Planting A Parkway Tree? Here’s What You Need To Know

Parkway trees are not “City trees”.

Most deeds show property ownership extending to the middle of the street. According the the City, “whoever owns the fee title in the parking or parkway owns the trees that grow thereon, which are a part of the realty, subject to the power to remove or regulate their growth when necessary to the enjoyment of the street for purposes of travel” (City of Los Angeles Bureau of Street Services 2008). The adjacent property owner is personally responsible for watering and routine maintenance of existing trees and trees they plant in the parkway, though the City may return to prune mature trees when funding allows.  The City, or one of its tree planting partners, will stake and water trees it plants in the parkway for about 3 years.   After 3 years, the adjacent property owner goes back to being responsible for watering and maintaining the tree, though the City may return to prune mature trees over the years when funding allows. They may also return to remove dead or diseased trees or trees that are lifting sidewalks as part of the sidewalk repair program.

Deforestation by Design

Despite ambitious tree planting programs, city tree canopies have been in decline.  Twenty-five to 40% of street trees die within the first 6 years after planting.  A smaller percentage of them will still be alive at age 35.  The high mortality rates have to do with poor planting techniques, but no single activity contributes more to tree mortality than a lack of care after planting.  Most people love to get a free tree for their parkway, but should consider that keeping trees alive, healthy and safe requires a considerable amount of care and maintenance that the City of Los Angeles does not provide.

Well executed planting projects start with appropriate design, site evaluation and tree selection. Site conditions and after care capabilities should dictate tree planting. Many trees are planted in compacted soil and under-irrigated, so they perform poorly following planting. Management of the landscape site in the early years following planting will also dramatically affect establishment rate and success of the planting. With poor planting techniques and/or poor soil conditions the establishment phase may take many years. It is common to observe trees that never establish, but rather simply hang-on for a few years and gradually decline.

Tree Species Selection

Irrigation capabilities, space (e.g. width, length, soil volume), maintenance and ecosystem services should all be considered when selecting tree species. Some tree species are better than others for biodiversity and improving the urban forest. For example, there are only 4 exotic tree species (American sweetgum, Southern Magnolia, Chinese Elm, Camphor) that actually improve the ability of the urban forest to support a diverse bird community. Native trees provide the most ecosystem services. Hence, the we should all work to promote the planting of native trees and select exotics.

Planting

More than half (some professionals say 80%) of tree health problems come from poor planting practices, and soil or root system problems.  decades.

1. When at all possible, plant the youngest tree you can because the larger the specimen that you plant the more chance for establishment problems such as settling, drying out, root rot or slow growth.

2. For rapid root establishment, the focus needs to be on planting-hole width and correct depth and modification for compacted soils. A saucer-shaped planting hole three-times the diameter of the root ball supports rapid root growth, reducing post planting stress. Ideally, soils in a tree’s entire potential rooting area would be modified and amended to a 5% organic content – Amending backfill soil in a planting hole will not substitute for modifying soil in the entire rooting area. In backfilling the planting hole, the best method is to simply return the soil and let water settle it.

3. Replace Parkway Ground Covers (e.g. boulders, decomposed granite, turf grass, artificial grass) with Mulch Beds. Trees planted in beds of mulch will be healthier than trees placed in turf or other ground covers. On newly planted trees, organic mulch can increase fine root development by 400% compared to grass. This results in 20% faster canopy growth.

4. What NOT to do: 1) Amendments and Backfilling. In all but exceptional circumstances where the soil is very poor, extensive research on trees clearly shows that there is no need to incorporate any amendments, fertilizers, living organisms, spores, dusts, powders, gels, humic acids, organic products, etc. into the backfill soil (Gilman 2001; Henderson and Hensley 1992; Ingram et al. 1981; Paine et al. 1992; Schulte and Whitcomb 1975; Smalley and Wood 1995). Water is the best amendment. Simply use the loosened soil that came out of the planting hole. The exception to this rule is where existing soil is so terrible or contaminated, such as in a parking lot island, parkway, or in a small cutout in a sidewalk, that all soil over a large area is replaced with good-quality soil. Amended backfill soil may be more supportive to root growth in the planting hole during the first two years. However, amended backfill may also hinder root spread beyond the planting hole. In tree planting, it is a common procedure to amend backfill soil with organic matter, but it’s really just a marketing technique for the nursery to recommend soil amendments with the sale of a tree. 2) Staking where trees are set on undisturbed soil and a ring of soil is firmed around the base before backfilling, is NOT needed. Trees establish more quickly and develop a slightly stronger trunk and root system if they are not staked at the time of planting. To hold a weak trunk upright should not be necessary on trees with a trunk diameter more than about 1.5 inches. If large trees require staking to prevent the trunk from bending, it probably indicates a lesser quality tree with poor trunk taper or poor root system. 3) Root Barriers. There are about 6 published studies on these devices that show that once the roots reach the bottom of the barrier they continue to grow out from underneath and back up to the surface once they reach the other side. The studies found that six feet from the barrier most roots were in the top 12 inches of soil. In other words, root barriers do not achieve the goal of preventing surface roots and protecting infrastructure. Trees outgrow root barriers. Root barriers can also create root defects such as circling and girdling roots. Do not install root barriers.

