Azaleas can thrive in a wide variety of growing conditions, which makes them useful in many different landscape situations. Many of these conditions are described here.
Particular plants grow best within a range of high and low temperatures. These ranges are known and published for many plants, using the U.S. Department of Agriculture cold hardiness zone numbers. Each zone covers a 10 degree Fahrenheit range of average annual minimum temperatures. A simplified zone map shows the zones across the United States, along with the temperature range of each zone. Compare your zone with the cold hardiness of the plants you are interested in to choose plants that will grow well for you.
Cold damage is primarily due to the minimum low temperature to which the plants will be exposed . Other factors include:
- Water – plants need to go dormant, or harden off, before the winter. To help them harden off, reduce water a month or so before the expected first frost. Then, after a few hard frosts, water them well to provide adequate soil moisture for the warm days that will come, which can dry the plant out even in the winter, particularly when combined with wind and sun exposure.
- Mulch – avoid mulching heavily in the fall until after the first frost. Since mulch serves as an insulator to slow the loss of heat from the soil, it can lower the air temperature by a few degrees just above the mulch.
- Age – young plants may be less cold hardy than older plants of the same variety.
- Health – a plant in poor health will be less cold hardy than a vigorous plant.
- Timing – a period of warm temperatures followed by a quick freeze is more damaging than a gradual drop to the same low temperature. Thus, an early freeze in the fall or a late freeze in the spring can cause more damage than those same low temperatures in the middle of winter.
- Duration – extended periods of low temperature may overcome the insulating properties of the soil and cause root damage.
Winter injury can take the form of frozen flower buds, which usually then turn brown. This ican occur with no damage to the plant other than a lack of bloom in the spring. With snow cover, this can appear as little or no bloom above the snow line, with more normal bloom below the snow line.
Injury to the plant itself is called bark split, The outer layer of bark splits away from the stem just above the ground. The damage shows up the following spring, and can kill the plant if the stem is girdled. Plants with bark split have been saved by applying melted paraffin or grafting wax over the split areas.
Roots are much more sensitive to cold than the plant stems or buds. In the ground, the soil insulates the roots against air temperatures below freezing. The roots of potted plants need similar insulation against cold temperatures. This can be provided by a cool greenhouse, a cold frame, or by placing them closely together and covering the pots with a heavy layer of mulch.
Heat damage is less obvious than cold damage. It typically shows up as a plant that simply does not thrive, and instead slowly declines and dies over a period of years. In general, plants begin sustaining damage at 86 degrees F., with the number of days above this threshold having a cumulative effect. Extended periods of such high temperatures appear to be more harmful than the maximum day-time temperatures, as do high night-time temperatures.
The American Horticultural Society developed a heat zone map similar to the USDA cold hardiness map. It maps 12 heat zones to show the cumulative number of days above 86 degrees F. While heat hardiness using these heat zones is beginning to be published for specific plants, just as for cold hardiness zones, the heat zone information is not yet as readily available.
Many deciduous hybrid azaleas appear to be more sensitive to high temperatures than evergreen hybrids. Hybridizers are working to improve the heat tolerance of deciduous hybrids by crossing them with the very heat-tolerant native R. austrinum.
The ideal exposure for azaleas is the variable shade provided by tall trees moving in the breeze–one minute, the ground beneath is in full sun, and the next minute it is in full shade. You can help achieve this by removing the lower limbs from the existing trees, at least as high up as you can reach with a pole saw.
Azaleas can also adapt to extremes. Many varieties, particularly deciduous azaleas, can handle more sun if they are given enough water. In general, azaleas grown in full sun will have shorter stems, and may have more blooms which will not last as long, and which may fade in a few days. If the leaves show signs of being burned by the sun (round brown spots on the leaf tips and edges) or the blooms fade quickly, move the plant to a little more shade. In full shade, many varieties will grow longer stems as they reach for the light, and they may not produce as many blooms, but the blooms will last longer.
