What the climate zone map can tell you about a plant’s growth potential

Travel is always a gratifying 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 when we find them thriving in locations where the conditions for growth are so much different from our own.  

When you take Biology 101 in college, one of the first terms you learn is “limiting factors to growth.” Where plants are concerned, the primary limiting factor is cold. If you look at the USDA climate zone or Sunset climate zone map, you will note 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 referred to as hardiness zone maps since hardiness refers to cold tolerance in reference to plants. 

Yet there is one important limiting factor to growth that these maps don’t acknowledge and that is humidity. This is amply illustrated if 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) are thriving in full-sun locations whereas, in our climate, full-day sun burns the foliage of these trees. 

If cold tolerance were the major limiting factor for growth, you might assume that Southern California would be more hospitable to Japanese maple growth than Washington or Alaska due to our much milder winters. Yet, the truth is just the opposite. And the number of sunlight hours Japanese maples receive is also irrelevant to their success. During late June in Juneau, the sun shines for 23 hours a day, so it’s not the amount of sunlight that burns Japanese maple foliage but the sun’s intensity along with the accompanying lack of moisture in the air.

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

It should also be mentioned here that sometimes it’s a lack of cold that diminishes a plant’s performance. The vast majority of sweet cherry, apple, and pear tree varieties, for example, will not produce fruit in Southern California because our winters are not cold enough for flowers to develop and where there are no flowers there is no fruit. You can grow any deciduous fruit tree in our part of the world but only those varieties that have a low winter chilling requirement — less than 300 hours below 45 degrees during the winter months — will yield a crop.

I am most grateful for the ability to identify any plant that comes my way thanks to plant identification apps. You take a picture of the plant in question and upload it to the app; within seconds, its identity is revealed. In downtown Seattle, I came upon a small tree that caused my jaw to slacken. It was covered with white blooms as brilliant as any you will ever see, even more brilliant than those on laurustinus (Viburnum tinus), a widely planted evergreen shrub in Southern California. Sure enough, this was a laurustinus cousin by the name of Japanese snowball (Viburnum plicatum), a deciduous species with leaves that turn purplish red before dropping in the fall. The fact that Japanese snowball loses its leaves in the winter explains why it is never seen in our area. Southern Californians have a deep-seated bias that favors evergreens to the near exclusion of deciduous ornamental shrubs and trees.

In Juneau, I spotted three ground covers that are seldom seen in our area although they deserve wider use. The first was moneywort (Lysimachia nummularia), named for the fact that its roundish leaves suggest coins. This is a species that grows rampantly in the shade with yellow flowers one-inch in size. The variety I saw looked like ‘Aurea,’ whose foliage is a shimmering golden color. The circular leaves of this plant are studded along trailing stems which makes it an excellent candidate for containers of all kinds, 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 carpet bugle varieties, one with foliage that is dark purple to nearly black in color. Blue flowers appear on upright spikes that are six inches tall. A final Alaskan ground cover rarely seen but highly deserving of wider recognition is common leadwort (Pulmonaria officinalis). The plant is a perennial that grows from rhizomes and does well in the shade. 3/4 inch flowers emerge pink but change to rose-violet before eventually turning blue. 

California native of the week: Catalina cherry tree (Prunus lyonii) is a highly attractive and underutilized species. Drought- and cold-tolerant, it makes an excellent evergreen hedge. Some classify it as a large shrub due to its girth of 10-15 feet, whereas it typically reaches a height of only 25 feet although, under ideal growing conditions, it could reach twice that height. It has glistening, heart-shaped leaves and assumes a domed form as it matures. If you like having birds around, this is the tree for you since its fruit are irresistible to avian creatures. The edible fruit is formed in late summer with a flavor that is more tart than sweet. Catalina cherry is a subspecies of holly-leaf cherry (Prunus ilicifolia). Holly-leaf cherry has somewhat spinier leaves and a form which is squatter and shrubbier than its Catalina derivative. On both species, fuzzy cream-colored caterpillar flower clusters put on a dazzling display in early spring.

Do you grow a ground cover of distinction that most gardeners might not know about? If so, please send your prescription for growing it to joshua@perfectplants.com. Your questions, comments, and tips, in addition to gardening predicaments and successes, are always welcome.