You have a heap of prunings, and you’ve just mowed your lawn. Can you use these materials to build a hugel?
Unless they’re poisonous! This might seem obvious, but it’s a very important point.
With all that wood breaking down into your hugel and feeding your veggies, it’s wise to know your poisonous plants. Please don’t put rhododendrons, azaleas, oleander, rhubarb leaves, or anything with a white milky sap into your hugel. I happily put pine and eucalypt in mine. There are some toxins in those woods, but given that our topsoil is probably full of the latter, and traditional raised garden beds probably made of the former, I doubt that it’s a major issue.
One more thing to note, is that if the waste is still green, you might have to leave the bed to rest for a few weeks, because the rotting vegetation will generate heat, as well as deplete nitrogen from the plants. Both which are bad for your seeds and budding seedlings. You might add a layer of nitrogen to your bed to offset any nitrogen depletion that occurs through garden waste decomposition.