Can you put a planter box around a tree?

Putting a planter box around an existing tree is certainly possible, but it takes some careful planning and preparation to do it properly without harming the tree. Here are some key considerations when installing a planter box around a mature tree:

Assess the tree’s health

Before deciding to add a planter box, take time to assess the overall health of the tree. Look for any signs of disease, rot, or pest infestation. Check that the tree looks vigorous with full, green foliage and no dead branches. Make sure the trunk flare at ground level looks intact with no girdling roots. If the tree shows any signs of poor health, remedy those issues first before taking on a project like adding a planter box.

Consider the tree’s maturation

Planter boxes are most successful around established, mature trees. Very young or newly planted trees can be harmed by distracting their root growth into a planter box too early. Large, mature tree roots tend to grow outward and downward rather than just under the trunk. This makes it easier to add a planter box without severing a high percentage of the important absorbing roots.

Determine optimal planter box size

Bigger is not always better when it comes to sizing a planter box around an existing tree. The planter box should be proportional to the mature size of the tree without occupying such a large area that you sever too many surface feeding roots. As a general guideline, the bigger the tree trunk diameter, the bigger the planter box can be while still allowing adequate root room. Here are some recommended planter box sizes based on tree trunk diameter:

Tree Trunk Diameter	Planter Box Size
Less than 4 inches	2 feet x 2 feet
4 – 8 inches	3 feet x 3 feet
8 – 12 inches	4 feet x 4 feet
12 – 18 inches	5 feet x 5 feet
18 – 24 inches	6 feet x 6 feet

Accommodate surface roots

One challenge with placing a planter box around an established tree is working around large surface roots. If these are severed or damaged, it can put the whole tree at risk. When laying out your planned planter box location, account for any visible surface roots. Use the following approaches to accommodate them:

• Shift the planter box location to avoid cutting through large roots
• Narrow the width of the planter box in areas with numerous roots
• Elevate the bottom of the planter box to lay over the top of surface roots without digging into them
• Encircle the root with a small bridge within the planter box area to avoid damage

Prepare the surrounding soil

Prior to installing the planter box, prep the area by digging out any grass, weeds, or poor soil surrounding the tree. Plan to dig down at least 12 inches deep around where the planter box will be installed. Loosen the underlying soil with a garden fork or rototiller if needed. Amend the native soil by mixing in 2-3 inches of high-quality garden soil or compost.

Install an open-bottomed planter box

For optimal tree health, it is best to use an open-bottomed planter box design. This allows the existing tree roots to still penetrate down into the native soil. Avoid lining the bottom with any material like landscape fabric or gravel that would obstruct downward root growth. Building the planter box walls atop the amended soil or laying pavers on packed gravel are good options.

Backfill amended soil into the planter box

Once the open-bottomed structure is in place, backfill the entire planter box with the amended soil mix. This will encourage the tree roots to proliferate within the planter box to access water and nutrients. Pack the soil lightly and leave a shallow basin shape near the tree trunk to help retain moisture.

Select appropriate plants

Not all plants are well suited for growing around existing trees. Avoid very water-hungry plants or anything with invasive roots that could compete with the tree. Shallow-rooted herbs, flowers, groundcovers, and leafy greens are good choices. Here are some other tree-friendly plants to consider:

• Ferns
• Hostas
• Astilbe
• Hellebores
• Coral bells
• Mondo grass
• Japanese forest grass
• Caladiums
• Coleus
• Impatiens
• Ostrich fern
• Sedum
• Heuchera
• Epimediums
• Peonies
• Ajuga
• Sweet woodruff
• Viola
• Vinca minor

Provide ample mulch

Cover the soil within the planter box with 2-4 inches of an organic mulch like wood chips, shredded bark, or pine straw. The mulch will help retain soil moisture and moderate soil temperature. Make sure the mulch does not pile up against the tree trunk.

Water sufficiently

Check soil moisture in the planter box frequently, especially during hot, dry periods. Water thoroughly as needed to keep the root zone and any plants moist. Aim to soak the entire root area and not just provide surface sprinkling. Install a drip irrigation system or soaker hose to make watering simple and consistent.

Avoid soil compaction

Excessive foot traffic over the planter box area can compress the soil and make it difficult for the tree to receive necessary oxygen. Limit foot traffic and do not pile heavy materials on top of the planter box around the tree. Occasionally use a garden fork to gently loosen and aerate the soil if it appears to be compacting.

Watch for signs of stress

Keep a close eye on how the tree responds through the seasons after installing the planter box. If you notice any worrisome symptoms like sparse foliage, pale leaves, dead branches, or slower than normal growth, the tree may be struggling. Be prepared to remove the planter box altogether if the stress continues.

Conclusion

Adding a decorative and functional planter box around an established tree can be done successfully, but takes careful planning and regular maintenance. Avoid harming surface roots, keep soil loose and moist, select appropriate plants, and allow adequate root space within the planter box. Monitor the tree’s health and remove the planter box if too much stress occurs. With a little extra care and observation, planting around an existing tree can be safe for the tree and enhance the landscape.