Newly seeds lawn is 6 inches tall after one month. When should I mow it. It’s Beginning of October now in Smokey mountains. Should I wai until the spring to mow. I am worried it will start falling over if it gets much taller. Lawn is large so I mow with a 60 inch deck zero turn riding lawn mower. I worry mower is too heavy for new lawn.
With most grass types, it's important to cut once it reaches 4-6 inches because it thickens up the grass. Each little shoot that germinates is a separate plant, and cutting it makes it branch out, like most other plants do, which means your grass does need cutting.
However, you are right that using your mower may cause damage - you only sowed a month ago, so the root systems won't be well established and may be ripped out by a heavy mower; the soil may compact down too much as well if you ride over it. I'd normally recommend using a hover mower or shears, but if the area is large, that doesn't sound like its going to work. A strimmer (think you call them weed whackers) might work, though they are not noted for giving a clean cut, but again, it means walking all over the area to do it which may be impractical. I don't know what to suggest otherwise, except to say yes, it does need cutting asap. You may just have to chance using the mower you have; if you do, ensure the blades are very sharp.
The goal at this stage is to get the grass to spread sideways. Grass produces one or more of tillers, stolons and rhizomes (Oregon State U.). Since you probably sowed a mixture of grasses you will probably have a selection of each. At this stage there is probably not much vertical growth left, but it would be helpful to flatten in some way the stand of grasses to damage the grass at ground level which will encourage this type of growth. If you have or can borrow a wide roller even if not heavy (say a length of culvert with a pole passed through and attached to a couple of cords) then you could get someone with a strong back to pull this randomly over the area. This will bruise the grass at the base and produce the desired effect.