In our climate onions are never ready before September. My friend grows onions at her farm every year, so it is not because of a mistake in timing or fertilizer in this case. There are two reasons - one is that we have cooler temperatures so it takes longer to get the "degree-days" for maturity. A second factor for tops staying green is that the season is usually dryer in June and July, with more rain coming in August and September.
I was at the farm on Monday and saw that they harvested half the onions the week before. The remaining half are still green though and are not flopped over. In other years I have seen them properly finished in the field ie fairly dried down, but not every year provides those conditions.
The same is true for garlic, that instead of drying down and losing half their leaves, many will stay green when you're waiting for clues to harvest. An experienced garlic grower told me, if you're counting leaves, just browning of the tips is enough sign for us, if three weeks or more have passed since the last scapes were harvested.
