Chapter 123 Prunus subhirtella

123.1 Weeping cherry, Higan cherry

Usuzumi zakura (Prunus subhirtella var. pendula f. ascendens) in Motosu, Gifu Prefecture, Japan. This is one of the oldest and largest living specimens of this species, and one of the genetic parents for many modern cultivars. Link to original image.

123.2 Soil: Moist Deciduous Mix

Ratio: 4 parts bark, 1 part grit

Add-ins: dolomitic limestone (LOW: ~2 lbs/yd), Tree-Tone (LOW or NONE)

pH: ~6.0 (mildly acidic)

123.3 Moisture

Weeping cherry likes a well drained soil with consistent watering during dry spells.

123.4 Light

Routine: Does best in full sun, but also forms tall, elegant weeping shapes when growing on the edge of a woodland.

Summer: Mid-day shade can keep the leaves from burning out by August. Even with shade though, they tend to lose their leaves in late summer or very early in the fall.

123.5 Cultivation Notes

123.5.0.1 Collecting

Weeping cherries are one of the first trees to bloom in late winter, even before red maples break buds for the year.

Once I learned what to look for, I realized there are large numbers of them growing on the edges of woodlots, in old fields, and along the fences around pastures. Many announce themselves with just a single flowering branch, with the rest of the tree tucked back in the understory. For the rest of the year, their leaves look so similar to our native cherries that they pass unnoticed. It is fairly easy to find a small tree during the early blooming season, and either dig it immediately or tag it for root pruning in fall, and digging up next spring. Do not be alarmed if you have to cut off roots; like many members of the rose family, weeping cherries will quickly regrow roots if they are kept cool and shaded for a few months after transplantation. The odds of success are higher if the tree has not yet leafed out.

123.5.1 Potting & Root Pruning

These trees naturally form dense mats of shallow fibrous roots that can block water penetration into the pots. At least twice a year I push a 1/4 or 3/8 inch diameter sharpened dowel into the soil several times, all the way to the bottom of each pot, to open channels for water penetration.

These trees also tend to fill their pots quickly, so should be checked annually to see if they need repotting. When repotting, be gentle while untangling the roots. Many roots will break off, and by the time they are untangled, you may not need to remove any more, and only have to trim the tips so they are more even.

The fine feeder roots will flatten and compact easily. Take extra time to work soil in and around the roots so they remain separated.

123.5.1.1 Top & Branch Management

Weeping foliage in summer. Link to original image.

During the growing season, cut back any upward-facing shoots. Only keep upright shoots if you want to increase the height. This pruning strategy helps the tree develop a natural-looking weeping canopy.

Cherries do not heal over large wounds well, so I try to avoid removing large branches, and start working branches when they are small. Large scars can be turned into uro cavities.

Fine branches can be cut back in mid-spring once leaves have expanded. I have had some branches die back if I cut them back too early. When in doubt I wait to prune cherries until later in spring, or even into June.

I do not leaf prune, because these trees lose their leaves very early compared to other trees, and the latent buds to not break until they have had some cold weather. Leaf pruning also reduces the number of flowers, which are the best part of this tree.

123.5.2 Feeding

Weeping cherries are susceptible to both transplant shock and root burn. They definitely do NOT want to be fed until 2-3 months after planting out. When repotting, be very careful not to use a soil mix that is too rich. You want free-draining bark rich soil, but not want to use anything more potent than an organic fertilizer like Tree-Tone, and you should limit even that.

I learned the hard way that their seedlings are VERY susceptible to root burn. I get much better seedling survival when I pot up seedlings in a modified version of my standard 4:1 soil mix without the added fertilizer or lime. They like acidic soil, and do not seem to mind being in low-nitrogen conditions for a couple months while they adjust.

After the roots settle, weeping cherries can be put on the same feeding plan as ericaceous trees and shrubs. Feed with slow-releasing granular-type fertilizer in spring (18-6-12 should be fine), followed a couple times each year by a liquid feed like Miracid or other other soluble food for acid-loving trees.

