Fertilising can be discussed in terms of Organic and Inorganic (man made). Both options are available to the rhodo gardener but both come with some pretty important messages. Rhododendrons require regular feeding, but their demand for food is specific. They do not like lime and if you have limechip on your driveways or pathways, lime can leach into the garden and cause them to turn up their toes.
Fertilisers assist with over plant health, they help to create a better shaped rhodo, and increase your chances of flowering. If a rhodo is under stress in the summer months when flower production takes place, the rhodo will go into survival mode and will put all its remaining energy into survival and forget about flowering.
If your rhododendron starts to turn yellow it is a strong sign that your rhodo needs feeding.
Organic Fertiliser Options
Some of the best things in life are free - and fertiliser can apply to this quote too. Rhododendrons are acid loving plants and will enjoy a regular dose of pine needles around them, pine needles are acidic and are the perfect mulch for rhodos.
You can successfully fertilise rhododendrons using organic materials however if a rhodo has turned yellow like the photo above it will be a slower recovery back from this colour to a nice green colour. Inorganic fertilisers can respond to the need for nutrients faster.
If you are going to fertilise using organic materials you need to take a holistic approach to the garden and be working on the nutrient needs of your plants around the year, not reacting to a problem and trying to create a quick fix.
You can burn and kill rhododendrons through using animal manure that is too strong, too close to the roots and too heavily applied. Organic animal manure needs to be well composted/rotted and often it helps to have the manure mixed with untreated sawdust or straw. You do need to know what you are doing, or ask a seasoned gardener, or get a wide variety of advice from rhododendron experts before making a decision to cover your rhodo garden in free poo.
Horse manure from a stable is very highly concentrated with urine and should be left to rot outside of the stable before being applied to a rhododendron garden.
Chook poo and Pig Poo is very very strong and we advise to give this kind of free fertiliser a miss. Some gardeners will use it with success - we recommend that you don't purchase it and if you are going to use it, then ensure that you test it and use it sparingly.
Sheep poo is a little less toxic and is a lot easier on the rhododendrons, the best is the rotted poo mixed with wool from under the shearing shed, often you can purchase this or get it for free if you are willing to help out a farmer - usually they will be delighted to have someone clean out under the shearing shed. When you apply the mix to the garden, keep it away from the direct circle that is around each rhododendron. Use the fertiliser on the outside perimetre of the rhododendrons and keep the area around the trunk and branches free of fertiliser.
Rhododendron roots are very fine and close to the surface, if you apply any kind of fertiliser on top of these fine roots the effect can be drastic. The first sign is that the leaves will go brown and crusty and look like they have been sunburnt. Once the leaves have been damaged they will not recover, however if the plant has not suffered too severe damage you will hopefully get a new crop of healthy leaves and you can either leave the damaged leaves to fall off by themselves or take them off yourself.
Inorganic Fertiliser Options
If you are happy to use inorganic options there are many available. We recommend the use of a slow release fertiliser. We sell coated balls, these slowly release nutrients into the ground over an 18 month period. So long as you follow the instructions for spreading you cannot cause damage to your rhodos like the photo of the burnt leaves above.
However!!!! We do know of a nursery worker who over applied fertiliser to a crop and killed the lot, so just remember fertiliser is like medicine, take what you need - overdosing can be dangerous.
During winter the granules will not release in very cold climates. So best to wait until spring before applying.