3, perennial ryegrass at 5 to 7, tall fescue at 6 to 8 and bahiagrass anywhere from 5 to 10. Those bands are working figures rather than one published table — an extension service's own sheet will not match them exactly, and the rate on the seed tag outranks both. The spread is not a difference in how much grass you get: it is a difference in what a pound of seed contains. Sowing a fescue rate of bluegrass puts roughly ten times the intended number of seedlings on the ground, and sowing a bluegrass rate of fescue leaves gaps that fill with whatever germinates next.

Because most of the lawn is already covered, and the seed you add is competing against established plants rather than filling empty ground. A thin lawn is not bare ground with grass on it — every seedling that lands there is starting under an existing canopy, in soil already full of roots, and the ones that survive are the ones that found a genuine gap. Doubling to a new-lawn rate does not fill more gaps, it just puts more seed where there is nowhere to go. The exception is a lawn that is more bare than grass, which is a renovation: strip it or kill it off and sow it as new ground at the full rate.

No, and well past the rate it gives you a worse one — this page flags anything over one and a half times the species rate. Grass seedlings compete with each other exactly as they compete with weeds, so an over-sown stand comes up shoulder to shoulder, stretches for light, roots shallowly and then thins itself out through the first hot spell — leaving a lawn that is patchier than a correctly sown one and less able to survive a drought. The dense wet canopy is also where damping-off starts, and a seedling stand lost to Pythium in the first fortnight is lost entirely. Density in a lawn comes from tillering over the first two seasons, which is a function of mowing height, feeding and water, not of how much seed went into the spreader.

This page works to 1 lb of actual nitrogen on every 1,000 square feet — a working rate, and the starter bag's own label rate wins — which for an 18-24-12 starter is 5.6 lb of the bag and for a 12% analysis is 8.3 lb. The rate is written in nitrogen rather than in bag weight because the same 1 lb comes from very different amounts of product, and reading the number off the front of the bag instead of dividing by the analysis is the standard way to double or halve a feed by accident. Worth knowing before you buy: several states restrict phosphorus on established lawns, with new seeding as the named exception, so the high middle number that makes a starter a starter is legal here and often not legal on the same lawn next spring.

Not a normal one. Pre-emergent herbicides work by stopping seeds germinating, and they have no way to tell a crabgrass seed from the fescue you just paid for — a spring application keeps the soil hostile to germination for weeks, and its label states how long to wait before seeding — which is the entire window you were sowing into. The two products that are formulated to allow it are siduron and mesotrione, both labeled for use at seeding. Otherwise the sequence is one or the other: seed in fall and preventer the following spring, or preventer in spring and seed in the fall after it has broken down.

Because most bags print their coverage at the overseeding rate, which is half the new-lawn rate. A bag stating that it covers 5,000 square feet will genuinely cover 5,000 of thin existing lawn and about 2,500 of bare soil, and the small print usually says so on the back in a place nobody reads in the aisle. This page reports what one bag of your chosen size covers at the rate you are actually using, which is the number to check the bag against — if it disagrees by roughly a factor of two, that is the reason.