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Photograph by Shari Kaplan
The blue dawn flower, sometimes called perennial morning glory, is a strong, fast-growing vine that can be used to cover trellises, fences or embankments.
Not all trees have what it takes to be street trees
By Tony Tomeo
It is ironic that the one person I know who hates trees more than anyone else is my own grandfather. Seriously! It is difficult to believe that we are related. I believe his disdain for trees resulted partly from his career in finishing concrete, which involved replacing curbs, gutters, sidewalks and sometimes driveways and patios that had been damaged by tree roots.
Although I disagree with Papa (my grandfather) that any tree within 50 feet of concrete must be removed, I know how important it is to select appropriate tree species for each application.
Street trees (those installed between the street and sidewalk) need to be compatible with the confining nature of their environment. Obviously, they should exhibit well-behaved root systems so that damage to concrete will be as minimal as possible. They should also be tolerant of adverse conditions, well-structured, structurally stable and clean.
Tree species that do not tolerate toxic fumes, such as pines, may perform well in the garden where air is well-mixed. Foliar damage, however, may result from regular, direct exposure to engine exhaust. Street trees must also tolerate reflected glare from pavement. For example, the flowering cherry trees installed as street trees in San Jose's Japan Town are mostly damaged by sunscald on their sensitive trunks.
Well-structured street trees are either small enough to avoid the flow of traffic, or produce the broad part of the canopy above the height of trucks. Low, broad trees, such as the Chinese pistache, require considerable structural pruning to avoid damaging or being damaged by tall trucks.
Structural integrity should also be considered because trees that may lose limbs in high winds or during warm, humid weather can easily squash a Pontiac. Limb failure may occur during warm, humid weather because heat stimulates vascular activity, and humidity decreases the amount of moisture lost by evapotranspiration. The weight of the water translocated to foliar surfaces, but not evaporated, may be more than limbs can support.
Cleanliness is also more important for street trees than for trees in the garden because no one wants to rake litter in the street or empty clogged gutters. Next week, I would like to discuss some of my favorite street trees.
Flower of the Week: Blue dawn flower
I am not certain if the blue dawn flower, Ipomoea acuminata, should be classified as an ornamental perennial or a weed. I had difficulty locating one when I wanted it for my own garden. Once it was established enough to bloom, however, it produced so many seeds that it now grows everywhere and competes voraciously with everything else it encounters.
Because they can climb quickly to 30 feet, dawn flower vines should not be permitted to climb into trees that could be overwhelmed. Instead, they are useful on large, sturdy trellises, embankments and even unsightly chain link fences.
The large, dark green leaves are softly textured. The 3- to 5-inch-wide flowers are bright purplish blue and may fade to pink if not desiccated by direct sun exposure or dry, warm air. New plants grown from seed do not bloom much during the first year, but make up for it the next.
Unlike other species of Ipomoea, blue dawn flowers will grow in even poor soil but prefer generous irrigation. They draw considerable moisture from the soil to compensate for a high evapotranspiration rate and a disproportionately lush foliar surface area.
The vines grow very easily from seed and may also be propagated by cuttings, division or layering of mature vines. Most people who enjoy them, however, are more interested in getting rid of unwanted seedlings than producing more.
A good example of established blue dawn flowers that exhibit both showiness and voraciousness may be seen on the embankment of southbound highway 880 just past the on-ramp from the Alameda. They have grown there mixed with Algerian ivy longer than I can remember.
Blue dawn flower is also known as morning glory or perennial morning glory. The other two species of Ipomoea, which are known by the same common name, are both summer annuals and may be a disappointment to anyone expecting them to perform like the blue dawn flower.
Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.
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Sunnyvale, California Since 1994
