Field growing is one of the techniques by which you can rapidly attain thick, tapering trunks with which to develop quality bonsai trees.
What is so important about a thick trunk you might ask. After seeing so many sticks in bonsai pots I would say its extremely important. You should keep in mind that as bonsai artists we are trying to create nature in miniature. Trying to capture and recreate the feeling you get when you stand in front of an old oak, an ancient camphor tree or a milkwood bent over from years of abuse by the wind you will notice with all of them they have fascinating trunks. The way they grasp the soil and how they stretch up into the heavens is a challenge to replicate in a small pot to say the least! So we need to use every visual device we have at our disposal. A thick, tapering trunk helps a lot. Does every trunk need to be thick? No. Sometimes a more feminine approach is required such as when styling Japanese maples. However we still require taper in the trunk to create visual interest and to recreate how the tree becomes thinner as it reaches up into the upper reaches of the canopy.
Field growing is not the only technique which can be used to create thick trunks, one can also use large wooden or plastic containers. For growers who live in a rented house, don't have access to much garden or for some other reason cannot use field growing, this will be the next best option. If you are limited to large containers then I would highly suggest our Professional growing medium. As its fast draining, loose and will result in rapid root and vegetative growth. BonsaiBoostwill be vital to ensure the tree grows healthy and strong during the period, or it will take longer to achieve the desired results.
However without a doubt, trees grow best when they are in the ground. For centuries this is how top quality trees of all species have been developed in Japan, and this approach is still practiced today.
Image caption. Hackberry trunk being developed in the ground. Notice the bags of BonsaiBoost.
The Hackberry above was developed from a one year old seedling treated as a cutting after the taproot was removed (To learn more about starting bonsai from seed purchase my e-book). The finer roots were spread out and the tree was planted in a plastic container for two years or three. In the fourth year field growing was started by planting the tree out into the ground.
Image caption. A brick growing bed, back filled with ideal growing medium
If you have the space you can build growing beds like these above. They are built from brick which create a void you can fill with ideal growing medium, which should be fast draining. This elevation of the soil level makes life a lot easier, especially on your back, when you are working low down on the trees.
Be sure to provide sufficient space between trees so their branches can develop as long as possible. The longer the branches become the more and quicker they will increase the girth of the tree. In the right growing conditions your trees may put on at least 1m of growth per year.
Image caption. Old slate tiles used to plant onto.
I have used tiles in my field growing for the following reasons:
It makes it easier to remove the tree from the ground as you don't need to dig underneath the tree. Just find the edge of the tile and chop the roots off there. Once these are severed you can simply lift the tree up. Easy peezy!
Tiles force the trees roots to grow horizontally instead of vertically down. This creates a very nice flare at the base of the trunk as the roots develop which gives a lot of character to the trunk.
If you don't have ceramic tiles you can use wooden boards or plastic sheets and even styrene, but tiles are more durable and you will be able to use them repeatedly.
Image caption. Mounding soil onto the tile
Mound some soil onto the tile. It does not have to be much, just enough really for the tree to have something to grow into.
Image caption. Placing the tree onto the tile
Place your prepared tree on top of the mound of soil. Work some soil into any gaps so that there are no voids. Roots will not grow into large pockets of air so you must be sure to eliminate them.
Mound some soil on the top and if you like you can mulch around the base also to ensure the soil does not dry out completely between waterings.
Image caption. Completed planting of a grove of trees
In the image above you can see several trees which have been planted in this manner. There is about 1m between them as they are quite developed trees already.
The trees are at their most vulnerable at this stage due to the fact that you have eliminated such a large amount of roots. Field grown trees have a lot more energy though than pot grown trees so species such as Chinese maple and Hackberry can take the harsh treatment with ease. Trees which are more fragile should be given more careful aftercare, including frequent light watering.
Hold off on fertilizer at first but after the tree has settled in and is showing sure signs of new growth you may go ahead and fertilize. Organic, slow release fertilizers are advisable as they cannot easily burn the sensitive new roots.
Development in the first year after planting the tree will not be very impressive but in the 2nd season it will greatly accelerate. In the subsequent years growth will speed up even further until in around the 4th year you will dig the trees up once again. During this time, emphasis should be on developing the branches and trunk line which means allowing growth to extend as long as possible. The longer the sacrifice branches become the more they will thicken the trunk.
