Trees are vital for cities. They provide cooling shade and purify the air. Yet, there is a shortage of trees within city limits, and sadly, most urban trees have an unhealthy seven-year lifespan. One might be tempted to grab a shovel and run out and plant a field of trees, but in the limited space of our urban forest, the planting of trees must be deliberate so they grow to their full potential and do not become obstacles if they grow too large for the space. To plan for trees within cities, Edward Kuprel, City Forester for the City of Edinburg, recommends introducing shorter, more compact canopies to provide cooling shade, beauty, and critical habitat for urban wildlife including butterflies, hummingbirds, pollinators, and other sometimes surprisingly beneficial wildlife species, like parrots! The best tree planting practices involve planting in raised beds to protect the tree’s roots from sitting in water. Kuprel likes to look at each planting bed as an ecosystem consisting of trees, shrubs, and groundcovers in a raised, mulched, composted bed, even in small spaces.
In many city areas, there is not enough room for a large, full canopy tree such as a Live Oak, Rio Grande Ash, Cedar Elm, or Texas Ebony due to proximity to buildings, roads, sidewalks, and other utilities such as power or sewer lines. Trees with compact crown canopies and root areas are able to take advantage of tight spaces such as those between the roadway and sidewalk. The perfect choice could be the Caesalpinia mexicana, or Mexican Bird of Paradise, which produces beautiful yellow flowers throughout the year. It is very effective under powerlines as it only grows 10-12 feet, yet is about as shady as a Wild Olive (Anacahuita), a Mesquite, or a Coma. That’s enough for shading a picnic table in your yard.
However, the Caesalpinia mexicana is often overlooked as a tree, more often seen as a multi-trunked tall ‘shrub’ growing along fence-lines in many valley backyards. “I try to encourage pruning this into a single-stemmed tree instead of a multi-trunked shrub,” says Kuprel. There can exist up to five codominant stems (all close to the same height and will be competing to be the tallest branches) when the goal is to have fence-height hedge shrub, but to get a compact crowned shade tree we need to encourage a single leader trunk, which can usually be identified as the central, straightest, and most healthy trunk. The first two to three years of aggressive structural pruning are crucial to make Caesalpinia mexicana capable of creating a shady area. “The biggest factor limiting how much we can cut off at this first pruning is to try to keep this chosen leader branch with the top ½ to 2/3 leaves and containing many narrow live branch-lets.”
Life is tough for a tree. A tree’s branches and food-making centers are often at the mercy of overzealous and untrained pruners, the trunk is susceptible to the whipping of lawnmowers and weed whackers ( known to be the second leading cause of death to urban trees in the first two years after planting), and even the roots can be hurt by improper care. The underground root systems of urban trees make up more than half of the tree, and regulate the first leading cause of tree death: under/over watering. Under-watering seems a likely culprit in our dry, desert-like atmosphere, but native plants are very drought resistant, and the tendency might be to accidentally over-water. When native trees sit in standing water, they cannot get oxygen and turn mushy (smelling like sulphur) and “drown.” Overwatering can be sometimes be made worse by having planted the tree too deeply into poorly draining clay soil surrounded by with concrete. The compaction of the wet soil by pedestrian or vehicular traffic on the critical root zone of the tree leaves no voids or pores in the soil. As trees take in oxygen through the roots from the water in the soil around them, soil compaction reduces oxygen and carbon dioxide exchange. “We need to plant in raised beds (which ideally should be mulched to a 2” depth) so that when the rains come, our trees will not have ‘wet feet,’” advises Kuprel. Raised planting beds (at least four by eight feet or a six foot circle) prevent this by allowing roots to spread, and raising the level with heavily composted beds topped with thick layers of mulch and groundcover plantings keeps the ground pervious (with aerated pores).
