Nothing better then a leek soup my mom used to make.
The bigger the better,.. cool crop & wet condition veggie, which is fine for our climate if you water in a dry spill but season might bee too short when planted from seed. Put your two thumbs together...about this thick they should be, then you have lot's of tender leek. I don't remember them going into seeds, perhaps they bolted for you,..too dry?
Light frost should be fine but I don't think they handle our cold winter.

I accidentally let a leek overwinter last year. We got above average snow cover though. This spring, I stumbled across it. The outside was rotten but the inner layers were still edible.
Last year, I think I harvested them in 1st or 2nd week of October...after a few -4 and -5 frosts but before negative double digits arrived.

I grew some a few years ago, I just took them out when they looked big enough.

Made leek and potato soup with them, it was amazing.

Not sure what you mean when you say you started them from seed -- if you started them indoors in trays around March 1, they should be plenty big now and you can harvest them anytime. If you started them directly in the garden in May then they are probably still on the small side and should be left until just before freeze-up in mid-late October. They are very frost tolerant, and assuming you mounded up the soil as they grew, then the whitish main edible part should be well-protected from frost anyway. I used to grow leeks, love the taste, but damn, I can eat onions all day, but leeks just don't agree with my digestive system :-(

I know I'm in a different zone than you guys but we left our leeks in the ground til about Mar. last year. As stated, the outside layers were a bit 'leathery' but the inside was tasty. We had some great leek and potato soup last winter. We did not bother to mulch but I think we will try that this year. Leeks left in the ground keep growing. Marg

I started them indoors and planted outside near the end of May. They were the cutest tiny little things at that time. I planted them in a trench in a raised bed and covered as they grew.

Sounds like I can leave them until about the middle of October, or even until Hallowe'en depending on our weather and/or when DH really wants to make his soup =:)

Thanks all !

I'm not sure that letting your leeks get frostbitten will "sweeten" them all that much; they're naturally mild and sweet.

I just harvested one of mine today. We had a terrible summer for tomatoes, squash, and corn, but apparently a great one for leeks: this one was the width of a corn stalk.