Eucalyptus radiata - Narrow-leaved Peppermint, Peppermint, Common Peppermint, Robertson's Peppermint (Southeast Australia) A medium sized tree with fine fibrous grey bark and leaves that smell of peppermint.  It has done well in parts of England, where it is said to take on an atractive weeping habit.  There is likely to be quite a bit of variation in cold-hardiness with this species, but a good guess would be a range of 6 - 16°F.  There are several subspecies, ssp. radiata, ssp. robertsonii and ssp. croajingolensis; which should all be tried for cold-hardiness.
Photos of Eucalyptus radiata

Eucalyptus recurva - (New South Wales) This extremely rare, protected species is known from only three individual parent trees.  The seed is said to have a very low viability, and the seedlings often show evidence of hybridisation with other species.  Indeed, the species itself (along with many euc species) may be of hybrid origin.  It is being preserved in a few Australian arboreta.  This species is closely related to E. parvula, but smaller in habit.  It too has small leaves, but they are curved at the ends (hence "recurva").  It is likely to be extremely hardy and tolerant of poor drainage, as is E. parvula.

Eucalyptus regnans - Mountain Ash, Giant Gum, Victorian Ash, Australian Oak, Tasmanian Oak, White Mountain Ash, Stringy Gum, Swamp Gum (Southeast Australia) This magnificent tree is somewhat well known because it has been recognized as the world's tallest nonconiferous tree!  The largest one ever officially measured was 373' tall, rivalling the coast redwoods of the United States.  There have also been some reports of E. regnans measuring as high as 464' in the 19th century.  Today it is not really known whether these reports were relaiable, and many people are understandably skeptical.  In any case, it is a very tall tree thriving in moist forests and mountain slopes in cooler parts of southeast Australia.  Sadly, some old groves of these giants are still being logged in the Styx Valley of Tasmania and other places.  It has dark green leaves similar to those of E. obliqua and, not surprisingly, makes rapid upward growth.  It is one of the "half-barked ashes," meaning that the bark is persistent on the lower trunk and smooth above.  However, in cultivation the bark may be light and smooth nearly to the base of the tree.  In addition to being a novelty for its size, it is also a very attractive tree; but it must be given moist soil and a site sheltered from wind.  It grows at lake margins and tolerates poor drainage in the wild, but I would not recommend using it in boggy sites in cultivation.  It is also sometimes grown for timber.  Although not generally considered a very hardy species, there are some provenances which may be able to take temperatures down to 5°F.  But unfortunately it does not develop a lignotuber and will not regrow if the top is destroyed in a freeze.  5 - 17°F.
Photos of Eucalyptus regnans
Eucalyptus regnans is also featured in Milligan Seeds and Trees Gallery: (trees)

Eucalyptus risdonii - Risdon Peppermint, Silver Peppermint (Tasmania)This rare peppermint comes from a very small area of Tasmania.  It has intensely blue, stem-clasping juvenile leaves which smell of peppermint, and grows quickly into a small tree with a narrow, open habit.  It has smooth bark and is quite floriferous at maturity.  Fairly adaptable.  Probably hardy to about 8 - 14°F.

Eucalyptus rodwayi - Swamp Peppermint (Tasmania) A Tasmanian swamp gum related to E. aggregata of the mainland, this medium tree has a dense crown of dark green leaves and tolerates poor drainage.  It has grey-brown bark and holds its lower limbs well.  It is one of the very hardiest swamp gums, and like E. parvula is suitable for cold wet areas.  -3 to +6°F.
Photos of Eucalyptus rodwayi
Eucalyptus rodwayi is also featured in Milligan Seeds and Trees Gallery: (young plant)

Eucalyptus rubida - Candlebark, Candle-bark Gum, Ribbon Gum, White Gum (Southeast Australia) This attractive large tree is widely distributed throughout Australia's high country.  It is found in subalpine forests, often in association with E. delegatensis.  Closely related to E. dalrympleana and roughly similar in hardiness, it is also an excellent ornamental tree.  It usually has smooth white peeling bark, often with some pinkish or salmon patches, especially in cool moist climates; and is also tolerant of dry climates.  3 to 12°F.
Photos of Eucalyptus rubida

Eucalyptus rupicola - Cliff Mallee Ash (New South Wales) This rare species usually grows as a multi-stemmed mallee.  It grows at very high elevations and is probably cold hardy into the single digits F.

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