I had more questions about the culture of those evergreen rhododendrons not called azaleas during 2000 than any other plant genus. So, while every nursery is different and there is no one best way to grow these beautiful shrubs, a summary of rhododendron production basics seems due.

Cultivars: When it comes to deciding which rhodies to grow, my crystal ball is cloudy. The biggest seller nationwide is probably 'Roseum Elegans' or 'English Roseum'. (They really are different cultivars that, unfortunately, may be hopelessly mixed in the landscape and nursery trades.) However, the cultivar that is best for you is the one you can sell for a profit, not necessarily the one that everyone else is growing and selling. The following cultural suggestions should work regardless of whether you are growing plants that appear as physically different as 'Chionoides,' 'Nova Zembla,' 'PJM,' and 'Scintillation' or even straight species like R. maximum.

Rhododendron Growing Substrates: As with other crops ALL aspects of culture are important. Perhaps none is more important than your growing substrate whether it is soilless media for a container grower or soil for a field grower. The substrate through which the rhododendron roots grow should be well drained yet retain moisture.

Phytophthora root rot remains the major cause of rhododendron death. This disease organism is most active at the pH and temperatures best for rhododendron growth. Therefore, the cultural factor we can most control to favor the plant over the disease is moisture and that requires both careful irrigation and well drained substrates. In southeastern container mixes, pine bark is usually the dominant component and is usually very well drained. In fact, it is so well drained that often we add something to hold the water in the mix longer as well as to provide some weight to keep pots from blowing over when they get a bit dry. I have seen very good quality hybrid evergreen rhododendrons grown in pine bark and peat, pine bark and sand, pine bark alone, peat and sand, composted hardwood bark, peat, redwood sawdust, composted Douglas fir bark (which looks like peat), and various other combinations including perlite and styrofoam. The best rhododendrons I see grown commercially in the rainy southeastern US almost always have pine bark with some peat, usually 10 to 15 % peat by volume in the growing mix. How you irrigate and fertilize these mixes is the key to avoiding soggy roots, poor growth and phytophthora. Overwatering is the fault of the grower, not the substrate or the crop.

For field growers, it may seem like substrate questions are easier than for container growers but they are not. It is easy to say you should only field grow rhododendrons in well drained soils. Unfortunately, there are not enough well drained soils up here in rhododendron country so we end up adapting what we have in the best way we can. Avoid soils with a pH above 6.5 and places where water does not drain. Water should disappear either into the soil, beneath the effective root zone of the plant (8 inches ?) or flow slowly away from the plant, never towards a rhododendron. In other words, avoid low spots and tight soils. A gentle slope is fine for getting rid of water but steeper slopes tend to bring erosion with them and, eventually, roots get exposed and plant growth suffers.

To improve drainage, raised beds have been used successfully and so have soil amendments. Remember that the raised bed needs to be at least as wide as the plant you want to sell, not look like a mountain peak. Also, if you are incorporating something like pine bark to improve drainage, whatever you incorporate needs to be throughout the raised bed and not just in the planting hole. More than 3 or 4 inches of pine bark tilled into the top 6 to 8 inches of soil . . . across the slope . . . may make the soil too well drained when we get the all too regular dry periods that are becoming common. Therefore, in all this discussion of sustrate drainage, I need to mention that occasional irrigation, particularly low volume irrigation, often grows the best plants. The problem always seems to be a compromise between having enough money to do everything right and doing enough right to grow the crop and make a profit!

Fertilizer: At no time should any of the granular fertilizers mentioned here be allowed to touch the leaves, stems or base of the trunk of rhododendrons whether in the field or in containers.

FIELD - Even before rhododendron transplants (liners) go into the ground, you need to consider their nutrient needs. If you will be growing in native soil, whether in transplant beds or field production, have the soil tested. Our native western NC soils are often very low in phosphorus, calcium and magnesium. Frequently, adding phosphorus to the soil is necessary and rarely, even adding limestone to add calcium and magnesium to the soil or raise soil pH is called for. Best plant growth will occur if these essential nutrients are applied and mixed with the soil before planting.

Whatever else you might need to do depends on the results of the soil test. The NCDA Agronomic Division can do soil nutrient analysis for you.

Once liners are in the field, a light top dress application of fertilizer is generally applied around the plant, a few inches from the base of the liner, in early to mid spring. In warmer locations or where plants are being irrigated, a light application can again be applied up until about the end of the first week in July. To fertilize later can run the risk of keeping plants from hardening off so that they can survive winter.

As field grown plants get larger the amount of fertilizer applied increases. The first year 0.25 ounces of nitrogen may be the application rate after planting and again in summer; 0.5 ounces of nitrogen the second year applied in late winter and again in summer and 1.0 ounces of nitrogen the third year in late winter but with only 0.5 ounces of nitrogen in summer is commonly used. If it is a dry year, use less fertilizer or don't apply the summer application unless you are irrigating. Please note that these rates are in ounces of nitrogen not fertilizer. The amount of fertilizer actually applied depends on the analysis. A 10% nitrogen fertilizer like 10-10-10 would require twice as much fertilizer as a 20 % nitrogen fertilizer like 20-10-20.

Potassium is rarely deficient in western NC soils when growing rhododendrons but, if it is, soil testing will alert you to the need for potassium. Absolutely necessary or not, potassium is usually included in the late winter application of fertilizer. In addition, when plants get large enough to be going to market, even if soils contain adequate phosophorus, it has been common practice in the mountains to side dress apply the summer application of nitrogen from 18-46-0 which is sold as diammonium phosphate or DAP. It is thought by growers that this will increase flower bud set and these growers have been happy with the results of using 18-46-0 this way.

