What influences fall color?

by Molly Nemec

Chisago County Master Gardener  

The variety of autumn colors depends on the temperature and amount of precipitation in late summer and fall.  The temperature before and during the time chlorophyll dwindles in leaves is a key factor.  Warm and sunny days, along with cool and crisp nights that aren’t too frigid, will create the most spectacular color display. The leaf produces a lot of sugar on warm days and the leaf veins slowly close during cool nights.  The closing keeps the sugars from moving out.  Sugar and sunlight produce anthocyanins, which turn leaves red and purple.  Carotenoids are always in leaves, causing yellow and gold colors to remain fairly constant from one year to the next.

Soil moisture also has an effect on tree color.  Early displays of color can be attributed to cool, wet summers.  Red foliage color may not be as striking when autumn is rainy and cloudy much of the time.  The leaves make less sugar in the reduced sunlight.  Sugar moves out of the leaves during warm nights and the leaves have less of it to form anthocyanins and thus, turn red and purple.  A mild drought can brighten the display; a severe drought may make colors less intense.  Sometimes, the leaves die early from lack of water.   Pests, diseases and environmental problems may contribute to lack of foliage brilliance, as well.

So what plants should you choose for great fall color?   Maples offer the most vivid fall color, bar none.  For red foliage, look for the cultivar “Autumn Blaze” (Acer plus fremanii).  This vibrant and very fast-growing hybrid combines the best traits of the parents, silver maple and red maple.  “Northwood” is another beauty with brilliant orange-red or red fall foliage, and showy red flowers along the branches in early spring.  It is also very cold hardy and is an excellent performer in northern areas.  “Firefall” is a University of Minnesota creation that is known for early fall color, ranging from orange to fiery red. The sugar maple (Acer saccharum), also known as the Wisconsin state tree, grows slower than the red maple.  It’s a long-lived tree and very desirable for its symmetrical form.  It turns anywhere from yellowish-orange to bright red.  The cultivar “Legacy” shows off impressive fall color. The amur maple (Acer ginnala) is an ornamental tree ideal for smaller spaces.  It has dainty leaves and exhibits colorful red fruit in summer.  Fall color is orange-crimson to deep purple.  Look for the varieties “Embers” and “Flame.”  The silver maple (Acer saccharinum) is the fastest-growing maple and makes a very good shade tree. The cultivar “Silver Queen” is gold in autumn. Among the oak trees, red oak (Quercus rubra) is an excellent choice for a yard, given its strong wood and glossy leaves that turn burgundy in autumn.  Pin oak (Quercus palustris) is one of the fastest-growing oak trees.  This tall, symmetrical tree with pyramidal form, displays burnt-colored leaves in autumn.  Though the pin oak and red oak varieties offer rich fall color, they are also susceptible to oak wilt.  Oak wilt is a fungal disease spread by beetles that affects the trees’ vascular systems.  Pruning oak trees in February and March, and not in summer, will help prevent this deadly disease from spreading.

In the white ash family (Fraxinus americana) “Autumn Blaze” and “Autumn Purple” are cultivars that were selected for their deep-green leaves that turn reddish-purple in fall.

There are several shrubs that can add dazzling color to your landscape.  Burning bush (Euonymus alata) is a large shrub with unsurpassed, bright-red to crimson-colored foliage in autumn.  Burning bush is a favorite among gardeners and is highly prized this time of year.  Highbush cranberry (Viburnum trilobum) sports small, fiery-red berries in late summer and red foliage in autumn.  Popular cultivars include “Alfredo” and “Compactum.”  Dwarf summer-blooming spirea (Spiraea japonica) is covered with flat clusters of flowers and yellowish-green foliage in summer.  It turns from soft pink to reddish-orange in autumn.  The cultivar “Goldmound” is a neat, compact shrub that looks nice near buildings and in mass plantings.

Autumn is a great time to explore nurseries in search of striking fall foliage for years to come.  Plan your landscape, not only for an array of autumn color, but winter, spring and summer, too.  Many of these aforementioned “all stars” will amaze and beautify your yard all year long.

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