Avoid regular 10-10-10 and similar quick-release fertilizers, especially right after repotting.

Weeping cherries bloom very early, and go dormant very early each year. I recommend skipping routine summer feeding in August or later in the season. Let them shut down early in the season and conserve their energy for flowing in early spring.

123.5.3 Pests & Problems

Horticultural references say these trees are susceptible to a large number of insect and disease problems. Potential diseases include leaf spot, die back, leaf curl, powdery mildew, root rot and fireblight. Potential insects include aphids, scale, borers, leafhoppers, caterpillars, tent caterpillars and Japanese beetles. Spider mites may also be troublesome.

This might be true for nursery cultivars, but I’ve not experienced anything like that. Mine have been relatively trouble-free. It may be the difference between a highly selected cultivar vs. growing them from seed. Eutypia blight is the one disease I have seen kill one my my larger established trees.

The main problem I have encountered with this species has been fire blight. Small seedlings can die, but saplings or larger trees do not usually die back completely. Typically I see individual branches dying back. If the tip of a branch is infected, I cut back at least 2 nodes above the dead portion, or all the way back to the main trunk. Don’t skimp on pruning back; fire blight has already migrated well back in the vascular system from any points where branches are dying.

Chemical Warnings

I have not seen any sensitivity to particular chemicals with this species. Most chemicals rated as safe for roses or cherries should be safe for this species.

123.5.4 Winter Storage

Store in mulch outdoors. Try to place in a shaded area so buds do not break in a mid-winter warm spell. No other special precautions are needed.

123.6 Propagation Notes

This particular cherry grows quickly, and is very long-lived under the right conditions. It is much shorter lived when grafted on other cherry rootstock, or if it does not have fertile, loose soil and regular water.

The seed-grown species is hardy in USDA Zones 5 - 8, and does very well here. The cultivar ‘Autumnalis’ (which blooms in fall) may be cold hardy to zone 4, while the cultivar ‘Pendula’ is only hardy to zone 6.

Named cultivars usually are propagated from semi-hardwood cuttings in summer or bud-grafted on related seedlings. Trees grafted on hardier rootstock may live longer than those started from cuttings.

Root cuttings of firm, young wood during the active growth phase in late spring/early summer. Take a 4-5" cuttings from the new growth. Remove the bottom leaves (keep the leaves on the upper one or two nodes) and dip the base of the cutting in a rooting hormone. Place the prepared cutting in a moistened soilless mix in a sunny indoor location. Cover the pot with a clear plastic bag so the soil mixture doesn’t dry out, but leave then end open for air circulation. In a few weeks, roots will form, and they can be potted up.

Personally, I grow most of my weeping cherries from seed, or collect trees I find growing wild, up to 1 inch caliper. The single dark blue or purple fruits are about the size of a lemon seed or raisin, and hang under the leaves. They appear around the first of May in this area. Collect them as soon as they turn dark purple or black, because birds will strip them from trees as soon as they find them.

Remove the skin and flesh from the seeds by placing them in a plastic bag and rubbing it between your hands to break the skins. Add water and wash away the juice and bits of flesh that break free. Soak the seeds overnight in more fresh water to loosen the remaining flesh. Rub the soaked seeds between layers of newspaper to remove the rest of the meat, and pick out the seed. Dry the seed for 1-2 days, then put them in a plastic bag and store until fall in the refrigerator.

In late September, soak seeds overnight in warm tap water, then sow about 1 inch deep in a 1:1 soil/bark mix in a dishpan. Cover the seeds and let the seeds stratify naturally outdoors for the winter.

