What can you put on vegetable garden to kill weeds before planting?

You can lay cardboard or similar over it for a few weeks which will kill many of them Frank. The deep rooted ones like Dandelions and Thistles I dig out by hand. (I don't use weedkillers at all on my veg patch)

Thank you good tip Frank

They make a roll of material that is called a weed barrier!

Hello Frank. If you use any kind of weed killer on the weeds, it will contaminate your entire garden. You can smother the weeds out with cardboard or old carpet. Before you plant your garden, pull everything out including all of the roots. Good luck Frank.

The cardboard is biodegradable, much better for the soil. It's a great barrier that takes time to break down. I use it on new and old beds. Only use untreated (no shiny finish) cardboard. Score it, wet it, throw some compost on top and mulch. Do that now and you should be able to plant by the end of May. Any weeds you get will be above the cardboard, dropped by birds, blown in by the wind, and they'll pop out easily. If you're putting in plants and not seeds you can sprinkle Preen on top, or buy the mulch that has Preen included. That will stop most weed seeds from germinating while you wait. Re-apply every 6 weeks or so. Happy Gardening!