Winter Storage

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By: Devon Cournoyer

Depending on where you live, fall clean up and winter preparation can mean many different things. For the purposes of this article I will focus on Wisconsin, where Badger Bonsai Society resides. I’ll discuss winter storage for hardy plants (juniper, pine, maple, etc.) as tropical species (ficus, schefflera, carrissa, etc.) could not possibly be expected to survive the winters of Wisconsin and must move indoors by September 1st or earlier.

Wisconsin runs the climate zones from 3-5 which means the temperature can be as low as -10 to -30º f. In my opinion, it’s best to assume that it can drop below -20, if even for a few hours, so the plants must be protected or they can dry out, roots can burst and pottery will shatter. Why not just bring them all inside you ask? Because they are hardy, outdoor trees which require a dormancy, a period of rest. If you keep your hardy trees inside, they will eventually exhaust themselves and die. You can overwinter your hardy bonsai in an unheated, rarely used shed or garage, nestled in garden beds, or in a climate controlled greenhouse kept just above freezing, say at 34ºf. Most of us will not have the greenhouse option, and if you did, you probably don’t need this article, so I will explore the outdoor options I mentioned.

I’ve had the best luck building bonsai shelters on the East side of my house, right up next to my foundation. The North side may work as well, though mine is not as accessible, so I use the East. The North and East sides will receive the least amount of temperature fluctuation, you don’t need sunlight, you need a constant temperature.

Starting around Halloween and finishing by Thanksgiving, I perform the following winter storage ritual, and my trees are alive and well come spring:

  1. I place the trees on a small platform of bricks and lumber, to keep them off the ground and discourage insects from taking up residents in the pots.
  2. Then I build a crude chicken-wire hoop structure, leaving the top completely open (this is VERY important), but tall enough to be higher then the tallest tree. This is to discourage the larger pests from making a home under my branches and feeding on my tree trunks through the season.
  3. Next, I wrap the outside of the fencing with a tarp (construction plastic will work too) on the three exposed sides (remember, one side is next to my house) and secure it to keep the cold, drying winds from attacking my bonsai. I water as I would this time of year. Don’t stop watering!
  4. When the maple trees in my yard drop their leaves, I back fill my structures with the fallen leaves. This keeps the trees insulated, breathable and you can water though the leaves.
  5. As the snow begins to fall, I start to construct an igloo of sorts around the sides of the structure, again leaving the top mostly open. I build up the walls right up to the side of my foundation and put some snow on top to keep the constant temperature.

Constant temperature, that is the name of the game! Assuming you work with species that can withstand our climate, keeping a constant temperature is how you keep your bonsai alive. The reason I keep the tops of my structures open is to keep from having a heat buildup during the day, only to drop and condense during the night. Keeping them next to my house ensure a little bit of warmth as my home will be warmer than my open yard. On the east (or north) side, keeps the least amount of sun. A shed works well as does an unheated garage. BUT, if you use your garage (open and closing the door two plus times a day) I would recommend another location as the temp exchange will be great as the big door open to park a car plus the heat from the car as it comes into the garage and cools off. This could be just enough to freeze/thaw/freeze your bonsai causing irreparable damages to the tiny branches and roots.

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Most Common Styles

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In English, the most common styles include: formal upright, slant, informal upright, cascade, semi-cascade, raft, literati, and group/forest. This list is compiled on Wikipedia.


Formal upright style, or Chokkan, is characterized by a straight, upright, tapering trunk.


Informal upright style, or Moyogi, may incorporate pronounced bends and curves in the trunk and branches of the tree, but the apex of the informal upright is always located directly over where the trunk begins at the soil line.


Slant-style, or Shakan, bonsai possess straight trunks like those of bonsai grown in the formal upright style. However, the slant style trunk emerges from the soil at an angle, and the apex of the bonsai will be located to the left or right of the root base.


Cascade-style, bonsai are modeled after trees which grow over water or on the sides of mountains. The apex, or tip of the tree in the Semi-cascade-style, or Han Kengai, bonsai extend just at or beneath the lip of the bonsai pot; the apex of a Full-cascade-style, or Kengai, cascade style falls below the base of the pot.


Broom style, or Hokidachi is employed for trees with extensive, fine branching, often with species like elms. The trunk is straight and upright. It branches out in all directions about 1/3 of the way up the entire height of the tree. The branches and leaves form a ball-shaped crown which can also be very beautiful during the winter months.


