Don't cut a hole, but an "X" in the plastic. Pull the flaps away and put the soil block underneath. This will prevent more weeds from emerging, since the soil will still be covered, and the growing plants will push the triangular flaps up and away. The onions may need to be given some extra room as they bulb, but your other crops should be fine.

This seems like a lot of work for spinach, though. If you plant spinach with a dense close planting, the plastic is unnecessary.

Be sure you have good tilth in the soil you are covering. If it is too hard, your root crops will have constrained growth once they reach the juncture between the soil block and the native soil.

I've heard that burning a hole will work much better than cutting, you won't have the fraying that cutting will allow for.

We burn X in our landscape fabric and plant soil blocks through that. One thing, you do not want large plants when planting soil blocks in plastic. I like my seedlings to have no more then 4 sets of leaves.

None of the crops you name do we do in soil blocks (beets and spinach hate to be transplanted so we always direct seed those crops. Onions do much, much, much better it if planted in 8" nursery pots as they really need the depth to grow decent roots.

We grow tomatoes, peppers, eggplant, melons, zucchini & basil in soil blocks and plant those crops on plastic. We also grow many other things in soil blocks but they are never planted on plastic

i use a bulb planter to make holes in mulch plastic.

Thanks everyone for your input.

bi11me - I don't know why I didn't think of making an X - it makes perfect sense.

In my area I am moving to growing more and more crops in plastic. I have quack grass - a terrible tendril type weed that only plastic slows down (nothing stops it). Plastic keeps it from creeping in from the paths.

Spinach - previously I wouldn't use plastic but with the multi plant blocks I thought there would be more opportunity for weeds with the wider spacing between blocks. Maybe I am worrying too much.