“Yet thus it was content to bloom,
In modest tints arrayed;
And there diffused a sweet perfume,
Within the silent shade.”
– Jane Taylor, “The Violet,” 1805
The English poet and novelist Jane Taylor is best known for writing the words to “Twinkle, Twinkle Little Star,” but she penned hundreds of other verses for children, many of them in which garden plants teach proper Victorian morals and behavior.
Unlike the violet, many of the plants in our summer vegetable gardens are not humble. Tomatoes, watermelons, cucumbers, eggplants, peppers, squash — they all shamelessly crave the spotlight, preferring at least eight hours of sunshine every day. A lack of sunlight prevents fruiting plants from setting flowers and slows the fruit from maturing. Also, plants weakened from not enough sunlight attract opportunist insects and diseases.
The challenge for many Rogue Valley gardeners is providing vegetable plants with plenty of sunshine while also protecting them from the summer heat. After all, heat-stressed plants are no more productive than sun-starved plants. Even sun-worshipping tomato and squash plants will not set flowers and will drop their flowers when temperatures reach past 90 degrees.
The effects of heat on garden plants prompted the American Horticultural Society to introduce the Plant Heat Zone Map. Zones 1-12 are based on the average number of days per year that the temperature will rise above 86 degrees, which is when plants start to suffer from the heat. Although Medford is designated in Heat Zone 6, last year we experienced 92 days with temperatures 86 degrees and above, which places our city at the edge of zones 7 and 8.
It’s helpful for gardeners to “be in their zone” when it comes to growing plants in the heat. Thousands of plants, including some vegetables and herbs, have been rated for heat zones, and some garden catalogues have begun labeling heat zones along with the more familiar USDA Plant Hardiness Zones. (These 13 zones are based on an area’s minimum winter temperatures; Medford is in Plant Hardiness Zone 8a/b.) Look for four numbers such as 6-9, 6-9; these numbers would mean the plant should survive the winter cold in Plant Hardiness Zones 6-9 and should withstand the summer heat in Heat Zones 6-9.
It’s also important to consider the vegetable garden’s micro-climate — conditions unique to a garden plot due to dips and elevations in the landscape and to nearby structures. My southwest-facing garden, for example, receives a lot of afternoon sun. The wooden fence, greenhouse and detached garage provide shelter from the wind, but they also trap in heat, resulting in a much hotter micro-climate in my backyard than elsewhere on my property. I keep the micro-climate in mind when deciding what to grow in the backyard garden and when to plant.
In some cases, I provide afternoon shade to guard against overheating. Although my harvests are probably smaller, at least the crops don’t cook in the hot sun. Some crops don’t mind part sun/part shade, including bush beans, beets, carrots, leafy greens, perennial herbs, onions, peas, potatoes and radishes. Partial shade means the plants still receive 4-6 hours of direct sunlight, ideally in the morning and early afternoon.
In “Original Poems for Infant Minds,” Jane Taylor offers little praise for the scarlet poppy growing “high on a bright and sunny bed,” and rudely commanding attention with its upheld “staring head.” Taylor cautions:
Yet no attention did it win,
By all these efforts made,
And less unwelcome had it been
In some retired shade.
— Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at firstname.lastname@example.org