To Bud or Not to Bud
Reprinted from the February 2001 Eugene Chapter newsletter
Why do our rhodies have a bumper crop of blooms some years - such as in 2000 - and some years not? In the bad years there are plants that don't have any buds at all. That's a question I've been pondering for several years now and have come up with some interesting points - and lots of questions.
The species I will consider in this article is the Rhododendron macrophyllum that grows all over the Cascade and Coast ranges in Oregon. I believe if we understand why the macrophyllum set buds - or don't set buds - we will better understand why our garden rhodies do or do not bloom. We know a few facts about the macrophyllum: 1)They grow in areas that get lots of rain and fog throughout most of the year. 2) The temperatures are not as hot as in the valley between the two mountain ranges. 3) They only grow at a certain elevation range. In the Cascade Range they grow mostly between 1500 and 6500 feet (450-1900 m) and in the Coast Range area from the Pacific Ocean on the west to the highest peaks and down to about the 1500-foot (450 m) level on the east side.
Why here and not in others? In the Cascades, where most of the macrophyllum grow, snow covers the ground from about October through May or June. The Coast mountain area varies a lot as far as snowfall is concerned - very little at sea level, while higher elevations can occasionally have as much as 9 feet (3 m) (winter of 1935-36). The western range also gets more rain than the Cascades - up to 150 inches (375 cm) or more with lots of fog off and on during the late spring and early summer.
The macrophyllum thrive in all areas in both mountain ranges but only bud and bloom well in open areas, caused generally by fire or logging clear-cuts. As the trees grow thicker and cause more shade, there are few, if any, buds. The temperature is not as hot in the mountains as it is in the valley areas.
These rhododendrons grow in a lot of duff which comes from deciduous trees leaves, needles from evergreen trees, fallen limbs and trees rotting on the forest floor. The duff ranges from a few inches to more than foot in depth. This helps retain the moisture in the soil, and as the years go by creates rich new soil.
The timing of the blooming season also affects the budding. The macrophyllum bloom in June and late July in the higher elevations. There are more hours of sunshine then than when buds are setting in our garden areas in lower elevations. The blooms in the mountain areas do not vary as much as in our gardens from year to year. I believe that this is the result of almost constant sun during the bud setting time in July and August.
Applying this to our hybrids and species grown in our gardens, what can we learn so that we can have better bud set each year? First, in the mountains moisture is available most of the year. If there is a long dry spell, the rhododendrons will suffer considerably. So we need to keep our garden plants moist all year round. I often begin watering in April and always in May even if it is still raining some. Remember, the new growth is starting in these months, so the plants need moisture and nutrition to stay healthy and produce as good a bud set as possible. Continually dry plants seldom have good bud set, though some plants require less water than others and still do well.
What else can we learn from these native plants to promote good bud set? Lots of duff on the forest floor keeps the moisture in the soil. In our area, which is mostly clay, I add a wheelbarrow full of bark mulch - I prefer cedar - into the soil when planting a rhododendron, as it helps keep the soil loose and aerated as well as holding moisture. I don't mean dig a hole and fill it with bark. It must be spaded up in an area 3 or 4 feet (1 m) in diameter, and the whole load mixed thoroughly into the soil. With such a good start, the plants can use an inch of bark over the root ball area every couple of years to retain moisture and keep weeds down. With lots of room for roots to grow, a good bud set can be expected if Mother Nature cooperates with lots of sunshine, but often the weather is quite nasty and dark in the spring. In that case, it seems there is not a good bud set.
Here is my question. Is it the cooler weather or that lack of sun that causes our garden-grown rhodies not to set as many buds? In the summer of 1998, our neighbor harvested many of his large trees just over the property line which had been blocking the morning sun and taking up a lot of moisture from our plants on the east side of our property. Until then, of the eighteen plants in the area, most of them, except for 'Double Winner' and 'Grierosplendour', had few or no blooms at all, regardless of how much I watered them.
After the harvest, there was a lot of sun from sunrise until 2 p.m. and no other trees for several feet from the property line to take water away from our rhodies. In 2000, the plants in that area were completely covered with blooms. Some had as many as seven buds on the end of one stem, and most of the plants had a bud on almost every stem. 'Double Winner' and 'Grierosplendour' had a few more blooms, but they had always performed well. The remaining sixteen plants received no different care, except they did not receive extra water. It was evident they benefited from more sun and few trees to suck up the water continually from them.
Which was most important, more sunshine or fewer trees? I don't know. Why did the two bloom quite well while the others didn't under the same conditions? Did the timing of the bloom and bud set have anything to do with it? 'Double Winner' is quite early, while 'Grierosplendour' is quite late - the other sixteen were mid-season bloomers. Or was it due to their genes?
Do genes cause some rhodies to bud well year after year despite the weather? It is generally the same plants that don't set buds well in the years of low bud set. Consider 'Scintillation'. It buds well year after year, and I use it a lot in my hybridizing program. Generally speaking, its offspring do quite well setting buds - and one had over 1000 blooms last year. Could it be in the genes? The next generation from 'Scintillation' crosses are also budding quite well so far, though two or three years isn't long enough for concrete evidence one way or another.
There are many other rhodies that are "good doers" in our garden, blooming year in and year out under good conditions. One that surprised me was 'Unique', since it is said to bloom well only every other year. I find that in our garden, it blooms about the same time each year. Is it genetic or in the way it is grown and fertilized? Another example is 'Taurus'. We grew it in the shade without the benefit of really adequate water - it bloomed sparsely. Six or seven years ago, I moved it into the sun and it bloomed fantastically - at least 1000 blooms this last year. Good care proved that 'Taurus' will do well, but does it have good genes also? Evidently, because it has not failed to bloom well in its present location, while others in the same area are sporadic bloomers. It follows that in hybridizing, therefore, one might want to consider choosing plants that bud well year after year.
In summary, there are a lot of questions yet, but let's go forward with what we do know. First of all, select plants that are good doers, the ones with a proven gene pool. Next, give them a good start with plenty of mulch in the soil at planting time and more applied in small amounts around the plants every couple years thereafter to hold in moisture and provide aeration for the plant. Don't scrimp on water - they need it all year round. A plant that continually lacks water will be straggly and less likely to set buds at all.
Plant in as much sun as the plant will tolerate. Some will not take full sun but will usually take more than you think if planted and watered properly. If planting near trees, remember shade is helpful at times, but those trees will take lots of water from your rhodies. Applying a little fertilizer will bring great results - too much will burn your plants. Check with other growers whose plants look healthy just which fertilizer is best for your area.
Mother Nature has a lot to do with bud set. I say that from my own experience as a nurseryman. People come to us with their rhododendron problems and just to talk in general. When I have a bad bloom year, they ask why. When I have a good year, the talk is that they had many blooms as well and are proud of their rhodies. With a whole area responding similarly, one can see that the weather had an important part in the matter. I firmly believe that a cold wet spring with too little sun will cause fewer buds to form for the next year's blooms. Fortify them with good care and you will be helping Mother Nature - between the two of you, you will have as good a bud set as possible!
Merle Sanders is a member of the Eugene Chapter.