Each time you lift the tree you will cut back the sacrifice branches and reduce the height of the tree, called a trunk chop. When you do the trunk chop it's not so important to make the cut at a particular angle and it's certainly not necessary to shape it. The fact is that the tree will die back to a point and from there it will issue new growth. So it is rather pointless to shape the cut only to have to repeat the process again in a year or two, at the point where the new trunkline has started. I know many books show this but I have learnt from experience that it's simply a waste of time, but it does sound like an excellent idea which makes it very believable.
Image caption. Field grown Chinese Maple. You can clearly see the three points along the trunk where it was chopped and then development continued again. Here I am preparing it to go into a wooden box I prepared for it.
The height of the trunk chop is dependent on:
how large you want the tree to ultimately become (If you want a short, stocky tree where you make the trunk chop will be different than when you want a tall, feminine trunk)
how many curves you want in it (Each time you make a trunk chop you change the direction of the trunk.)
and how thick you want the tree to be. (You don't want to cut the trunk until it has thickened sufficiently)
Don't chop the trunk until the part which remains is at least 2/3rd's or more the final thickness you want. Very broadly speaking you might want to make your first trunk chop at about 1/3rd of the final height of the tree, but this is really dependent on what style you are creating.
As you draw closer to the end of the process ie the trunk has the girth you desire, you might decide to shorten the time the trunk is growing in the ground before you dig it up and prune. The reason for this is because you also now need to consider any scars and cuts you make, as once the tree is in a bonsai container the large scars created during field growing will be almost impossible to get rid of. I would suggest allowing branches to develop just a little further along the trunk, above any major cut. This will cause sap to flow aggressively at that area and will result in quicker healing.
To avert large scars in the latter stages of trunk development you might also remove branches before they get too large, reducing the size of the resulting cut. It is also imperative that you use a sharp saw to make any cuts with as this will result in quicker healing.
After removing the branch with the saw I find that using a root cutter is best for removing the remaining branch stub, as it is a hardier tool than a branch cutter.
Image caption. Typical roots when practicing field growing
While in the ground the tree will grow strongly and roots will develop equally as strong. After several years in the ground the roots will have thickened considerably and will be very coarse. Feeder roots which are fine and can collect nutrients and moisture from the soil is what we need to encourage for pot cultivation, not the thick storage roots. You will also find that trees which remain for many years in the ground will only have these thick roots with the finer roots having died off closer to the trunk.
So every 3 or 4 years it is advisable to dig up the tree and remove the coarse roots. To do this I use a chainsaw and root cutters.
A good quality root cutter is an indispensable tool for field growing of bonsai trees as it can be used for reducing roots and stubs of branches.
To encourage feeder root development the tree should spend a season in a large plastic or wooden container. While in this container growth will be slower and more controlled. Fine roots will be issued by the tree from the thick roots which you cut when you last lifted it from the ground, but they will also emerge from the base of the trunk.
Image caption. After trimming the roots
The above image is of a Hackberry which was growing in one of my beds for a number of years. You can see how severely the roots can be cut back on field growing stock, due to the energy levels they would have built up. Its important that these root cuts be made, if at all possible, at the correct angle. The angle which is preferred is horizontal. The reason for this is that when the new roots emerge from the cut edges there will eventually be no visible scarring as the scar is essentially underneath the tree. If on the other hand you make the cut at a 45 or so degree angle, it will take many years for the cut to disappear entirely.
You will also note the large cuts created when I removed branches which were used to thicken the trunk. These should be sealed properly to prevent the live cambium/plant tissue from dying back at the edges creating an even bigger area to be healed. As you get closer to finally ending the field growing process you should focus more on these cuts healing over because once in a bonsai pot this healing will be a lot less.
Branches which developed during field are mostly useless once you begin styling the tree. Its very tempting to keep them many times, but they are generally too thick, too straight and have too little taper to be of any use. So it's best to start with a clean trunk and develop the branches you want to keep in the final tree, while you can control their growth better.
Image caption. Developing finer roots in a container
Here is another celtis which was developed in the ground for some years. Its in a large plastic container where it can develop many fine roots before returning to the ground for further trunk development after a season or so.