UNITED STATES—Mild climates allow more flowers to bloom through autumn and winter here than in most other parts of America. That is why cool season annuals like pansies and violas are so popular. Cyclamen can be planted now too. None will be obscured by snow. By the time cool season annuals start to fade, warm season annuals will be replacing them. There is something to bloom in every season.
There are a few disadvantages to mild climates, though. Many plants rely on significant winter chill to stay on schedule. Inadequate chill limits the cultivars of apples and pears that are productive here. Not many spring bulbs will naturalize. Prior to winter, some deciduous plants are hesitant to resign to dormancy until they experience a chill that is cool enough to convince them it is autumn.
Some deciduous plants recognize a specific temperature as credible evidence of a change of seasons. Others want a specific temperature to be sustained for a specific duration or repeated for a few nights. Shorter days and longer nights are taken into consideration by species who want to confirm what they deduce from the weather. Different plants use different methods of observation.
That is why deciduous plants who develop foliar color before defoliating in autumn do so on their own terms. Weather conditions that promote excellent color among birches may not be the same that cause flowering cherries to color well. Warmth and minimal humidity that sometimes prompt premature and blandly colored defoliation of sycamores might enhance later color of sweetgums.
Sweetgum, Chinese pistache, flowering pear and ginkgo are the most reliable trees for foliar color in autumn. Ginkgo turns only brilliant yellow. The others exhibit an excellent mix of yellow, orange and red. Crape myrtle can be about as colorful, but is not always as reliable.
Of course, there is more to these and other deciduous trees than their colorful foliage in autumn. After all, they are trees. Their particular characteristics and appropriateness must be considered before adding any of them to a landscape.
Highlight: European white birch
Like a flower girl scatters rose petals ahead of a wedding procession, European white birch, Betula pendula, tosses its small deltoid leaves soon after turning soft yellow with autumn chill. Color may not last long on the trees, but becomes a delightful mess for those who appreciate such assets. The primary allure though, is the slender strikingly white trunks, accented with black furrowing.
European white birch is very informal, but also elegant enough for formal landscapes. To best display their gently leaning white trunks, they are popularly planted in relaxed groups. Their canopies are neither broad nor dense, so a few fit together nicely. As lower branches get pruned away, pendulous upper branches sway softly in the breeze. Mature trees are mostly less than 50 feet tall.
Himalayan birch, Betula utilis or Betula jacquemontii, which has become more popular than European white birch since the 1990s, has a completely different personality. Its strictly vertical trunks and upright growth are appealing separately, but incompatible with European white birch. When adding trees to an established grove of any birch, it is very important to procure more of the same.
Horticulturist Tony Tomeo can be contacted at tonytomeo.com.