Be Careful with Shore Pine in the Winter-
Shore Pine is one of North America’s most beautiful two-needled pines, with short bright green needles and great bark. It has similarities to many other smaller pines which often have multiple crowns, like pinyon pines. Shore pine does have one distinct difference however, and that (appears to be) cold hardiness.
I say ‘appears to be’ because it’s more of a hunch. It does seem that at least some populations of Shore Pine are not as cold hardy as its relative, the continental, mountain-dwelling Lodgepole Pine. In some respects this is not a surprise, since many Shore Pines live close to the Northwest coastline in the same sort of zone as the Japanese Black Pine does. Anton Nijhius of Vancouver Island, Canada collects many Shore Pine, and says that some of them live at 4,000 ft in a lot of cold on the Island, so there is definitely some room for argument here, and possibly different strains.
That Shore Pine does fantastically in the ground in Portland Oregon, where it can get down to 10 F in rare storms, and yet does less well in pots (occasional branch death, tree death) supports the idea that it is a root hardiness issue. But again this is a bit of conjecture.
Japanese Black pine roots begin to die at about 12 F / -11 C. Given what I’ve seen and experienced, and other anecdotal stories from others who otherwise have excellent horticultural skills in colder climates than mine, Shore Pine bonsai can be even more sensitive to cold than Black Pine bonsai, and so my recommendation is that Shore Pine bonsai should be protected from strong freezes. Perhaps even 50 miles inland from the coast, in more continental climates where everything is more severe, Shore Pine should be protected in the winter.
Given the natural beauty of the plant, there are simple solutions that can allow us to enjoy this native tree as a bonsai. If you live in a region that frequently gets below 25 F / -4 C in cold snaps, consider taking protective measures starting in fall.
For more, please visit these posts about winter and our bonsai:
Post-note: Cold hardiness for trees in pots is a massively different matter than cold hardiness of plants that are normally put in the ground, and that we read about in gardening manuals. A plant with a cold hardiness of Zone 4 might not survive in a small pot sitting on a bench in that zone. Root hardiness is the issue that bonsai practitioners need to be aware of, and that information is less easy to find.
Great stuff Michael. This is the ground truthing that is needed for practitioners using our native North American material. Many thanks!
I have never had a cold hardiness problem with Shore Pines or lost any due to low temperatures. Not saying that they are cold hardy!
However I and a few of my friends have lost in the past Shore Pines in the early spring which we blamed on cold hardiness but was actually a lack of water. Examining the rootball confirmed this. They need to be watered in March, April even though there may be frost or low temperatures if the weather is dry they need water.
Even though they are pines and take summer drought and less watering they need that early spring water or they die. You get an El Nino year or a dry early spring with warm temps. and an early kick in the season the trees have an increase in transpiration (evaporation from plant leaves) and respiration (process of releasing energy for plant growth).
Err on the side of caution
When discussing root hardiness and winter temperatures, you might consider the difference between ground and air temperatures. Trees in pots setting on the ground and mulched in will maintain a temperature which is very close to ground temperature and nowhere near what air temperature may be during a few colder days. Now, those living in colder zones may experience colder ground temperature later in winter but again, mulch will make a huge difference. In my case here in zone 7, my ground under my mulched in trees typically doesn’t even freeze let lone get anywhere near root killing temperatures even though we may occasionally see 10 degree air temperatures for short periods of time. Surrounding un-mulched or bare ground will freeze but again rarely ever get below around 25 degrees F. Now, those that leave their trees on benches will encounter soil temperatures close to air temperature.
Dave Bogan – S. Indiana zone 7.
I have updated this Shore Pine post with links to past posts about how to care for bonsai during the winter-
The problem with the black pines isn’t just the roots: the predominant mycorrhizal symbiont, Pisolithus tinctorius, also dies about that temp. A plant with that association will not survive that die-off, and will slide downhill the next Spring.
Good post Michael.
Trees have genotypes and phenotypes. In the UK we are looking to use forestry trees from northern France to help survive climate change, same species but different genotype. Using seed from the same species but further south will almost certainly die with the first heavy frost, same species different genotype, different growth.
Thank you for this detail, appreciated-
If you are concerned about the cold hardiness of roots of the shore pine (or any bonsai), may I suggest protecting the bonsai by burying the root mass in the ground, in the shade and protected from the winds. This works in the Great Lakes area, where few bonsai would survive if not protected.
That is a first stage protection, but in continental climates it can be borderline. I have updated this Shore Pine post with links to past posts about how to care for bonsai during the winter-
The differences that you and Anton have noted is what we call in forestry a “provenance”, a species that has adapted to a specific locations over time with much different environmental conditions within the natural range of the species. The ones more inland at higher elevations may receive much harsher conditions (much drier in summer, lots of heavy snow in winter etc), than say the ones on the extreme west coast (wet, moist all year). These conditions may also effect the overall characteristics of trees within that subregion….example – on the west coast of Van. Is., the trees within these stands have much different bark I.e. corky, as compared to the ones on the east coast which has scaly bark.
I would love to know how many hours of dormancy a Shore Pine requires. I have one (thanks Anton) and live in Winnipeg (zone 3a). Winters are hard and I try to bring my trees in and grow under lights as soon as possible. This was a great success last year, but I worry it was a one off and over time I will not give trees enough of a break. I’ve found dodgy info on most of my species, but very little on Shore Pine. Help?
A very good question and I’d love an answer myself. I think your concern is justified.
It’s a mildly hardy tree, but not an extremely hardy one. In my experience and from talking with others it seems that the best idea is to keep it cool through the winter and protect from any real cold snaps. In your area I imagine the winter is one long cold snap. The problem seems to be that this species, like my suspicion is with Japanese black pine, can’t read continental swings in season and so it is susceptible to cold that otherwise, if fully dormant, it might be fine with. These are just guesses. The lights…I think keeping your tree roughly in the 1-3 C range through the winter should be possible without light. I have one student who lives in Montana and rolls his trees in and out of a dark room (that doesn’t freeze) and into the sun for the above-freezing days in fall and spring. It’s a lot of work but that’s what I’d do with your shore pines in Winnipeg. A greenhouse that you can temperature control is another idea, though a lot of energy will be needed to do so. Hope that helps a bit but I think you’re wise to strategize with this species.