Answer: Congratulations on your new home! Sod installation in September is rather iffy. The roots of the grass will grow as long as the soil temperature remains above 40F, but it will struggle. Since you'll be getting snow soon (which will hide the lawn), I'd recommend waiting until spring to install sod. When spring arrives you can either use a vegetation killer such as Round-Up to kill off the grass and weeds or you can rent a rototiller and till the entire area up. Or, you can simply till the area and then rake out all the old grass and weeds. Either way, remove the debris from the area and rake it level. Then install the sod and keep it well watered. The sod should root in a matter of weeks and provide you with years of trouble-free lawn.
Cottonwood trees are probably the fastest growing in your region, but they are deciduous so you'd lose the privacy in the wintertime; Douglas Firs grow quickly and are evergreen, but they can grow to 45' tall. If you have the room, they are great additions to any landscape. Linden, elm, ash, aspen and maple are all fast growing, but they are all deciduous. You might try cruising your local garden center to see what else they offer.
Best wishes with your new landscape!
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