Literati style, or Bunjin, is characterized by a generally bare trunk line, with branches reduced to a minimum, and typically placed higher up on a long, often contorted trunk. This style derives its name from the Chinese literati, who were often artists, and some of whom painted Chinese brush paintings, like those found in the ancient text, The Mustard Seed Garden Manual of Painting, depicting pine trees that grew in harsh climates, struggling to reach sunlight. In Japan, the literati style is known as bunjin-gi (文人木, bunjin-gi?). (Bunjin is a translation of the Chinese phrase wenren meaning “scholars practiced in the arts” and gi is a derivative of the Japanese word, ki, for “tree”).


Root-over-rock style, or Sekijoju, is a style in which the roots of a tree (typically a fig tree) are wrapped around a rock. The rock is at the base of the trunk, with the roots exposed to varying degrees.

Growing-in-a-rock style, or Ishizuke, means the roots of the tree are growing in the cracks and holes of the rock. There is not much room for the roots to develop and take up nutrients. These trees are designed to visually represent that the tree has to struggle to survive.


Multi-trunk style, or Ikadabuki, has all the trunks growing out of one root system, and it actually is one single tree. All the trunks form one crown of leaves, in which the thickest and most developed trunk forms the top.


Raft-style, or Netsuranari, bonsai mimic a natural phenomenon that occurs when a tree topples onto its side (typically due to erosion or another natural force) and branches along the exposed side of the trunk, growing as if they are a group of new trunks. Sometimes, roots will develop from buried portions of the trunk. Raft-style bonsai can have sinuous, straight-line, or slanting trunks, all giving the illusion that they are a group of separate trees — while actually being the branches of a tree planted on its side.


Group (or forest) style, or Yose Ue, comprises a planting of more than one tree (typically an odd number if there are three or more trees, and essentially never 4 because of its significance in Japan) in a bonsai pot. The trees are usually the same species, with a variety of heights employed to add visual interest and to reflect the age differences encountered in mature forests.

Metasequoia Glyptostroboides

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Metasequoia Glyptostroboides or Dawn Redwood, is a deciduous conifer. What makes this great material for bonsai is the way it tapers from a heavy broad root flare.

Dawn redwoods are hardy from zone 5 to zone 8. They will winter over well up here in Wisconsin (zone 4 and 5) with ample protection from the harsh wind.

Native to China and thought to be extinct for many years until a small grove was discovered in the 1930’s in the interior of China. Since the rediscovery they have been widely propagated throughout the world and are now are used as an outstanding landscape tree.

Dawn redwood is not very particular as to the soil they grow in but do prefer a well draining, acidic potting medium.

Do not over fertilize, as too large amounts of nutrients will cause long spindly shoots. Water as you would most of your bonsai but do not let stand in water.

Dawn redwood is pruned by pinching off the ends from the new shoots forcing them to branch out.

Small Dawn redwood made a very nice grove and a little larger (1″ to 2″ trunks) make a nice 3 tree grouping, and of course a large heavy trunk one will make a good formal upright, or one with shari up the front and some jins from the lower branches. They can also be trained into a flat top like the old bald cypress that grow in the swamps of the deep south.

“Bonsai Bob” Eskeitz

Ilex Vomitoria

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Ilex Vomitoria, or Yaupon Holly, is a small spreading evergreen tree that grows upright or as a large shrub reaching 15-25 feet tall. The foliage is small gray-green and leathery. Ilex needs well-draining soil without regard to pH levels and will thrive in loam, sandy soil, or even clay.

Yaupon holly is a native of North America. They are excellent to work with and will make quality medium and shohin size bonsai. They bud back readily on old wood.

Yaupons are easy to take care of and thrive in full or partial sun. Yaupons seem to lean toward the ‘old oak tree’ style when shaping for bonsai. Small flowers will appear on second year wood in the spring. Yaupon are only hardy in zones 7-9 so a lot of winter protection is required. You can treat it as an indoor up here.

Pests include leaf miners, mites, aphids, and scale but none are really a serious threat to the health of plants and no known diseases affect the yaupon.

Watering daily in the hottest part of the year is very important since missing a day may cause leaf drop and twigs to die back. In nature, yaupons grow along stream and river margins as well as in more arid regions such as sand dunes. However they require consistent watering to promote good growth.

It is critical to fertilize carefully in the spring when new growth is at it’s peak. You can encourage smaller leaves in March, April and May by using 1/4 strength soluble liquid fertilizer or one tablespoon of organic fertilizer to a gallon of water every two weeks.

To maintain the shape of yaupon holly bonsai, trimming is one of the most important things to do in the spring and summer. Cut the new growth back to two sets of leaves when six to eight pairs have developed. This is the most effective way to control the design of your tree. Good ramification will be evident after only a few years of this clip-and-grow training. Constant care must be taken with older trees so strong shoots don’t develop and dominate the design of your bonsai.