Irrigation: Establishment Phase

No matter how much time and effort goes into the ‘perfect’ planting hole, street trees that are not irrigated after planting are doomed. Adequate watering during establishment (approximately 5 years in our climate) is literally a matter of life and death for new trees. Unlike established plants, research clearly shows that recently transplanted trees and shrubs establish quickest with light, frequent irrigation. Frequent irrigation after planting encourages rapid root growth that is essential for tree establishment. Irrigation helps maintain and encourage the desirable dominant leader in the tree canopy on large maturing trees. Instead of a dominant leader, trees that are under-irrigated during the establishment period often develop undesirable, low, codominant stems and double leaders that can split from the tree later and cause risk to person and things.

5. Apply water over the root ball a minimum of three times each week during the first few months after planting. Water bags empty within 5 to 10 hours of filling, which is too rapid to reduce frequency of watering so any plans to fill bags every 7 to 10 days is insufficient. Water bags would require a minimum of 2 fills per week. More frequent (as much as daily) watering is required for trees planted during the warmest
and driest months.

6. Provide Frequent Irrigation Until Plants Are Fully Established. Irrigate 2 to 3 times a week. At each irrigation, apply 2 to 3 gallons of water per inch trunk diameter. With good planting techniques and soil conditions, the establishment phase takes one to two growing seasons per inch of trunk diameter. A two-inch diameter tree typically takes two to three years, a three-inch diameter tree typically takes three to four years, a four-inch diameter tree typically takes four to five years. Note: There is benefit from applying additional irrigation outside of the root ball area. This cannot be accomplished with watering bags.

7. What NOT to do: Use High-Profile Water Bags such as Tree-Gator original unless you remove them between filling. When the bags are full, they press against the trunk, creating a humid, dark environment that’s only made worse when rainwater seeps into the space. The image below shows a rotting trunk, allowing insects and disease into the living tissues – in the tree’s rotting trunk.  City staff have attempted to solve this problem by attaching the water bags to the tree stakes instead of over the trunk as designed. That’s a problem because the rootball of the tree is at the base of the trunk and the bag needs to be right over the rootball so it does not dry out.  If it does, the tree takes much longer to establish or it never establishes, has structural and health issues, or topples on people and things, or dies.

Irrigation: Young & Mature Trees

ALL TREES NEED WATER! No matter how drought tolerant a tree species is, all trees in man-made landscapes must be watered by people to survive. Lack of water can cause decline and death on young and mature trees. Even large trees more than one-hundred years old can decline with a lack of water. Foliage on drought stressed trees thins and drops beginning at the top center part of the canopy. This can be largely prevented with
timely application of irrigation, and appropriate soil management. It is better to apply water preventively before the canopy begins loosing foliage than to wait until these symptoms appear. Many newly planted and mature trees in the southwest decline or die from extended drought. Young and mature trees alike can benefit from irrigation in dry weather. One or two years of drought can be tolerated by many well established trees only if roots can explore soil unrestricted by curbs, buildings, roads, and other root barriers.

1. Young Trees – Irrigation is most efficiently applied to the soil surface using drip or bubblier system which is designed to deliver water slowly, minimize evaporation, and prevent runoff. Apply about 5 gallons of water per 1 inch of trunk diameter twice per month, twice the width of the root ball. During a drought, water once weekly until normal rainfall occurs.

2. Mature Trees – For mature trees apply about 10 gallons of water per 1 inch of trunk diameter (measured at breast height). Evergreens need heavy watering going into the winter, and need watering during winter droughts. It’s important, particularly with mature, established trees, to water the entirety of the soil volume, even the part under paving. If there is no automatic irrigation system (bubblers, drip), I suggest using a soil watering needle with a watering hose connected. Add water to wet the top 12 inches of soil every 2 to 4 weeks in extended drought. Apply water to
all soil under the canopy if possible. Inconsistent, infrequent or insufficient watering forces the tree to produce only the foliage that it can sustain, which reduces the trees capacity to cool the air, and remove carbon and pollution from the atmosphere while taking vital food and habitat away from wildlife. A subsurface application using an injection needle can be useful if soil is covered by hardscape such as asphalt or concrete, or if soil is compacted. Davey and other companies perform subsurface watering with a high-pressure soil probe, a tool attached to a hose that’s plunged into the soil so water reaches the root zone. This service costs about $165 for a large tree or a couple of smaller trees. Residential water pressure is typically too weak to penetrate the soil in a useful way, especially if the soil is very compacted.