CONTAINERS - Soil tests are rarely used in container culture because experience has shown that nearly all fertilizer elements essential for plant growth will need to be added. Micronutrients or minor elements can be added by incorporating them into the soil mix, which is the most common technique, or by using a fertilizer that contains minor elements. If you are having your growing substrate pre-mixed, you can have the minor elements blended into the mix. The same is true of limestone. If you need limestone, mix it with the growing substrate before planting rhododendrons. However, check your irrigation water first (The NCDA Soil Testing Laboratory can help with this.). Your irrigation water may be providing all calcium and/or magnesium that your crop needs. If your irrigation water is providing all the calcium and/or magnesium you need, adding limestone or the improper kind of limestone could actually cause problems.

Most commonly container grown rhododendrons are fertilized with a controlled release fertilizer with a 180 day release pattern in the mountains and a 270 day release pattern in the foothills and piedmont. One application per year in late winter or early in the growing season generally provides all the nutrients a plant needs but, in some very warm or very rainy seasons, supplemental fertilizer from a soluble source may need to be used so that plants are not 'hungry' entering the winter.

Liners: Transplants or liners are the small plants that are planted in pots or the earth to grow into the larger rhododendron plants that you sell. Buy full sized, vigorous liners that are true to name or grow your own.

Most liners for hybrid evergreen rhododendrons will be grown from rooted cuttings. These cuttings may have been collected from shrubs directly or have been grown from tissue culture. In most situations, a rooted cutting is not ready to be planted in the field and a microcutting from tissue culture may not even be ready for a pot as small as a quart. Therefore, the second thing you should do after getting your liners is to plant them where they will survive and grow into larger liners or transplants. Field grown evergreen rhododendrons have usually been grown either in gallon containers or transplant beds for at least a year before they are large enough to withstand the environmental rigors of field production, even if you have irrigation. Well-rooted cuttings in 4 inch or larger pots are certainly large enough to be transplanted into two or three-gallon containers but it may take a microcutting a season to achieve this size. Plan accordingly.

The first thing you should do when you get liners is grade them. If there is a problem, let you supplier know when you get the liners. If a liner has a weak root system, disease or other problems, get rid of it. Do not spend more money trying to turn it into a high quality salable plant. It is not worth the expense and you are probably going to fail.

When planting rhododendron liners, be sure that they are planted at the same depth they were growing before you got them and NO DEEPER. They may survive but not thrive in well drained container mixes but in field soils most will start declining shortly after they are planted, particularly if the soil is soft. In recently tilled, soft soil the plants will settle and be even deeper in the soil. If you have forgotten the discussion about Phytophthora root rot, please go back and re-read it. Planting too deep can lead to root rot along with other disease problems.

The same rules apply if you are growing native rhododendrons. The only difference is that you will probably be working with seedlings rather than cuttings. Discard the culls, plant liners at the proper depth, not too deep, and grow them into larger liners that are more likely to make you money in a field or container nursery.

Sun vs. shade: It is possible to grow high quality evergreen rhododendrons profitably in the full sun in the mountains but it is difficult. It is even more difficult in the NC piedmont but cultivars exist that are tolerant of bright sunshine. They will survive and grow an attractive plant in full piedmont sun if kept cool and moist enough. However, it is much easier to grow a high quality hybrid rhododendron in the lower mountains and the piedmont by growing them under shade. Growers will argue over whether it is better to use knitted shade vs woven shade or lath shade or natural shade. They may disagree about black shade vs. white shade but they are usually not arguing about where they should use ANY shade. The shade level most often chosen is around 50 % and, more often than not, is black. The reason black is chosen is probably because it is most easily available. I have seen good evergreen rhododendrons grown under all of these shade regimes.

Field grown plants are rarely shaded beyond the transplant bed growth stage. However, much better looking plants of the native Rhododendron maximum develop if this species is shaded during summer afternoons so you may see them planted adjacent to a woodland that will cast afternoon shade.

Winter protection: When growing under shade, most container and transplant bed growers like to remove the shade when the weather cools and sunlight becomes less intense in the fall. This is done to "harden" plants or let them go dormant naturally to be able to survive winter. Exposing plants to full sun and frosts will ensure that they are fully dormant.

Container rhododendron grown plants are frequently covered during the winter either in a plastic covered greenhouse or a floating bed cover to protect them from the worst cold as well as to keep them salable for spring by reducing frost burn. Roots are much more sensitive to cold than tops of plants so plants in containers with their roots exposed have a greater chance to be injured by the coldest weather of winter.

Generally, growers of H-1 rated plants in the piedmont are not as worried about winter cold injury as the yellowing of leaves and tip burn that may occur if plants are not protected from cold winter winds. Rhododendrons with yellow leaves that are supposed to be green are very difficult to sell in spring garden centers even if you assure the customer that they will turn green again . . . which is usually the truth.

Field-grown plants are not covered in the winter because their roots are in the ground and stay warm enough. However, field-grown plants are still subject to sun scald/frost burn. We have been doing research into ways to prevent sun scald/frost burn for the past two decades and have found nothing that can be sprayed only once or twice that will keep the leaves green until the next spring. We have also not found any fertilizers that will keep those leaves green. In fact, the only effective treatment we have found is to move the plants to the shade if you plan to sell them in the spring. These fall dug plants will need to be watered and the rootballs protected from winter cold but the leaves will be green if you get them out of the sun while the leaves are still green in the late fall.

Pest management is best discussed by the appropriate professionals. Avoidance of pests is the best policy for insects and diseases on evergreen rhododendrons. You will see leaf spots occasionally, lace bug, mites and midge injury may appear. Learn tolerance and thresh-holds for when it pays to spray. For weeds, planning is essential. In the field, competitive perennial weeds should be killed before planting then less competitive annual weeds may be tolerated the first year. However, by the second year in the field and from the start in containers, a pre-emergence herbicide program should be employed.

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