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« on: August 16, 2019, 02:32:56 PM »
I would separate cold hardiness from vigor. Vigor is displayed by rate of growth as well as duration of growth expressed by number of flushes and growth per flush. Cold hardiness consists of a number of factors: A. early initiation and retention of full dormancy, B. actual tissue resistance to freezing and freeze damage. I don't see vigor as being preventative of cold damage, and may be the opposite by exposing late, immature, succulent growth to freeze damage.
Recovery from freeze damage is a different matter, and vigor may indeed assist in repairing and overcoming winter damage. Therefore I would separate resistance to cold injury from subsequent spring recovery.
Vigor is desirable, but needs to by curtailed in the latter part of the season in order to allow the plants to achieve early and sustained dormancy in preparation for winter. A major difference between zone 6b Citrus and warmer areas is that achieving and maintaining dormancy is easier in the colder climates. The bottom line is whether the 6b plants can endure the cold under full dormancy.
An other factor is the plants ability, or lack thereof, to re-establish vigorous growth in the spring in cold wet soils after winter. Some plants appear to be stunted and slow in recovery.
In regards to Poncirus vs. it's hybrids, there are hybrid plants that are genetically more vigorous, as well as many that are stunted especially in the F2 generation. I find vigorous Poncirus hybrids to easily outpace pure Poncirus in growth. This may be a contributing factor in the greater hardiness of pure Poncirus.
Of the 69 survivors (with partial intact stem) out of 3,000 zygotic F2 citranges on trial, the tendency was for mid-range to dwarfish plants to predominate. None of the very most vigorous plants were survivors, these tended to continue growing too late in the season.
I am not a grower of pure Citrus species or cultivars and can't really comment on their vigor.