3. What NOT to do: Use Water Bags Long-Term. Watering bags are specifically designed to deliver water to the root ball of newly planted trees. Bags are NOT a permanent installation – they are meant to be used on as-needed basis, as means of watering a newly planted tree or shrub. The bags are most often used during the initial 1 to 2 growing seasons after planting. Young and Mature, established trees, need water delivered to the entirety of the soil volume, even the part under paving, where roots grow.

Maintenance:

1.Mulch. Generally trees thrive with mulch under their canopies and beyond. At minimum, a 3 inch to 4 inch thick layer of mulch should be maintained directly over the rooting area to conserve moisture, moderate soil temperatures, suppress weeds, and prevent soil compaction. Mulch keeps turf away so mowers are less likely to injure the trunk and compact the soil. Mulch reduces moisture evaporation from the soil surface. This can improve root growth, especially during warm dry weather. Mulch also suppresses weeds and turfgrass that can compete with tree roots for
moisture and nutrients. Mulch adds organic matter to the soil system and appears to encourage earthworms and other beneficial soil organisms. Killing the turf and adding mulch can improve health on declining trees. Do not pile any mulch against the trunk and keep it off the root ball on newly planted trees (keep mulch about 12 to 18 inches from the trunk for any size of tree). Replenish the mulch as it deteriorates (usually twice a year is sufficient). Routine maintenance is required to ensure mulch is never up against the trunk.

2. Pruning Young Trees. In the parkway landscape, the abundance of sunlight all around the canopy encourages trees to develop multiple, competing trunks or leaders. This type of structure is susceptible to mechanical breakage and can reduce life expectancy. But trees with one dominant leader and small well-spaced branches are less likely to suffer this type of mechanical failure. The dominant leader structure also makes trees better able to retard the spread of decay within the tree.

3. Pruning Mature Trees. In the most general terms about 25% of the live canopy is the absolute maximum you should remove at any one time. Two of the main factors determining the appropriate amount of live canopy to remove are the age of the tree and the overall condition or health of the tree. The upper limit of 25% is most appropriate for young and mid-sized trees that are well established and in generally good health so they will be able to recover from the shock of having up to a quarter or their live canopy removed. As the age of the tree approaches maturity, even if the
older tree is in generally good health, the upper limit should be decreased to 10% to 12% of live canopy because the pruning process, in general, can be a serious shock to the tree and anytime you initiate wounding on a tree, which is what you are doing when you prune, there is a codive response (tree tries to compartmentalize the cuts). Older trees have more trouble compartmentalize the wounds.

4. Modification for Compacted Soil. Compacted soil leads to root sidewalk conflicts and unhealthy trees. Several methods can be employed to improve soil conditions including vertical trenching, incorporating organic matter, and air or water injection into soil. 1) Vertical trenching dug with use of an air spade cuts trenches like spokes in a wheel from near the trunk toward the edge of the canopy. New topsoil is gently placed, not packed, back into the trenches. Roots grow back into the loose soil added to the trenches. Trenches can be dug with air spades to avoid damaging roots.  2) Incorporating organic matter into the top soil layers can loosen compacted soil and can improve soil biology. Organic matter can be incorporated into the soil using an air excavation device. Begin by loosening soil with the Airspade or Supersonci Air Knife. Next, add several inches of well-decomposed organic matter and mix it with loosened site soil using the air tool. 3) Air and water can be injected into soil using a compressor and hollow soil probe. Some tree companies use this technology for injecting fertilizer into soil. This has been done on dry compacted soil with the intention of creating cracks for air movement. Some horizontal soil cracks form as a result of this treatment but research has yet to show a positive tree response from this treatment.

5. What NOT to do: 1) Prune Live Branches in a Drought, Heat-Wave or When Tree is Stressed. When a tree is unhealthy, or diseased pruning live branches is not recommended because this removes live tissues forcing the tree to expend energy to defend against the pruning cuts. Removing live foliage also reduces the capacity of the tree to grow once rains return. 2) Prune Newly Planted Trees before they are established (approximately 5 years in our climate). Newly planted trees need time to get over the transplant shock.