Well, I kind of messed up . . .
I have planted Walla Walla Sweet Onion seed in August before. It has been a few years but, if I remember right, it was at least 3 times. Sowing seed in August is how I understand that the onion growers in Walla Walla do it.
In my garden, about 200 miles north of Walla Walla, it worked once and then not again :/. The plants would bolt in the spring! The last time, 100% of them bolted!
For the last couple of years, I have grown a nice hybrid sweet onion, Ovation. It has real good keeping qualities and that is one thing I like about it. My idea was to try sowing some Ovation seed in August - problem was, I don't have any seed! Too late to order, I decided to go with Esteem Hybrid, instead

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Esteem hasn't
proven itself in my garden, at all. I tried sowing it very late at the beginning of the growing year - a couple months later than the Ovation, Walla Walla and Utah. They have all done well and formed large bulbs. The Esteem nearly died during transplant - too hot! Cucumbers have now invaded their space and it looks like there is no chance of them forming anything but tiny bulbs :/. I will leave some of the plants this winter. I may pull some and see if they'd work as "sets" next year . . . and, now, here I go planting some seed on the last day of August

!
Anywhooo, I thought I'd share and suggest that some of you may want to sow seed for next year's onions. It would sure be nice for me to not have to start flats of them in the (unheated) greenhouse in February. I'm not betting the farm on what I've got right now, tho'! For one thing, it has NOT cooled down here! If they need "cooler," they may not get it for a couple of weeks. But, maybe by June 2014, I'll be able to decide if Esteem warrants any esteem . . . hope so

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Steve