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Size the planter to the root ball, not to the tree. The interior opening should be 2 to 3 inches wider than the root ball, and the interior depth should leave at least 3 inches of potting mix beneath it with the top of the root ball sitting 1 to 3 inches below the rim. Measure the nursery container the tree arrives in, then compare it with the interior figures for each size, never the size in the product name.
A tree's height tells you very little about the planter it needs. Two trees of the same height can arrive in very different containers depending on the species and how long the grower kept them. What has to fit is the root ball, so that is where the sizing starts.
What size planter does a tree need?
One whose interior is 2 to 3 inches wider than the root ball and deep enough to put at least 3 inches of mix underneath it. Both figures come from NC State Extension's guidance on plants grown in containers: for pots 10 inches or wider, a container 2 to 3 inches wider than the plant's root ball is ideal, and the plant should go in a container large enough to leave at least 3 inches of potting mix under the root ball for further growth.
The third figure comes from the manufacturer. Jay Scotts' owner's guide sets the plant's root ball 1 to 3 inches below the planter's rim, which leaves room to water without it running straight over the edge. Put the three together and you have the whole calculation:
OpeningRoot ball width plus 2 to 3 inches.
DepthAt least 3 inches of mix below, plus the root ball, plus 1 to 3 inches up to the rim.
How do you measure the root ball?
Measure the container it is growing in now: the widest point across the top, and the height from the base to the soil line. If the tree is balled and burlapped rather than potted, measure the ball itself at its widest point.
Do this before you choose a planter, or ask the nursery for the dimensions of the container before it ships. A trade size such as a 15 gallon is a label, not a width you can plan to, and pots sold under the same label can differ in shape. Tall narrow pots and wide shallow ones need different planters even when they hold the same volume.
Why the size on the label is the wrong number
The size in a planter's name is an exterior dimension, and the root ball goes inside. The wall has thickness, so the opening is always smaller than the name, and on a tapered form it narrows again below the rim.
A Montroy cube sold as 24 inches has an interior opening of 21.2 by 21.2 inches. That is almost three inches less than the name suggests, and it is the difference between a root ball that drops in and one that does not. We publish the interior opening for every size on its product page and on our planter specifications page, so you can compare the number that matters.
What most buyers overlook: the depth
Most tree planters are bought on width, and depth is where they fail. A root ball that fits across the opening can still sit too high, leaving no mix beneath it and soil heaped above the rim.
Here is the calculation worked through for a tree arriving in a pot 17 inches across and 15 inches tall. It needs an opening of at least 19 to 20 inches, and a depth of at least 19 inches: 3 below, 15 for the root ball, 1 to the rim. These sizes clear both figures on their published interiors:
- Montroy cube, 24 inch: opening 21.2 by 21.2 inches, interior height 22 inches. Ships by ground.
- Lima tapered square, 24 inch: opening 20.4 by 20.4 inches, interior height 22 inches. Ships by ground. It tapers, so check the root ball against the narrower base.
- Granada rectangular, 36 by 24 by 24 inches: opening 32.6 by 20.6 inches, interior height 22 inches. Ships by freight, and leaves room either side for underplanting.
- Rio Grande cylinder, 22 inch: opening 19.2 inches, interior height 20 inches. It clears both figures with almost nothing to spare, so treat it as the lower limit.
There is a gap worth being honest about. Several planters with a wide enough opening, including the 24 inch Asheville cube, have no published interior height. Jay Scotts' spec sheets give interior length and width but no interior height for any model, and the figures we do publish come from a secondary source that notes dimensions may vary by up to 2%. Where no interior height exists, we leave it blank rather than estimate it. If depth decides your choice, ask us before ordering and we will tell you what is published for that size.
How big a planter does a fruit tree need?
NC State Extension lists minimum container sizes for fruit: 20 to 25 gallons for apples and peaches, 15 gallons for grapes, 10 gallons for figs, and 2 by 2 by 2 feet for blueberries. They also note that fruit trees in containers produce less than the same trees planted in the ground.
Planters are sold by dimensions, not gallons, so the conversion is worth doing yourself. On its published interior, the 24 inch Montroy cube holds at most about 43 gallons (21.2 × 21.2 × 22 inches, divided by 231 cubic inches per gallon). The 20 inch holds at most about 23. Those figures are ceilings, because any taper in the walls reduces the real volume, so the 20 inch sits on the edge of the apple and peach minimum and the 24 inch clears it comfortably.
Which shape suits a tree?
Straight-sided boxes, cubes and cylinders give the most usable room for their footprint, because a straight wall does not narrow below the rim. Tapered and bowl forms read softer and suit a single specimen, but their opening is quoted at the rim and narrows below it, so a root ball that clears the rim still has to fit lower down.
Tall forms such as the Saint Tropez put a tree at eye level without a stand, but a tall planter concentrates weight over a small base. Our guide to tall outdoor planters covers stability and delivery for those sizes. For a pair or a row, a long box such as Granada holds one continuous root run instead of separate root balls.
Soil, weight and winter
Use potting mix, not garden soil. Jay Scotts' Outdoor Planting Tips guide says to use only potting soil, potting mix or container mix, because top soil is heavy, becomes heavier when wet, and puts outward pressure on the planter. The owner's guide also lists heavy garden soil and fine sand among the things not to use as a bottom layer.
Decide where the tree will stand before you fill it. Once the soil, the root ball and the water are in, the planter is a small share of the total weight, and moving it means emptying it. Our guide to filling a large fiberglass planter sets out the method and the lightweight alternatives to a full-depth soil fill.
Outdoors, the planter needs drainage holes. Jay Scotts drill them free of charge at order time, to their standard of 1 to 2 inch holes every 12 inches of planter space, and a drilled planter is final sale. In a freezing climate the planter is not the weak point, the roots are: NC State Extension notes that pots holding fruit trees or bushes that stay outside through winter may need insulation to protect the root ball from freezing. Our article on fiberglass planters in winter covers the rest.
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Fiberglass tree planters
Every size lists its interior opening, so you can check the root ball before you order.
Frequently asked questions
What size planter does a 7 foot tree need?
The height does not decide it; the container the tree is growing in does. Ask the nursery for that container's width and height, then choose an interior opening 2 to 3 inches wider and a depth that leaves at least 3 inches of mix underneath and the root ball 1 to 3 inches below the rim.
What is the best planter for a tree?
One whose published interior clears the root ball on both width and depth, with drainage holes if it stands outdoors. Straight-sided cubes, boxes and cylinders give the most usable room for their footprint, because their opening does not narrow below the rim.
Can a tree live in a fiberglass planter permanently?
It can, provided the planter drains and the roots have room. Watch for roots pushing out of the drainage holes or a dense, circling root ball; NC State Extension treats both as signs the plant needs a larger container.
Should I fill a tree planter with topsoil?
No. Jay Scotts say to use only potting soil, potting mix or container mix, because top soil is heavy, becomes heavier when wet and puts outward pressure on the planter.
Do tree planters need drainage holes?
Outdoors, yes. Jay Scotts drill them free at order time, to a standard of 1 to 2 inch holes every 12 inches of planter space. A drilled planter is final sale, so decide before you order.
How deep should a tree planter be?
Deep enough for the root ball plus at least 3 inches of mix underneath, with the top of the root ball 1 to 3 inches below the rim. For a root ball 15 inches tall, that means an interior height of at least 19 inches.