Raised beds also offer opportunities for trees to work with companion plants. “I always want to include a tree as a part of my mulched and composted, raised ecosystem with companion plantings, which helps a city tree to live for its normal healthy lifespan of 25 years or more in the case of the Caesalpinia, and 75 to 100+ years for the Mexican Olive,” he says. Caesalpinia acts as a ‘nurse tree’ for slower growing larger trees. Using Caesalpinia around the base of the tree can also help deflect away damage from chewing and scratching animals and human damage. Kuprel is experimenting with the planting of a Caesalpinia next to a slower-growing, large canopy shade tree. It provides the slower growing tree with some protection from overhead heat and sun as well as some cold protection against the northern wind.
Trees and shrubs of various heights in the planted ecosystem give the raised bed depth with many canopies protecting each other from the elements, and in time becomes self-sustaining. The ecosystem will attract hummingbirds, butterflies, pollinators, and other beneficial urban wildlife. Caesalpinia pulcherrima, cousin to the Caesalpinia mexicana, is thought to have evolved together with certain butterflies and even some hummingbirds. It can be planted on edges of the Caesalpinia mexicana planting beds as its reddish flowers, like the tubular-shape flower of the pulcherrima, reinforce the possibility that hummingbirds and certain butterflies will come to your garden!
When we design raised, composted, mulched planting beds with a tree with shorter, more compact canopies in the center surrounded by some of the above mentioned companion plants, we not only help our urban environment, we also create vegetated areas acceptable to power, cable companies, and city utilities (both overhead and underground). This ensures that care taken when planting these trees will not be in vain, and these trees will not be uprooted in the future. “Our group, Edinburg Forestry Partners, has convinced even the most skeptical planter that raised beds allow for better growth and less damage from flooding rains,” says Kuprel. Even small planting beds can be raised, and Forestry Partners come in all shapes, ages, and sizes. Kuprel encourages all homeowners to plant a raised bed with companion plants. “I hope these tips will help you have happy, healthy trees and plants. Happy Planting!”
Side Bar 1: Suggested combinations for attracting butterflies and birds to raised beds:
-Between the plantings of the Caesalpinia pulcherrima on the outside edges of the 4’ x 8’-10’ bed, with the Caesalpinia mexicana structurally pruned to be the tall shade component, companion under-plantings of Betony and/or Gregs Mistflower (Eupatorium betonificolium & greggii)- as one of the two will usually adapt well to a site
-Turk’s Cap (Malvaviscus arboreus) tends to be a hummingbird magnet almost without equal.
-The Mistflowers are a super nectar source, especially to Monarch and Queen butterflies and keep the garden full of fluttering orange wings. The tropical as well as its less-showy and native milkweed cousins are another mainstay for Monarchs.
-Pigeonberry (Rivina humilis), Tropical Milkweed (Asclepias curassavica) and Mexican or Katy Petunia (Ruellia simplex) can fill in the smaller lower gaps between the mainstay shrubs and they all attract more than their share of butterflies and birds, especially hummers.
-Petunias and Pigeonberries will bring in native birds, hummingbirds, and smaller butterflies.
side bar 2: HOW DO I MAKE A RAISED BED PLANTING?
- Clear off all grass, dig out roots, and loosen soil in a 6 foot circle or 4X8 rectangle
- Mix in leaves, compost, grass clippings & other decomposed organic material (not grass roots)
- Make a huge mound one to two feet above ground level by shoveling in dirt from the outside edges (mixed in with compost, humas, or other good organic soil conditioner), on top of your loose material from step 2. Stomp lightly to solidify.
- Dig hole for tree inside of mound so most of the root ball is above the ground.
- Fill over tree roots, stomp and jump hard to solidify tree and remove air pockets.
- Create a water dam (lip on edge) around the circle. This will allow when watering that the water will not runoff and will stay around the tree. If you make the ring in a 2’ radius around the trunk it will hold at least 2 gallons of water.
- Water well every 3 days or twice a week with at least one gallon- or even better two gallons for the first 3 months, except when ground is wet from rains. Water once a week after 3 months at least through the first year.