Seedlings appear in spring, but are not ready to be potted up for several months. I have lost nearly all of my seedlings in the years that I tried to pot them up too early. I get better survival when I leave the seedlings in their dishpan for a full year, and repot them their second spring. Also, I get better survival when I keep as much soil from the original container around the seedling roots as I can. They seem not to tolerate bare rooting well. Seedlings go into 4-inch pots for 1-2 years then 1-gallon pots. When they have filled a 3-gallon pot they are ready to start styling in earnest.

123.7 Artistic & Styling Notes

Weeping cherries look their best when they have one central leader with multiple cascading branches from the apex. So I do not prune the leader until it is at least 3-4 feet long. If the tree seems to want to go higher, I support it with a light stake and let it keep going. Usually the central leader reaches a certain height, then starts sending out secondary branches that weep naturally. Most of the styling work will be clipping out the occasional branches that want to spread laterally instead of drooping down.

When growing these trees in pots or fabric bags, I’ve not been able to achieve a trunk caliper much larger than 1.5 inches in diameter. To get them to thicken further, these trees definitely need time in open ground.

123.8 Additional Information

This species is one of my three favorite trees, and the one I most look forward to seeing bloom each spring. So I’ve spent a lot of time trying to figure out how to bring it into wider use in bonsai.

123.8.1 Natural History

If you want to get confused really fast, try figuring out the taxonomy of flowering cherries. Many references classify our weeping cherry as a naturally occurring hybrid of Prunus incisa and Prunus spachiana. That means it should be called Prunus x subhirtella.

Widely grown in Japan, their naturalized range includes the islands of Honshu, Kyushu, and Shikoku. They are thought to have been introduced to Japan from China in the 8th century AD.

According to almost all propagation guides, _P. x subhirtella__ cannot be cultivated from seed, only from green wood cuttings.

There is a problem (actually, TWO problems) though. If they only grow from cuttings, where do the naturalized trees that we see growing in our local woods come from? I solved that puzzle by testing the seed (once I learned how to beat the birds to them), and found that our local weeping cherries produce 80-90+% viable seed. That raises the second problem: WHY do they produce mostly viable seed if they are hybrids?

The official explanation, courtesy of the USDA’s Natural Resources Conservation Service Plants Database, is that this particular weeping cherry is not a hybrid, but a true species. According to USDA, commercial cultivars of P. subhirtella have naturalized in several US states.

How did a non-seeding cherry cultivar become naturalized in old fields and woodlots where no one planted them? It is very likely that weeping cherry escaped more than one time. Many commercial cultivars of P. subhirtella do not bear fertile seed in any quantity, but they also probably are not genetically sterile (as we might expect for a hybrid.) Most likely the original ornamental cultivars were selected for low seed viability, in much the same way that modern cultivars are. Inevitably an occasional flower is going to revert back and set fertile seed. If just one or two fertile revertants from a nursery-grown tree grow to maturity, they would provide more than enough fertile seed to naturalize into woodlots. Other weeping cherry cultivars DO set viable seed that will germinate slowly if cold stratified. The cherries often are eaten by mice and birds, so I suspect that local wildlife has been hard at work carrying seeds from intentionally planted trees far and wide.

123.8.2 In Cultivation

The Miharu Takizakura is another ancient weeping higan cherry tree in Miharu, Fukushima, in northern Japan. This tree also contributes to the genetic heritage of modern cultivars. Link to original image.

If you buy one of these trees from a nursery, make sure that you are getting a tree on its own roots, or one that has been low grafted, not high grafted.

Standard practice in production nurseries is to graft or bud scions of weeping trees onto a 3-4 foot tall straight rootstock. This high graft may work in a yard (though personally I think they look hideous), but they are useless for bonsai.

Look for a tree that has some movement starting within a few inches from the ground. If you can see a graft line close to the soil line, the weeping portion was likely low grafted on a hardier rootstock (typically bird, sweet, or wild cherry (Prunus avium)). Provided the graft union is not lumpy or out of proportion, a low grafted tree can be trained easily. If there is no visible graft line, the tree probably is on its own roots.