While roses are considered by many gardeners to be the queen of the garden, rhododendrons are said to be the king. Their large trusses of flowers can be of almost any colour of the rainbow and will bloom sometime between April and June in southern Ontario.
Rhododendrons slowly evolved in moist acidic boggy environments or as high altitude well-drained plants. The result is that rhododendrons require some special growing conditions. Siting is the first place to start. They require some shade. This can be achieved by planting them under evergreen trees or on the east or northeast side of a building.
Rhododendrons can also be grown successfully under deciduous trees, on the north or northwest side of a building but only if extra are is given to ensure adequate moisture and winter protection. Come to think of it I have even seen some gardeners successfully grow rhododendrons in full sun in the middle of their front lawn. I would not recommend you try this! If you wish to try this use rhododendron ‘Nova Zembla’ because it is more sun tolerant. If you are going to put your rhododendrons on more exposed locations to the winter sun and wind then always use the more hardy rhododendrons. Rhododendrons have their own hardiness rating system. Yes, like all rules there are some exceptions i.e. P.J.M, Olga, and the new varieties now just coming out of Finland are hardier than the H-1 classification.
These will vary with your location and environmental conditions
H-1 Hardy to -32 C
H-2 Hardy to -26 C
H-3 Hardy to -21 C
H-4 Hardy to -15 C
Soil preparations are critical to the successful growing of rhododendrons. They require a rich well-drained high organic content soil mix. You can provide this by mixing into the existing soil a very large amount of peat moss. The peat moss should be 50% by volume of the soil. This will produce an acidic soil of about 6.0 to 6.5 pH. Check this estimate with a pH meter. After checking the pH of the soil mix you will want to adjust the pH to 4.5 to 5.5 using garden sulphur according to the label directions. Never use aluminum sulphate to adjust the pH of the soil for rhododendrons because in acidic conditions the aluminum can become toxic to the rhododendrons especially to younger plants. You should also add some leaf mold and coarse sand to the soil mix to encourage better growth.
When you plant rhododendrons like this Duke of York prepare a hole at least 2 to 3 feet wide and 6 to 8 inches deeper than the height of the root ball. If you think extra drainage will be needed place a 4 to 6 inch layer of coarse gravel in the hole ensuring that you still have the required 6 – 8 inches between it and the bottom of the root ball. Back fill with your soil mix and gently firm the mix around the root ball while never pushing or standing on the rhododendrons root ball.
The finished grade should be 2 – 6 inches above the original grade. You should now water in your plants with a transplant fertilizer with a rooting hormone in it. A mulch of pine bark, cocoa beans, pine needles or oak leaves at least 2 – 4 inches thick can now be applied.
After the flowers are finished you must disbud. This is the removal of the spent flowers and must be done very carefully because the new growth is immediately behind the spent flowers, though some buds will also occur in the leaf axils. Their removal directs the plants energies towards producing new growth and the flowers for next year.
After disbudding gently remove the mulch by hand carefully so as not injure their very shallow root system. Now add a layer of mulch. Alternatively you can just add the new mulch over the old mulch. Do, which ever you feel is best for the rhododendron. Feed your rhododendrons at this time with an acid based fertilizer. Do not feed them after July 1st. Feeding after this time encourages late season growth that may not harden off before the winter sets in and it reduces flower set.
If winterizing your rhododendrons is required by the environmental conditions i.e. northwest location then just before the ground freezes, pound in some wooden stakes at least 1 foot out from the ends of the branch tips and taller than the plant. Now after the ground is frozen staple a single layer of burlap to the stakes 2 inches above the ground. The top should be open to allow the snow to enter and insulate the plant. In very cold climates you can carefully stuff leaves in between the burlap and the leaves only after the ground is frozen.
Always remember never cultivate around the rhododendrons because their new feeding roots grow up and hence the need for an annual mulch. Cultivating around them can kill them.
Truly rhododendrons are the kings of my garden!