I have three forsythia bushes we planted up against the house. They are pretty sparse. They don't appear any thicker then the day we bought them 2 years ago.
what time of year should I prune my forsythia?
Also How should I prune them to keep the more natural wild look? Would pruning them to encourage a square shape be any different.
They will grow "wild" (eventually) if you just ignore them. Shrubs can take a year or two to get used to different soil and a different climate when you first plant them.
If you really want to prune them, do it after flowering. Pruning in winter won't harm the plant, but it will cut off the flower buds on last year's growth, so you won't get any flowers.
Remove any dead wood. Cut weak stems back to ground level. Finally, cut about a quarter of the length off the wood that just flowered, cutting just above some buds, which will then grow new stems and flower next year.
If they aren't making much growth I wouldn't bother about pruning them. I've worked in gardens where the forsythias haven't been pruned for years, and they still flower happily enough. Make sure they're in a reasonable position - decent soil, some sun, not too close together. The important thing is to keep the soil around each plant (say a metre diameter circle) weed and grass free, so there's no competition for water and nutrients.
