8 Step Acer Palmatum Repotting Routine with John Innes No 3 in a Glazed Pot

Late February to mid March, before the buds swell, is the window most growers at the RHS and specialist nurseries such as Junker’s Nursery work to for repotting a deciduous Acer palmatum. The sap has not yet risen, so root disturbance costs the tree very little. Work too late, once the leaves are out, and you trade a dormant pruning wound for a tree trying to push growth while its root system is in shock. The other defensible window is October, after leaf drop, though spring is the safer default in colder regions.

Why a glazed pot rather than terracotta or plastic matters more than it looks. A glazed ceramic pot loses far less water through its walls than unglazed terracotta, which suits a maple’s hatred of drought stress, but it also drains and breathes less, so the compost mix has to do the aeration work the pot will not. That single fact drives almost every decision below.

Step 1: Lift, and read the root ball before you decide anything

Tip the pot on its side and ease the tree out by the trunk base, not by pulling the branches. A maple that has been in a 30cm glazed pot for three years will usually come out as a solid cylinder, the compost barely visible behind a felt of pale roots. That felt is the diagnosis. If you can see more root than soil at the sides and base, the tree is pot bound and the repot is overdue, not optional.

Look for circling roots running round the inside wall. These strangle a tree slowly over years and are the reason a maple can look healthy at five and decline sharply at eight. Tease a few free with a wooden chopstick or a root hook so you can see the structure. If the bottom third is a dense brown mat with a sour smell, that is anaerobic compost, and it tells you the old mix held too much water for a glazed pot.

Step 2: The John Innes No 3 mix, and why you cannot use it neat

John Innes No 3 is a loam based compost with the highest nutrient charge of the three John Innes grades, formulated originally at the John Innes Horticultural Institution. The loam gives it weight and water retention, which is exactly what an open terracotta pot wants and exactly what a sealed glazed pot does not.

Used neat in a glazed container, John Innes No 3 stays wet at the core long after the surface dries, and that is where root rot starts. The fix is to open it up. A reliable ratio for a glazed pot is roughly 60 percent John Innes No 3 to 40 percent additional drainage material, by volume. Horticultural grit at 3mm to 6mm is the standard choice; some growers split that 40 percent between grit and composted bark for a lighter mix on a large specimen.

Mix it dry in a tub or trug and check the feel. A handful squeezed should hold its shape briefly then crumble when poked. If it stays in a tight ball, add more grit before the tree goes anywhere near it.

Step 3: Comb out and reduce the roots

With the tree out and the mix ready, comb the root ball from the bottom and sides with a hook or chopstick until perhaps a quarter to a third of the old compost falls away and the outer roots hang loose. This is the step people skip, and skipping it means the new compost never bonds to the old core, leaving a dry pocket the water skirts around.

Step 4: Prune the roots with clean, sharp blades

A pair of Niwaki Okatsune shears, the same brand many use for topiary work, holds an edge well enough to cut roots cleanly, though a dedicated root scissor is better still. Cut back the longest circling and downward roots by up to a third, always with a clean cut. A torn root heals slowly and invites the same fungal problems as soggy compost. Sterilise the blade with surgical spirit between trees if you are working through several, since maples are prone to verticillium wilt and a contaminated blade spreads it.

Keep the fine feeder roots near the trunk; those do the water and nutrient work. The thick anchor roots can be shortened hard. Aim to remove enough that the tree sits in its pot with two to three centimetres of clearance all round for fresh mix.

Step 5: Crock the base and build the first layer

Glazed pots usually have one central drainage hole, which clogs easily. Lay a piece of plastic mesh or a single broken crock over it so the hole drains but compost does not wash out. A two to three centimetre layer of pure grit across the base then gives the water somewhere to go before it reaches the central hole, which is the difference between a pot that drains in seconds and one that holds a reservoir at the bottom.

Add a mound of the 60/40 mix on top of the grit, high enough that when you set the tree on it the old soil surface sits about two centimetres below the pot rim. Settle the tree, rotate it to its best face, and check the trunk is vertical from two angles.

Step 6: Backfill, working the mix into the roots

Trickle the mix down the sides a handful at a time, working it into the gaps with the chopstick as you go. Air pockets left here collapse after the first watering and drop the tree below its proper level. Firm gently with fingers, not by stamping, until the pot is filled to two centimetres below the rim. That gap is the watering reservoir; without it, water sheets off the surface and over the edge.

Step 7: Water in slowly, then leave it alone

Water from a can with a rose, slowly, until water runs clear from the base. Then water a second time. The first pass settles the mix and reveals any sinkage; top up with more compost if the level drops. A freshly repotted maple should go somewhere bright but sheltered from direct midday sun and from wind for two to three weeks while the cut roots heal. Wind dries the foliage faster than the damaged roots can replace the water, and that is what scorches the leaf margins brown.

Hold off feeding for six weeks. John Innes No 3 carries enough nutrient charge to cover the tree until early summer, and feeding into fresh cuts only risks burning them. A liquid feed at half strength from June onward, every two to three weeks through the growing season, keeps the colour without forcing soft growth that flops.

Step 8: Mulch, and set the watering rhythm for the season

A thin top dressing of fine bark or a layer of moss keeps the surface from capping and slows evaporation in a glazed pot that already holds heat. Through summer a potted maple in full leaf can need water twice on a hot day, and the glazed wall that saved water in spring now traps heat against the roots. Move the pot into afternoon shade if the leaves crisp despite watering.

The repot itself buys you two to three years before the root felt returns. What it does not fix is placement. A maple repotted perfectly and then stood against a south facing wall in a dark glazed pot will still cook, because the ceramic absorbs and holds the heat the roots cannot shed. The open question for anyone with a treasured specimen is whether the pot that looks best in the garden is the one the tree can actually survive in through a heatwave.