Zeedman
Garden Master
Each successive soybean Maturity Group is about 10 days later than the one before it. Those Maturity Groups are:wow, that's a large difference! 30 days would likely put a plant into the frost zone here. why such a large range and not something more like 15 days?
000
00
0
Then the Roman numerals
I
II
III, up to X (with X being tropical)
Because soybeans are somewhat photo-period (daylength) sensitive, there is not a direct correlation between Maturity Group and DTM. I've tested that several times, by using early transplants vs. later direct seeding. Flowering began at nearly the same time, and at most, the transplants only gained 1 day for every 2-3 days started early. The last time I tried this, with transplants started 2 weeks earlier, dry down was nearly identical.
In my zone, the highest Maturity Group which I can reliably grow to maturity is Type II... and more often than not, those are a race against frost. This year, I planted a Type II soybean 3 weeks late, (was really crossing my fingers there ) and it still matured at about its normal time.
Several years ago, I tried to grow a soybean in a higher Maturity Group than is recommended for my zone, (Type III) using transplants. Contrary to expectations - especially given my success with growing long-DTM pole limas through the use of transplants - it didn't help. The plants were exceptionally bushy, but still didn't bloom until very late in Summer, and the pods were not even close to maturity when frost arrived. Having seen this excessive foliation in other tropical vegetables I've grown (such as hyacinth beans, rice beans, and some yardlongs) I've come to associate that with the response of short-day adapted varieties to the longer Summer daylength at my latitude.
An interesting observation. Normally, the USDA climate zones are irrelevant for vegetable gardening, because those only refer to Winter hardiness. However, those climate zones also generally follow the dates of first frost. The USDA map for recommended soybean Maturity Groups tends to follow climate zones, rather than latitude. Assuming that all soybeans of a given variety & planted at the same latitude will flower at the same time, those grown in warmer climates - such as the U.S. East Coast - will have more time to mature before frost. Presumably that would apply to gardeners close to the Great Lakes as well.