I live in zone 5-6. Iceland. Problem is that hard freezes come often without any snow. And in early and late summer, freezes still occur, just after mild weather.
I started my first perennial broccoli in 2011 from seed. I could harvest broccoli that summer. Then, a few plants survived into 2012, and again I harvested a few broccoli heads. In 2013, the plants are still alive outside, even after two summer harvests. One plant even did flowered and produced a bit of seed, and kept alive afterwards, but it is recommended to never let the plant go to seed.
I have no idea how long can this continue, but it might depend in both: 1) severity of your winter, and 2) how short is your summer. I think a mild winter and a short summer are favourable to induce and select for the perennialization of broccoli.
The difference to normal broccoli, is that normal broccoli always dies after a summer harvest. But perennial broccoli sends new growth from its root and side shots, but new heads will be smaller than in first year. So far it has extended growth into its 3rd year, so it is really a perennial, but probably short term. I do not know how long it will remain alive.
Here in Iceland some normal broccoli plants did not flowered last summer, and they are still alive. I think they will produce broccoli next spring, as weather warms, and who knows if one or two might perennialize as the 9 star variety. I also let many of them cross and set seed, in hopes of getting more perennial types.
in Portugal, sheltered terraces facing eastwards, high water table, uphill original forest of pines, oaks and chestnuts. 2000m2
in Iceland: converted flat lawn, compacted poor soil, cold, windy, humid climate, cold, short summer. 50m2
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