Rhododendron Hardiness - Winter 1948-49
As our last winter was quite severe. In fact, one of the coldest for several years. It may be of interest to the readers to know how the various rhododendrons survived the freeze in our nursery here. We are located 10 miles north of Seattle, where we had a temperature of eight degrees above zero for one night, and several nights with ten to twelve degrees above.
Of the larger sizes of rhododendrons, from four to eight feet tall, and of which we have several hundred, including species and hybrids not a single one was killed by the freeze. A few of them were lightly touched, such as: species morii, the hybrid white arboreum x white calophytum, R. 'Dolly', R. 'Raffill' and R. 'Nereid' x discolor.
Why the last mentioned cross should be effected by the freeze is a mystery, as this cross is a result of crossing the three species dichroanthum, R. neriiflorum and R. discolor.
Some of the finest orange flowers have been produced from crossing the last-mentioned rhododendron, and many of you readers have admired these plants and flowers at the Seattle rhododendron shows.
I have noticed that the rhododendrons from which the scions were removed last fall, were the ones mostly effected by the freeze. Among the smaller sizes of rhododendrons from a foot to four feet tall, the largest number touched by frost were noticed in the crosses 'Fabia' x 'Tally-Ho' and griersonianum x campylocarpum. Why the 'Fabia' x 'Tally-Ho' should freeze when the parent plants of that cross were not touched, is another mystery. Some of the plants of 'Fabia' x 'Tally-Ho' where in the frame-house, and others were, standing in the open. I believe the plants in the frame-house froze because they were in a growing condition when the frost came.
The cross griersonianum x campylocarpum have not produced any outstanding flowers. It has been my experience that a narrow leaved rhododendron species does not cross well with a round leaved species. souliei x griersonianum has not proved to be any better with us.
When I started growing rhododendrons, and finally got them big enough to plant outside, I had the habit of planting the less hardy kinds in among the taller ones for protection from the cold, but I soon noticed my mistake. I found that the less hardy kinds, planted in the open, where they had a chance to harden off, withstood the freeze much better than the plants which were crowded in among the tall ones.
Rhododendrons should have full circulation of air at all times, and they should have some sun, also. It is the sap in the wood which causes most of the rhododendrons to freeze here on this coast, where rainfall is heavy in early winter.
I would like to mention a few rhododendrons less hardy and which we had planted in an open bed. These consisted of R. johnstoneanum one to two feet tall and not a single one froze. There were among them a good sized plant of 'Lady Chamberlain', one R. ciliatum, a few small grafts of Rhododendron venator all escaped the frost.
One row of small plants of Rhododendron 'Dolly' were touched only lightly, but the larger sizes of 'Dolly', from which I had removed the scions last fall froze quite badly. Most possibly, when the scions were removed it forces the plant into growth in the late fall.
Another thing I have noticed: certain young sizes of rhododendrons which have been used for crossing year after year and without being given a rest, will grow weak and will freeze easily. It is very essential to the well-being of a rhododendron to remove every flower-spike as soon as it has finished flowering.
As last summer was very wet, dark and cold, our rhododendrons did not produce their flower buds as profusely as usual. This was, very noticeable with us, as we have so many tall fir trees shading the plants.
While we are on the subject of frost, I recall the winter of 1934-35, when the temperature went down to zero one night. At that time, I had my small seedling plants (rhododendrons) standing in flats in frames, but I had them covered with common building paper, and as that covering proved to be dependable, we have used it ever since. We think it is better than glass covering, and the plants do not mind being kept in the dark for a month or two, while dormant. We do not put the covering on until temperatures go down to 20 degrees above zero or there about. As soon as the weather gets milder we lift the sides of the paper along the frames to give the plants a good airing, but we cover up again, if we notice another freeze is in the air. This way, we saved every one of our one and two years old seedlings which were planted in flats and placed in frames after transplanting last summer, and of which we had about fifty thousand plants.