It’s a triple whammy for gardeners.


Heat. The last 12 months have been the warmest ever recorded in the contiguous United States, according to the National Climatic Data Center.


No relief is in sight. The National Center for Atmospheric Research says the recent weather pattern strongly indicates Americans can expect future summers of unrelenting heat.


Drought. More of the United States is in moderate drought or worse than at any other time in the 12-year history of the U.S. Drought Monitor, according to the National Drought Mitigation Center at the University of Nebraska-Lincoln.


Luckily for gardeners, there’s something they can do about the plants they see wilting in the heat.


All they have to do is make a few simple changes in plant selection, maintenance and garden design, says David Ellis, editor of the American Horticultural Society’s bimonthly magazine, American Gardener.


The secret of garden design

Smart garden design takes into consideration the water needs of the plants, Ellis says.


For instance, he suggests that gardeners place plants with the highest water needs closest to the house. They can be easily observed there and watered at the first sign of heat stress. Plants that are more self-sufficient should be placed further from the house.


An effective design that Ellis employs in his own garden is to create a meadow effect with tough prairie plants.


Prairies, Ellis points out, include a variety of flowering plants and once had a much wider range than they do now. The trick with creating a meadow garden, he said, is to give the plants enough water their first year to get them established.


Some of the plants growing in Ellis’s small meadow-themed bed in Maryland include:


  • Sweet black-eyed Susan (Rudbeckia subtomentosa)
  • Meadow blazing star (Liatris ligulistylis)
  • Obedient plant (Physostegia virginiana)
  • Northern dropseed (Sporobolus heterolepis)
  • Blue wild indigo (Baptisia australis)
  • Lance-leaf coreopsis (Coreopsis lanceolata)
  • Pale purple coneflower (Echinacea pallida)
  • Indian grass (Sorghastrum nutans)
  • Switch grass (Panicum virgatum)
  • Pink muhly grass (Muhlenbergia capillaris)

Why plant selection matters

The meadow design, Ellis says, incorporates an important strategy that can be used throughout the garden: choosing plants that are relatively self-sufficient. Plants native to different regions of the country, for instance, are especially well adapted to local conditions, including extremes.


For self-sufficient non-natives, Ellis says its best to check with local nurseries rather than try to offer generalized ideas. In his Mid-Atlantic region, some examples of self-sufficient plants include lavender (Lavandula spp.), catmint (Nepeta racemosa 'Walker's Low'), leadwort (Ceratostigma plumbaginoides), golden dwarf sweet flag (Acorus gramineus 'Ogon'), barrenworts (Epimedium species) and Lenten roses (Helleborus x hybridus).  The first two are for areas of the garden that receive full sun. The latter four plants prefer shade or part shade.


Other plants that would make excellent candidates for surviving tough summer conditions are Mediterranean herbsm such as rosemary, and succulents, such as Sedum spectabile ("Autumn Joy"), or groundcover sedums, such as gold moss stonecrop (Sedum acre). Some of the hardy ice plants (Delosperma spp.) are worth trying in the East, Ellis says, although he adds that they are getting a reputation for invasiveness in the West.


The best local source for regional drought tolerance is a nearby botanical garden, Ellis advises. The plants in their display garden are a good indication of plants that will thrive in that particular region, he says.


If there isn’t a botanical garden near you, or if you would like to do online research, Ellis urges home gardeners to check out the Plant Heat-Zone Map on the American Horticultural Society’s website. The map lists plants for their heat tolerance in the same way that the familiar U.S. Department of Agriculture's Plant Hardiness Zone Map serves as a guide to plant cold hardiness.


To determine your heat zone, click on the Heat Zone Finder in hypertext at the top of the website and enter your ZIP code. To view the map, click on Downloadable Heat-Zone Map. Instructions on determining heat tolerance of plants you would like to grow are provided in the section headlined: Using the Heat-Zone Map.


The most comprehensive source for heat codes and cold-hardiness zone, Ellis says, is the American Horticultural Society’s “A-Z Encyclopedia of Garden Plants,” which includes hardiness and heat codes for more than 8,000 plants. This will be available in a digital form in a few years, he added. Some other publishers have also begun listing heat zones in their books. 


Another source for heat zones is on plant tags themselves. Major wholesale nurseries, such as Proven Winners, are adding heat zone codes to the tags on the plants they are shipping to retail nurseries, Ellis said.


How to deal with drought

Plants are composed of anywhere from 50-90 percent water. When they suffer heat damage, the cause is always because of an insufficient amount of water being available to them, according to the AHS website. Turgid leaves are a sign that a plant has sufficient water and is able to take in carbon dioxide from the air through tiny, open pores on the underside of the leaves and make food.


“The plant uses the carbon dioxide to photosynthesize and make food — or fruits or seeds,” says Mark Whitten, a senior biologist at the Florida Museum of Natural History. “These pores have little lip-like valves that can open and close,” he continued.  “But when these pores are open to take in CO2, the plants also lose water. The hotter it is, the faster the plants lose water, just like we do when we sweat. If they lose too much water, the plants wilt and die. If they close the pores to conserve water, then they can’t take in CO2 and can’t make food.”


“Think of inflating a bicycle tire,” says Ellis. “Then think of what happens when the air goes out and the tire goes flat.” That’s what happens with plants through transpiration, he says.


When plants wilt from lack of sufficient water, they stop growing, stop producing and will die if their cells are not replenished with water.


The best way to get moisture to the plants, Ellis says, is to apply water at ground level with a soaker hose. The idea, he said, is to give the plants a deep soaking. Water that seeps deep into the soil will help plants develop a deep root structure, which helps them survive prolonged periods without rain.   


The best time to water, Ellis said, is early in the morning. This is the coolest time of the day, and there is less evaporation while temperatures are relatively cool than later in the day when the temperature is at or near its peak. The second best time is right at dark.


He advises against using sprinklers because a significant amount of water will be lost because it will evaporate from the leaves into the air before the leaves can absorb the water.


For patio container plants, Ellis suggests adding water gels to the potting mix. The gels absorb water and release it slowly to the plant roots, reducing the number of times the plants will need to be watered.


Another option for patio containers, Ellis said, is a self-watering pot. These types of containers have a water reservoir from which water is absorbed up into the pot and to the root zone. Like the gels, these specialized containers will reduce the need for frequency of watering.


Another way gardeners can help their plants survive excessive heat and drought is to mulch their garden beds. The mulch will help reduce evaporation, insulate plant roots from the high temperatures and reduce or eliminate weeds, which compete with desirable plants for water and nutrients. “In the eastern half of the country, organic mulches are ideal,” Ellis says. “In the western regions, gravel or stone is often more appropriate.”


Even when gardeners do all the right things, they can’t always beat the triple whammy. Some plants will reduce their yields even when gardeners give them sufficient water.


“Tomatoes,” for instance, “don't set fruit when temperatures are over 90 degrees,” says Ellis. 


But there’s a cure for that, too — the rejuvenating, cooler temperatures of fall.