I live in a rental and there are three olive trees/bushes in the back yard. They are all different sizes. With one being reasonably sized, a little taller than 6 ft. Otherwise there is one that's pretty scrawny and one in between. I would assume these were planted well over 7 yrs ago as the house has been a rental for that long and the owners are absentee.

Other than the variations in size and the small yield I see nothing overly unhealthy about them. There are a couple of dead twigs on them that I need to prune.

I would like to help them the best I can but don't know much about gardening. What would be suggested to invigorate them? We get pretty dry down here for the next couple of months. How regularly should I water them?