The planting guide says May to July. It looks like it is going to turn cold this weekend, time to plant I think. I have never grown it. They say plant around the shortest day of the year and harvest the longest - sunlight hours that is. Some people say that isn't true.
I planted early last year and lost most of my crop early on because it was too hot. This year I planted mid April and apart from having to shade the crop a couple of days the temperature is perfect. The crop is looking great and growing fast.
I'd be thinking you may need to experiment as well as hope nature doesn't decide to play games with the seasons. You might next year consider staggering 2 bulbs every 2 weeks for say 6 weeks and recording the results. That way the following year you should nail it.
Good luck and keep trying. Determined gardeners eventually find the sweet spot.
I was going to suggest staggering the planting over a few weeks. Seasons can vary from year to year so much. The year before last it didn't turn to cool/cold weather until well into July here. Last week we had night temps (6.7 and 7.9) within 2-3 degree of our normal coldest winter temps.
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