It is very common for rhododendrons to grow well for a while, then to start dropping foliage,
become yellow and die back. This can nearly always be traced back to root rot caused by poor drainage.
The plants are fine until their roots hit the subsoil clay or reach the permanent water table. When that
happens the roots can no longer spread to find nutrients and they may start to rot.
Regular feeding will keep such plants alive, but it isn’t really a cure. The answer is a well-drained site
and extra preparation, especially with larger-growing plants. It’s all very well to prepare the top 20 in
(50 cm) of soil for a 5 ft (1.5 m) high plant, but taller plants have larger and deeper root systems.
Although rhododendrons are very shallow-rooted compared to most plants, a tree-sized specimen will
still be anchored by a substantial root system.
Good drainage is best taken care of before planting time. Because you took the time to work in
plenty of compost before planting, the drainage should be reasonably good, but check it just to make
sure. Dig a 20 in (50 cm) deep hole and fill it with water. If the water disappears within four to six
hours the drainage should be adequate. If after that time there is still water in the hole or if the hole fills
with water as you dig it, you need to think about improving the drainage or planting in raised beds.
A raised bed can be a naturally higher part of the garden, an artificial mound or a boxed bed. If poor
drainage is still a possibility or you have very hard clay subsoil, field drains may need to be laid. These
should drain the soil to at least a depth of 3 ft (1 m). If you can go deeper, so much the better. It is also a good idea to incorporate a mild all-purpose fertilizer prior to planting. Fresh animal
manures and harsh chemical fertilizers can burn tender young roots so use mild fertilizers and make
sure they are worked well into the soil.
Only when you are satisfied that the ground is thoroughly prepared is it time to plant. Newly
planted rhododendrons need loose soil in order to make quick root growth, so make sure you dig a hole
that is at least twice the size of the plant’s root ball.
Thoroughly soak the shrub before you remove it from its container or the roots may adhere to the
sides and suffer damage. Plastic bags may be cut away or carefully eased off and pots usually come
away cleanly if up-ended and given a firm rap on the rim. If the shrub appears pot bound, gently loosen
up the root ball. Otherwise just lightly work your fingers into the root ball to allow moisture to
penetrate and spread a few of the lower roots to get them growing in the right direction.
Place the rhododendron in the hole and check the soil level. It should be at the same level it was in
the container. Gently firm the plant into position with your heel as you replace the soil but don’t ram
the earth back into place or you’ll undo all the work that went into loosening it up.
Mulch around the plant and stake large rhododendrons to prevent wind rock. In dry areas or
on sloping sites it is a good idea to make a rim of soil or mulch around new plants to act as a reservoir
that can be filled when the plants are watered.
Newly planted rhododendrons need ample moisture, but don’t drown them. If the plant is slow to come
into growth, yet you are watering and feeding, it may be that the root ball is still dry at the center. That is
because most nurseries now raise their rhododendrons in containers of soil-less mix rather than
growing them in the open ground and lifting them just prior to selling. Potting mixes can be very difficult to re-wet once they have dried out. It is easy to think you are
watering deeply when in reality the surface soil around the plant is getting wet while the
all-important root ball remains dry.
The only way to check this is to probe around the roots. If those closest to the main stem are not
noticeably dry, you may have to lift the plant to check the roots. This will also highlight any
problems with root-feeding insects or root rot.