You'll definately get larger clumps faster if you don't thin them. However, a lot depends on the type of grass you've sown. If it's a grass with distinctive characteristics, like color, you may want to grow plants from individual seeds, and select for the strongest characteristic representation. In other words, if you sow Blue Fescue, and grow clumps from multiple seeds, there will be blades within the clump which are more or less blue in color. You also need to be careful how you do the thinning. If you just cut off the blades of some Fescues, they'll grow right back, with more blades than before.

Most grasses will germinate with one or two blades, but will quickly, during the first growing season, expand the number of blades. You should sow more than one seed in each cell, or to each square inch of an open seed flat. Check the web for specific instructions on the number of seeds per cell for the varieties of grass you are sowing. Jelitto, linked below, is one source for such information. If I can't find appropriate information, I use the ultimate width of the grass blade as a guide. If it's a fine-bladed grass, I sow more seeds per cell, and if it's a wide-bladed grass, fewer seeds. When I grow grass from seed, I transplant the small seedling clumps to nursery pots, usually 4" or 1 quart pots, to grow them on for at least the first season. Then they go into nursery beds for at least another growing season, before eventually being moved to display beds as almost mature plants. What grasses are you sowing? Here is a link that might be useful: Jelitto

Based on your requirements I would suggest; painted concrete! That is my usual suggestion to anyone who wants to grow living plants under lousy condions & with no maintenance needed. It is an impossible request. More kindly, I would definitely say NO to the idea of seed. Any of the weeds already growing in this area will have seeded in the soil. As seedlings you will be unable to tell the difference between the plants you want & those you don't. Really, if you don't want to do any work, an inorganic solution would probably be the best. How about putting down a landscape fabric topped by mulch or river rock? If you still wanted plants you could put a holes in the fabric, improve the soil slightly in those spots & then plant a hardy tough grass. For the sunny spots Little Bluestem (Schizachyrium scoparium)might survive your conditions and would be trouble free for your neighbour. Shady dry spots are a little more difficult....... good luck, A.

Most ornamental grasses which are reliably hardy in your zone, will create viable seed, from which you can grow more plants. However, grasses, especially variegated and specially colored named cultivars will probably not grow true to the special characteristics of the parent plant. I have hundreds of grasses which were grown from seed. Here's a partial list: Anthoxanthum odoratum.. Sweet Vernal Grass Briza maxima.. Annual Quaking Grass Briza media.. Quaking Grass Calamagrostis brachytricha..Fall Blooming Feather Reed Grass Carex comans 'Bronze Form'..Bronze New Zealand Hair Sedge Carex grayi.. Gray's Sedge Carex muskingumensis.. Palm Sedge Chasmanthium latifolium.. Northern Sea Oats Cortaderia selloana.. Pampas Grass Deschampsia caespitosa.. Tufted Hairgrass Elymus hystrix.. Bottlebrush Grass Eragrostis spectabilis.. Purple Love Grass Festuca gigantea.. Giant Fescue Festuca glauca.. Blue Fescue Festuca mairei.. Maire's Fescue Festuca scoparia.. syn. Festuca gautieri..Bearskin Fescue Helictotrichon sempervirens..Blue Oat Grass Hierochloe odorata..Sweetgrass, Holy grass Lagurus ovatus.. Bunny Tails - Annual Luzula nivea.. Snowy Woodrush Luzula sylvatica.. Greater Woodrush Miscanthus sinensis 'Central Park'..Maiden Grass Miscanthus sinensis.. 'New Hybrids'..Maiden Grass, Eulalia Grass Nassella tenuissima.. Mexican Feather Grass Panicum virgatum.. Switch Grass Panicum virgatum 'Strictum'..Switch Grass Pennisetum glaucum 'Purple Majesty'..Ornamental Millet..Annual Pennisetum alopecuroides.. Fountain Grass Pennisetum alopecuroides 'Moudry'.. Black Seeded Fountain Grass Pennisetum orientale..Oriental Fountain Grass Saccharum ravennae..Ravenna Grass Sesleria caerulea.. Blue Moor Grass Setaria faberi.. Giant Foxtail - Annual Sporobolus heterolepis..Prairie Dropseed Stipa capillata.. Feather Grass Tridens flavus.. Purple Top