The more space you are able to mulch, the better for your tree. Some landscape standards call for mulching to the ‘drip line’ of the tree, or to the edge of the tree’s leaf canopy. This may be impractical if you have a large tree in your yard whose canopy spans the entire lawn, so a smaller circle of mulch around your tree may be more appropriate. The larger your mulch area, the greater long-term improvement in tree health and growth.
Street trees planted in 3‑by-10-foot tree pits (standard Pittsburgh tree pits) receive the most benefit from mulching the entire 30 square foot area.
Mulch wide, not deep, and do not allow mulch to touch the trunk of the tree.
Tree roots need oxygen, and if the mulch layer is too thick it can limit the amount of oxygen available to the roots below. Therefore, limit the thickness of mulch around trees to a maximum of 3 inches.
It is extremely important NOT to pile any mulch up against the trunk of the tree. Even though this misguided practice is widespread in the landscape industry, it will cause decay in the bark of your tree and can kill a young tree after only a few years.
Tree Pittsburgh uses strictly hardwood mulch. This is a local product made up of old trees, wood pallets, saw mill waste, and other recycled wood that has been chipped up into small pieces. This material breaks down at a good pace and stays put when applied in tree pits. Pine bark mulch is okay, but tends to wash out of tree pits too easily.
We do not recommend cypress or rubber mulch.
Cypress mulch is not a sustainable product and should be avoided. Cypress trees are removed from around the Gulf of Mexico, and hurricane-prone areas suffer greater damage in storms as each one is removed.
Rubber mulch does not provide many of the benefits mentioned above. It will not decompose and will remain mixed with the soil for many, many years. There is speculation that rubber mulch may absorb heat and cause damage to the tree.
- Begonia (Elegant flowers that come in a large variety)
- Coleus (Variegated foliage, purple flowers)
- Lobelia (Small edging plant)
- Pansy (Some are winter hardy)
- Viola Torenia (Looks like a small pansy)
- Impatiens (Popular colorful flowers that will do well in your tree pit, sun or shade)
- Monkey Flower Mimulus (A variety of colorful flowers)
- Browallia (Fabulous blue color)
- Fuchsia (Variety of species, most annuals bear a red tubular flower)
- Elephant’s ears Bergenia cordifolia (Holds good winter color)
- Coral Bells Heuchera
- Spotted Deadnettle Lamium maculatum (Excellent groundcover)
- Lilyturf Liriope muscari (evergreen)
- Foam Flower Tiarella cordifolia (Vigorous groundcover with white upright flowers)Plant suggestions for tree pit gardens came from the New York City Parks & Recreation Website.
- Sweet Alyssum (Mat forming, small yellow or white flowers)
- Dusty Miller – Senecio cinerara (Beautiful snow white and shiny silver color)
- Licorice Plant Helichrysum
- Portulaca-Rock Rose, Moss Rose (Good drought-tolerant plant)
- Geranium (An old standby that requires little water)
- Scented Geranium (For leaf form, color and small flowers)
- Heliotrope (Beautiful scent)
- Cosmos (Dwarf – height 24”)
- Ageratum (Low growing with small purple flowers)
- Blue Marguerite Daisy
- Lantana (A variety of unique flowers)
- New Guinea Impatiens
- Ox-eye Daisy chrysanthnum leucanthemum (Beautiful white flowers)
- Bugleweed Ajuga reptans
- Snow in Summer Cerastium tomemtosum
- Lilyturf Liriope muscari
- Sedum Sedum albuonor acre (Only low growing)
- Thyme Thymus serpiphylluur or pseudolanugipsus (Mat forming varieties)
- Can dissuade dog owners from letting dogs relieve themselves in a tree pit.
- Serve as reminders to owners to water the plants and the tree that was recently planted.
- Prevent cyclists from locking bikes to a newly planted tree.
- Look great!
Guidelines for Tree Pit Gardening:
- Do not plant vines or plants that will creep up the tree trunk or onto branches.
- Plant at least one foot away from the tree’s trunk to avoid disturbing the roots.
- Plant shallow rooted plants that won’t compete with the tree’s nutrients.
- Remember to water the tree pit enough so that both the plants and the tree get water!
- Consider the needs of pedestrians and people exiting parked cars. If your tree pit is next to a parking space that is not exclusive to you, do not plant large items that would act as obstacles to those exiting vehicles. Also consider placing pavers or other flat stones in the tree pit. Not only does this give people a comfortable place to step across, but it reduces the amount of compaction that the soil will experience by being frequently stepped on.
Tree Pit Approved Plants
- Choose plants that require little watering. Key words to look for are “drought tolerant” and “xeric conditions.”
- Use small plants and bulbs – large plants require large planting holes, which damage tree roots. In addition, plants with large root systems compete with the tree for water and nutrients.
- Do not add more soil to your tree pit. Raising the soil level will harm the tree.
- Mulching a tree pit is always good for your tree and plants. Mulch keeps the soil moist and prevents weeds from sprouting in tree pits.
- NEVER PLANT: Bamboo, Ivy, Vines, Woody Shrubs, Evergreens or Invasives! They are all major competitors for water and nutrients and can stunt or kill a tree. See a list of invasives in Pennsylvania here (http://www.dcnr.state.pa.us/forestry/InvasivePlantBrochure.pdf).
Bulbs are a nice addition to tree pits, and neighborhoods often plant them in the fall during tree plantings. Many bulbs will return each spring, bringing color to your neighborhood. They are both affordable and easy to plant.
Winter can be a harsh season for newly planted trees. Take a little bit of care with your trees this winter so that they will be nice and healthy in the spring.
Give trees fresh mulch in the fall. A new layer of mulch, kept away from the trunk of the tree in the shape of a donut, insulates the tree’s roots during the coldest months. If you missed giving your trees new mulch this fall, be sure to give them fresh mulch in the spring.
Be Careful with De-icing Products. Rock salt (sodium chloride), or de-icing mixes that contain rock salt, prohibits a tree’s finer roots from absorbing water, nutrients and oxygen. If a tree’s soil receives a lot of salt in the winter, the tree will likely suffer in the spring and could potentially die. Alternative products such as calcium or magnesium chloride are less harsh on the tree’s roots but can still cause harm if used excessively. Follow these tips to avoid salt damage to your trees:
- Use the recommended amount of product. Many people over-apply de-icing products in hopes that it will do a better job of melting ice. Often the ice is melted at the same rate but the excess product finds its way into tree pits.
- Be careful when spreading the de-icing product. When applying de-icers along the sidewalk, be sure to avoid spreading product onto the soil of a tree pit.
- If shoveling salty slush off of the sidewalk, do not pile it on tree pits.
- If you live near a busy road that is frequently salted, consider installing a temporary barrier made of burlap or plastic to block salt from being sprayed into your tree pit by passing vehicles.
Remove holiday lights when the season ends, and do so carefully. Many trees set their buds for new leaves and flowers in the fall, so be careful not to knock the new buds off branches when putting lights up or taking them off the trees. Do not leave holiday lights tied around trees year-round. Lights that are left on too long can end up strangling or “girdling” the tree, keeping nutrients from getting to its branches and killing it.
Prune Dead and Damaged Branches. Winter is a great time to prune as the trees are dormant and their structure is easy to see. If you think now is a great time to prune your young street trees, contact Jake to schedule a pruning workshop (firstname.lastname@example.org).