How to isolate street noise from the garden?
Foliage won't do much unless it's very dense and a fairly wide strip--say 50 feet thick or more. Instead, you'll need to look to solid-panel fencing to block and deflect sound energy. 

The problem is likely that there are other homes nearby, which will reflect sound from several directions. You may not be able to accomplish significant noise abatement with that consideration. 

As they say in real estate, the three most important assets of a property are location, location, and location. This is a case in point. My current home has a highway a few hundred yards away, and we hear it almost constantly. Eventually our ears tend to become accustomed to ambient sound and it's less of an annoyance. 

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