Grasses are pioneer plants, they're often the first to move into areas. They provide shade for seeds and seedlings, trap soil and provide humus. As larger scrubby plants begin to grow they start to shade out the grasses. They provide more shade for seedlings that don't like direct sun in their early stages. Slowly even larger plants take over. It's part of nature's process of colonising barren areas. However, if any of those stages stalls due to lack of rain/moisture or whatever it might not develop further. And these processes can take a long time depending on factors like rain/moisture/soil, etc.

So you need to look carefully at the area and understand what is happening there now. It's at the stage that nature has reached and has stalled. You need to know what it needs to get the process moving on.

I'm in the middle of a process to get nature going where climatic factors had stalled it for many thousands of years. There's been a lot of trial and error, but it's all edging forward. Be prepared to learn from your mistakes, because no doubt there'll be a lot of them along the way. Good luck.

Here's a much better place to ask your question. People on his forum actually live in HOVE.

http://www.punaweb.org/Forum/default.asp

Where in Hawaii is this? I know many people who's gardens were planted on a lava flow... most just trucked in 'soil' or crushed lava rock and planted them in that... had the additional advantage that way of keep the weeds/grass down (which do manage to pop up in old lava flows rather commonly). If it doesn't rain a lot, you must be referring to the Kona side of the big island? Most other areas of Hawaii have a lot of rain. Most tropicals seem to love the acidity of lava rock, though not true for all plants... you can adjust soil acidity and ingredients as needed if you grow all your stuff in imported soils/ or in planter boxes.