“Bonsai Bob” Eskeitz

Ficus Retusa

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The Ficus Retusa, also known as Ficus Microcarpa Nitida, is one of the easiest trees to take care of. It is a native of Malaysia and other southeast Asian countries. It is a tropical and must be kept inside when the temp gets below 40 degrees. Failure to give protection from the cold will cause leaf drop and may result in branch die back and possibly the demise of the plant itself.

Ficus retusa grows to about 60 feet in the wild, but it is easily trained as a bonsai. The bark is a smooth gray with a lot of aerial roots if you make sure it gets a lot of humidity. One year, I went on vacation and had some in a lattice house which received about 50% shade and I had an automatic water system on twice a day for 30 minutes each time. I came home after two weeks and had new aerial roots of about 2 inches on most of the plants; however, when I put them back in full sun the new aerial roots never made it to the pot. If you are growing them somewhere that has a lot of humidity never let the aerial roots grow out of the pot and into the ground. If this happens it will cause the bonsai to put out a rapid growth of branches and become a bush. Foliage is dark green and quite dense and grows upright.

Retusa grow best in full sun with plenty of water. Fertilize with an organic or all purpose fertilizer every week. To prevent die back, prune only when the tree is actively growing. With terminal pruning the reuse will bud back easily. The milky white latex that is secreted when pruning branches may cause an allergic reaction to some people, so it is best to avoid contact with the skin.

Ficus retusa likes to be repotted in the hottest part of summer. The tree is not particular in what type of soil it is potted as long as it is a well draining mix. The PH of the mix does not seem to be a major factor here.

Ficus retusa does not seem to be seriously bothered by insects or fungal pests. It is quite possible to find an occasional scale but the milky sap of the tree seems to discourage most insects. As a precaution, you can spray the tree with an insecticide twice a year.

Ficus retusa is an ideal tree to use for beginners. That is the reason I use them a lot at schools and garden clubs where people are not familiar with bonsai. As I say to people, “If you kill a ficus, you have tried to.”

“Bonsai Bob” Eskeitz

Thuja Occidentalis

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Thuja Occidentalis, or Northern White Cedar (Arborvitae), grown throughout Wisconsin, except the southwest portion. It grows usually in moist areas where it is often found in dense stands. However, farther north and in Canada it appears on well-drained slopes, usually in with hardwoods.

The bark is thin, gray to reddish brown, separating in long, vertical, narrow shredding strips.

The foliage is scale-like; length 1/8 to 1/4 inches, arranged to make the small branches flat. In the fall Thuja can shed about one-third of its foliage.

The fruit is small oblong cones that mature in one season, is yellowish-brown, 1/3 to 1/2 inches in size, with six to twelve scales and borne singly or in large clusters on ends of branches. Seeds are 1/8 inch long, with 2 narrow wings almost circling the seed.

The wood is light, soft, brittle, coarse grained, durable, fragrant, and pale brown in color. It is especially important for making fence posts (which I hope none of you have to do with your bonsai), building poles, rot-resistant lumber, and shingles.

Note: It is one of the preferred and important species for deer browse in the winter, so please do not feed the deer.

When used for bonsai the driftwood style is popular for this “cedar” since the wood is relatively soft, nicely grained and easy to carve.

Pruning and wiring can be done throughout the growing period. Pinching of new growth results in short, dense foliage. Although budding back on old wood has been noticed in nature, it has not been seen in bonsai. Fall shedding can be controlled some by timely pruning and pinching in late summer (September).

Repotting may be done in the spring, always before the first of June. Repot about every three years. The potting mix must be well draining. Thuja has very fine roots that will rake out quite easily. Thuja will tolerate root pruning well.

Fertilize with a high nitrogen 30-10-10 fertilizer which will give a deep green color to the foliage; apply every two weeks in the growing season. A lower nitrogen fertilizer like a well-balanced 20-20-20 can also be used. Different brands of fertilizer are recommended.

Thuja is tolerant of being kept a little dry, but if you are not sure, water as you would the rest of your bonsai, but do not keep it extra wet.

There are no pests or diseases to speak of when in bonsai.

Thuja should be grown in full sun. Winter care is simple, as they are very hardy, tolerating below zero temperatures. Smaller trees could be dug in the ground to the top of the pot and mulched, or winter over as you would the rest of your outdoor bonsai. Wind protection should be provided as with all bonsai up here in the north.

This is one of the easiest species to use for bonsai.

Robert “Bonsai Bob” Eskeitz