Tips and Tidbits

July Tids and Tidbits

Posted on July 11, 2016 at 6:10 AM

July is all about Heat, Heat, Heat, and MORE Heat…


As we swelter to keep flower beds, potted plants and young trees/shrubs watered, mulched and alive in spite of the vicious heat, we should remember that it isn't easy being green here in summer. It may not sweat and swoon like an overheated gardener dragging a garden hose, but like us, our landscape is also under assault by the extremes of climate.


Plant the wrong plant here, (or even the right plant in the wrong place or in the wrong season), and it may limp along for a while during our more temperate months. But in the teeth of a subtropical south Louisiana summer, those plants will suffer and likely die, even if we're keeping them hydrated.


In winter, there's often a rush to protect favored plantings against plummeting temps when the rare, hard freeze is forecast. But but all too often, there is no thought given to our gardens as they bake and broil in summer. And yet, too much heat is as deadly as too much cold; it just generally delivers a slower death. While freezing weather can kill in a cold snap - with physical damage immediately visible - death-by-heat is more like wasting away.


"Plant death from heat is slow and lingering ... Flower buds may wither, leaves may drop or become more attractive to insects, leaves may appear white or brown (chlorotic)," advised the American Horticultural Society (AHS). "The plant may survive in a stunted or chlorotic state for several years. (But) when desiccation reaches a high enough level, the enzymes that control growth are deactivated, and the plant dies."



The sad fact is, hundreds of the plants that call to us from the pages of national magazines cannot tolerate the heat and-or humidity of our summers. We may be able to grow some of them during cooler months, but others demanding cooler weather or dryer conditions will never survive in this land of heavy rain and high humidity. We know it takes just the right people to survive in our coastal Louisiana climate, and it takes just the right plants to survive along with us. Don't throw away money to watch ill-suited-plants and flowers needlessly sizzle into oblivion. We must understand our climate, and we must accept that climate dictates what will survive and thrive in our yards.


It all begins with the numbers, in knowing how hot it generally gets here in summer, how cold in winter, and how long those those extremes generally last. Lots of smart people have already done the research and answered these questions in a trio of maps that can help guide landscaping decisions. Just triangulate the data.




The American Horticulture Society's (AHS) Plant Heat Zone Map divides the United States into 12 zones based on the number of "heat days" expected to occur yearly in each zone. The communities surrounding Lake Pontchartrain are all in Zone 9, meaning we experience from 120 to 150 "heat days" with temperatures above 86 degrees, which is the point at which the AHS says "plants begin suffering physiological damage from heat." These are numbers to remember. By way of comparison, for example, Seattle is in Heat Zone 3 with only seven to 14 heat days. Obviously, our terrain experiences a lot more heat stress than Seattle and much of the country.




The Heat Zone Map is NOT the map we have seen printed for years on the backs of seed packets and routinely referenced by people who grow, sell, plant, lecture or write about plants and gardens. That one is the USDA's storied Plant Hardiness Zone Map, which provides data about the coldest expected temperatures in each region. On its latest updated map, the United States Department of Agriculture (USDA) divided the country into 13 zones that reflect a 10 degree temperature change from the adjacent zone; each is is then subdivided into five-degree sub-sections noted as "A" and "B."

In south Louisiana, that means expect low temps of 20 to 35 degrees, depending on whether you are in zones 9A, 9B or 10A. Just enter your zip code in the USDA map, and check your designation.


Together, these companion maps provide temperature parameters, the expected highs and lows, by zip code. Ah, it sounds so simple. If only cold hardiness and heat tolerance were the only variables in the balancing the equation of successful gardening. Instead, they are but the beginning.


CLIMATE MAP explains a more holistic Climate Zone Map based on multiple variables. "A plant's performance is governed by the total climate: length of growing season, the timing and amount of rainfall, winter lows, summer highs, wind and humidity," according to Sunset researchers. Their map divides the country into 28 zones and puts south Louisiana in Zone 28, which encompasses a band of land along the Gulf of Mexico running from the Houston area east through south Louisiana and Mississippimand north Florida to south Georgia and the Charleston, S.C., area on the Atlantic Coast. In delineating this zone, Sunset considered year-round rainfall and humidity, and our region's virtually frostless winters that are nevertheless subject to periodic blasts of frigid Arctic air.


The American Horticultural Society says thousands of garden plants have now been coded for heat tolerance in accordance with the AHS map, and more will be coded in future. Heat Zone map information is expected to become commonplace, right alongside the USDA cold hardiness information, in gardening literature and in nurseries.


But go ahead and start checking now. When planning and shopping for new garden plants and flowers, start looking at plant tags and labels for four numbers that look like this: 3-8, 8-1. These particular numbers mean that if you live in USDA Zone 7 and the AHS Zone 7, you can plant tulip bulbs in your outdoor gardens and leave them in the ground year round. Of course, here in south Louisiana, we're interested in plants for Zones 9A and 9B, even a bit of 10A in some small areas along the water.


(You can access both the USDA Hardiness Map and Sunset Climate Page on the AHS website at or google them directly. Although the AHS map is not available online, you can order a hard copy of the map via the AHS site.)



Written by Sheila Grissett, Your Happy Garden Contributing Writer


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