What the Climate Zone Map Can Tell You About a Plant’s Growth Potential – Whittier Daily News

Japanese maple crimson queen. (Getty Images)

Traveling is always a satisfying experience for us plant lovers because of the many opportunities to learn something new about the botanical world. Not only are we introduced to plants that don’t grow in our climate, but we also see familiar plants in a new light as we see them bloom in locations where the conditions for growth are so very different from our own.

When you take biology 101 in college, one of the first terms you learn is “limiting factors to growth.” With plants, the main limiting factor is the cold. If you look at the USDA Climate Zone or Sunset Climate Zone Map, you will notice that one zone is separated from the next by “average winter temperature.” Zone 1 has the coldest average winter temperature and Zone 13 (USDA map) and Zone 24 (Sunset map) have the warmest average winter temperature. These maps are sometimes called hardiness zone maps because hardiness refers to cold tolerance in relation to plants.

Yet there is one major limiting factor to growth that these maps fail to recognize, and that is humidity. This is amply illustrated when you spend a day in Seattle, followed by a day in Juneau, Alaska, as I recently did. In both Seattle and Juneau, Japanese maples (Acer palmatum) thrive in full sun locations, while in our climate all-day sun burns the foliage of these trees.

If cold tolerance were the main limiting factor on growth, one might assume that Southern California would be more hospitable to Japanese maple growth than Washington or Alaska because of our much milder winters. Yet the truth is quite the opposite. And the number of hours of sunlight Japanese maples receive is also irrelevant to their success. In late June, the sun shines 23 hours a day in Juneau, so it is not the amount of sunlight that burns the Japanese maple leaves, but the intensity of the sun and the associated lack of moisture in the air.

Out of curiosity, I called Hunter’s Nursery in the town of Big Bear Lake to see how the Japanese maples were doing in Sunset Zone 1B, the coldest zone in Southern California. I was told that Japanese maples do fine in Big Bear as long as they are grown in shade.

It should also be mentioned that sometimes it is a lack of cold that reduces a plant’s performance. For example, the vast majority of sweet cherry, apple and pear tree varieties will not produce fruit in Southern California because our winters are not cold enough to grow flowers and where there are no flowers, there is no fruit. In our part of the world you can grow any deciduous fruit tree, but only those varieties that have little need for winter chilling (less than 300 hours below 45 degrees during the winter months) will produce a harvest.

I am very grateful for the ability to identify every plant that comes my way thanks to plant identification apps. You take a photo of the plant in question and upload it to the app; within seconds his identity is revealed. In downtown Seattle I came across a small tree that caused my jaw to weaken. It was covered in white flowers that were as brilliant as you will ever see, even more brilliant than those of laurustinus (Viburnum tinus), a widely planted evergreen shrub in Southern California. Sure enough, this was a laurustinus cousin called Japanese snowball (Viburnum plicatum), a deciduous species with leaves that turn purple-red before falling in fall. The fact that the Japanese snowball loses its leaves in winter explains why it is never seen in our area. Southern Californians have a deep-rooted preference for evergreens, to the virtual exclusion of deciduous ornamental shrubs and trees.

In Juneau I saw three ground covers that are rare in our area, although they deserve wider use. The first was moneywort (Lysimachia nummularia), named for the fact that its round leaves suggest coins. This is a species that grows rampantly in the shade with yellow flowers of 2.5 cm in size. The variety I saw looked similar to ‘Aurea’, whose leaves are a shimmering gold color. This plant’s rounded leaves are strewn along trailing stems, making it an excellent candidate for all kinds of containers, especially hanging baskets.

Carpet bugle (Ajuga replans) is another Juneau ground cover with a mat-like growth habit that also does well in pots. There are many species of carpethorn, one with leaves that are dark purple to almost black in color. Blue flowers appear on erect spikes six inches long. A final Alaskan ground cover that is rarely seen but greatly deserves wider recognition is the common leadwort (Pulmonaria officinalis). The plant is a perennial that grows from rhizomes and does well in the shade. 3/4 inch flowers turn pink, but change to pink-violet before finally turning blue.

Californian, resident of the week: Catalina cherry tree (Prunus lyonii) is a very attractive and underutilized species. It is drought and cold tolerant and makes an excellent evergreen hedge. Some classify it as a large shrub due to its 3 to 4 meter girth, while it typically reaches a height of only 25 meters, although it can grow to twice that height under ideal growing conditions. It has glistening, heart-shaped leaves and takes on a domed shape as it ripens. If you like having birds around, then this is the tree for you, as the fruits are irresistible to bird creatures. The edible fruit is formed in late summer and has a taste that is more sour than sweet. Catalina cherry is a subspecies of the holly leaf cherry (Prunus ilicifolia). Holly-leaved cherry has somewhat spinier leaves and a shape that is stockier and bushier than its Catalina derivative. In both species, the fluffy cream-colored flower clusters of the caterpillars create a dazzling display in early spring.

Are you growing a distinctive ground cover that most gardeners may not know about? Send your growing recipe to [email protected]. Your questions, comments and tips, in addition to gardening problems and successes, are always welcome.