Japanese maple questions and answers – 2
Acer palmatum 1a
Japanese maple question: I have an inherited collection of Japanese maples. They vary in age (7 – 20 years) and type and I have been told that they have all been re-potted (3 – 5 years ago). I know that most if not all now need re potting and I was wanting to know a bit more about this. I had intended to do this when the leaves started to fall in autumn but have since read that spring would be better. One article suggested that one third of the old root-ball should also be removed. Can you advise? Many Thanks.
Answer: The recommended re-potting time is early autumn (September), whilst the roots are still in active growth. This allows the plants to establish themselves quickly in their new pots and continue producing storage roots for optimum growth next spring. Disregard any article that says cut off one third of the old roots – you will end up losing at least 50% of the root system as the majority of roots are at the base of the root-ball. A by-product of this amputation is going to be little or no new shoot growth next spring and also the possibility of major die-back.
Acer palmatum 1b
Acer palmatum 1a, above, and the close-up, Acer palmatum 1b, right, show a plant that will need a pot one and a half to twice the diameter of the existing pot. Heavily congested roots have already caused some die-back, as can be seen just above the base of the trunk. Also visible is the original planting depth, as indicated by the change in bark colour. Replanting should take the root-ball back to that level and cover the currently exposed roots.
Acer palmatum 2a
The next plant, Acer palmatum 2a shown left, needs a pot wider than tall to give it stability and also going to a size twice as large. When dormant, a heavy prune removing the last three years growth of the lowest branches will encourage more and thicker growth in the middle. One point that I would draw readers attention to are the separate feet this pot and all the others are resting on: these ensure that any roots growing through the drainage holes are air-pruned and don’t find their way to the ground and grow into the soil.
Acer palmatum 2b
The close-up, Acer palmatum 2b on the right, shows a moss and liverwort infestation on the surface as well as roots growing out of the root-ball due to compaction and lack of space within the pot. When potting on, the surface should be gently scraped away and re-potted to cover the existing roots.
Acer palmatum 3
The next plant, Acer palmatum 3 pictured left, is suffering from serious competition for moisture, caused by the ornamental grasses planted with it. This needs to be removed and the temptation to put anything in with the maple when it is re-potted should be ignored. Signs of stress are already visible, as indicated by the dead branches. The two lowest branches on the right plus the two branches nearest the trunk hanging down on the left should be removed. The stake can also be removed as it serves no purpose and the ties are cutting into the bark. Again, pot size needs to be doubled and for long term planting it’s best to avoid wooden barrels as they will rot and disintegrate. Any preservative used to prolong their life will be toxic to the roots.
Acer palmatum 4
The same comments regarding wooden tubs apply to Acer palmatum 4, on the right. The staves are already starting to rot at the top.The branches on the left should be pruned back in winter so that they are level with the top of the container; this will help to thicken up the plant in subsequent seasons. Also requiring attention are the shoots with the palmate leaves that are growing from the under-stock; these should be removed this winter. They can be left on for the time being as they will help build up the storage roots once the plant is re-potted, ideally into a pot double the diameter.
Acer palmatum 5
The last plant, Acer palmatum 5, illustrated left, shows considerable die-back and an unnaturally open habit due to moisture stress caused by the heavily compacted potting mix, the poor drainage of which has encouraged an infestation of moss. Re-potting into double the existing size will suffice for the next few years but this plant is ultimately going to be too big to use as a container plant and should eventually go in the ground; planting can be done the same time as re-potting the other Japanese maples.
As for the potting mix to be used, multi-purpose, ericacious, tree planting or John Innes type composts on their own should be avoided. They will quickly become compacted due to the volume and weight and the subsequent lack of drainage will soon cause problems again. Garden soil opened up with added gravel, whilst a good alternative, will make the pots so heavy as to be impossible to move.
The best option by far is to use a standard multi-purpose potting compost mixed 50/50 with small decorative bark chips and a few spoonfuls of fish, blood and bonemeal as a base fertilizer. During the growing season, if you are using Tomorite on your tomatoes, this can also be used at half strength on your Japanese maples.
A more lightweight mix can be made from two parts multi-purpose potting compost with one part medium grade perlite and one part coarse grade perlite. The medium grade is readily available but the coarse grade will usually only be stocked by hydroponics suppliers, although these are quite common; a quick glance through Yellow Pages will locate one or more near you.
All the pots should be mulched – a sheet of polythene cut to shape and held down by a thin layer of coarse gravel. This will ensure moisture levels remain fairly constant, plus the porous mix will allow good drainage with no worries about over-watering. Moss and weeds are also prevented from establishing and competing with the maples for water.
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An interesting article, John, as usual. Thanks for taking the time to write this blog, and share your experience of growing maples. If you don’t mind I would like to ask a few questions to follow on from the points raised above.
Do you recommend replacing the existing potting medium, either partially or totally, when potting on a Japanese maple? (Often referred to as bare root repotting)
With a maple that has not been repotted for several years, how do you deal with the circling roots? Do you recommend simply repotting the rootball as is, and allowing the new roots to find their own way out into the new space?
What happens when the Japanese maple to be rehoused has already been potted on several times during its life, and is in the largest size container you feel comfortable dealing with? Would you advocate root pruning under these circumstances or is there another way round it, short of planting into the ground?
What size are the “small decorative bark chips” you recommend, and is there any advantage to using bark that has already been composted, such as can be found in some brands of peat-free compost?
Replacing some or all of the growing medium is not necessary as the roots won’t grow back in to the existing root-ball once they’re surrounded by new. It can also be the case that removal is going to be awkward and damage the finer roots, particularly if the plant has become pot-bound. Roots will always grow out into the new medium, no matter how congested, so there is no need to try and tease out the roots, although a heavily congested root-ball can be slow to root out. This is where potting on in late summer/early autumn will produce the quickest re-establishment.
Root pruning or planting in the ground are the only choices when an optimum container size has been reached. The disadvantage of root pruning is, as I mention in the article, the loss of much of the active root system and a consequent reduction in top growth the following year. Of further note is the explosion of top growth the year after once the new growing medium is colonized which brings us back to square one again and a cycle of feast and famine as far as the plant is concerned.
A variation on continual root pruning, and a far kinder option for the plant, is to do it once and then plant into a cage such as that suggested in the article ‘growing a potted Japanese maple in a hot climate’. This will encourage a more natural root growth pattern and eliminate circling. Air pruning pots have been available commercially for a number of years but their retail availability is very limited at present and only come as straight sided, as opposed to a homemade cage that can be made to fit whatever shape of pot you choose.
The ‘small decorative bark chips’ are those used for mulching beds and borders, contain only a small amount of shredded wood and are typically between 1/2 and 2 inches in size. This provides a rigid framework within larger pots and prevents any compaction that will impede drainage. I personally avoid peat-free potting mixes so cannot really comment, but from work I’ve done on growing media in the past, anything less than 30% bark will have no appreciable effect on drainage. Chip size also needs to be related to the size of pot.
I shall go into the subject of potting mixes in more detail shortly, as they are the most critical component as far as the well-being of a Japanese maple is concerned but I hope the above has gone some way towards answering your questions.
Thanks, I root-pruned most of my larger maples this year and experienced first hand the “consequent reduction in top growth” you mention. Otherwise they seem healthy though. I am planning to make and trial some air-pruning inner containers from landscape fabric or similar, should be functionally similar to the cages.
Looking forward to the piece on potting mixes, I like to use as much soil/loam in my mixes as I can whilst retaining good drainage with grit and bark etc.