Azalea sun/shade tolerance is affected by your latitude. In the south, fewer azaleas can thrive in full sun than in the north. Similarly, fewer azaleas, and particularly deciduous azaleas, can thrive in full shade in the north than in the south.
Azaleas have shallow roots, and prefer moist, well-drained soil with a pH between 4.5 and 6.0. If in doubt, you can submit soil samples through your County Extension Service to have the pH and nutrient levels tested, usually for little or no cost. The test results will also have information about changing the pH if needed, which will be appropriate for your soil type and climate.
Check how well your soil drains by filling a hole with water. If it is empty in a few hours, it drains well. If not, you have so-called “heavy” soil. In heavy soil, don’t plant azaleas into small holes filled with amended soil, since the holes will hold water and be death traps for the plants. Instead, you can build raised beds, or amend a large area, or “plant high”, ranging from an inch or so above ground level to planting on top of the ground, and mounding amended soil around the root balls.
Azalea roots need access to both moisture and oxygen. In wet soil, the roots will grow closer to the surface to get oxygen. In dry soil, the roots will go deeper to get moisture. In reasonably well drained soil, the roots of evergreen azaleas tend to stay in a well defined mass of fine feeder roots from the surface down to around 12 inches deep. They seldom extend beyond the width of the plant, and usually stay within a foot or so of the trunk. Roots of deciduous azaleas may range deeper and further from the trunk in their search for water.
Sand and clay are the soil extremes, and neither are suitable for growing azaleas. Sand has large particles with large spaces between them. Gravity quickly drains excess water from it, there is rather little surface area to hold a film of moisture for roots, and there is plenty of space for oxygen. Clay has small particles with a lot of surface area. Gravity takes a long time to drain excess water from clay, there is a lot of surface area to hold moisture, and almost no space for oxygen. The ideal soil, called loam, has a mixture of large and small particles. That mix allows excess water to quickly drain out by gravity, and has a lot of surface area moisture and spaces for oxygen. Sand and clay can be improved by mixing them together, to provide the desirable mix of particle sizes. They can also be amended by adding organic matter and humus (decomposed organic matter), as much as half by volume, to improve their fertility.
About an inch of rainfall each week is ideal for azaleas. Supplemental water may be needed if the rainfall is much less than that, especially if there is no rainfall for extended periods. Especially watch for signs of dryness on recent transplants, on azaleas which have been planted high, and on azaleas which are in full sun or locations exposed to drying winds. Fortunately, drooping leaves show the need for water well before the plant dries out completely, and watering it slowly and deeply usually restores it within hours.
Mulch insulates the soil to even out soil temperature extremes (which may undesirably lower the air temperature at the base of plants in the fall). It also helps to keep the root zone moist, reduces water runoff, and reduces weeds. Perhaps even more importantly, decomposing organic mulch adds humus to the soil. Humus improves the ability of the soil to accept and hold water, it helps get oxygen to the azalea roots, it maintains the acidity of the soil, and it supports the very beneficial mycorrhizal fungi that absorb nutrients from the soil and move them into the azalea roots.
The best mulch is naturally decomposing wood and leaf mold from oak or pine woods. Other good materials are leaves, pine needles, wood products such as twigs, wood chips, ground bark, wood shavings, excelsior or sawdust, and shredded crop rubbish such as straw, peanut hulls or corncobs, and well-rotted animal manure (which may have an odor). Other than its appearance, the major factors to look for are the absence of weed seeds, and a mix of different sizes to provide porosity.
Leaves will decompose and become humus in about a year, while other mulches can last several years before they are decomposed. The bacteria and fungi that decompose the mulch use up nitrogen and phosphorus. To maintain the levels of these elements, it is good to add them at the rate of several handfuls of ammonium sulphate or ammonium nitrate and a handful of superphosphate to a bushel of “raw” or fresh mulch. This is more important with sawdust than with other materials, and less fertilizer is needed if the mulch is already somewhat decomposed when applied.