4 Best Fertilizers for Rhododendrons | How and When to Use

Rhododendrons are evergreen shrubs that make excellent landscape plants and can be used to dramatic effect to help create a picturesque woodland garden. Some smaller varieties are perfect for containers or planters and can be just what is needed to add interest to a shady or sheltered area of your garden.

One thing that is common to all Rhododendrons is their ability to add a splash of colour from spring through to early summer with their magnificent blooms.  

Getting the best from any Rhododendron plant means providing the right soil conditions as well as catering for its nutrient requirements. Here’s all you need to know about buying the best fertilizers for Rhododendrons and just how and when to use them.

Choosing Fertilizer for Rhododendrons

Since rhododendrons need regular mulching and soil that has been enriched with organic matter, a light fertilizer during their growing season to encourage blooming will be sufficient for their needs. That is unless you find that you need to supplement any lack of nutrients in the soil in addition to composting and mulching.

The roots of rhododendrons form an intricate network close to the soil surface and can easily become damaged or burned by the use of fertilizers. Avoid heavy usage, harsh chemicals or the need to dig fertilizer into the soil.  

Granular Vs Liquid Fertilizer

Granular feeds typically need to be sprinkled into the soil that surrounds plants and then dug in. Chemicals in the granules usually get activated once they become wet and are almost always offer a slow-release option to feeding.

If you are using granules, look out for those that don’t need digging in and always try to avoid any heavy digging in and around your rhododendrons as this can cause damage to the roots.

Liquid fertilizer generally needs to be diluted in water before using and offers a fast-acting approach to fertilizing. Making sure you use the correct ratios as per the manufacturer’s instructions will help to avoid causing potential damage as the liquid is absorbed into the soil and the roots of your plants.

Liquid Concentrate or Spray

The beauty of liquid concentrate fertilizers is they can be used as part of your plant’s watering regime. Simply mixing the concentrate in a watering can with the correct quantity of water can save time and effort and combine watering with feeding.

Fertilizer sprays are usually ready to use straight from the bottle and offer convenience and avoidance of messy mixing. Whilst you can use what you need and then store it in a safe place for later, they are usually more expensive litre for litre than most other fertilizing options.

Fertilizer Granules or Powder

Fertilizer granules and powder are the most cost-effective methods of feeding your rhododendrons. Depending on the manufacturer’s instructions they either need to be sprinkled or scattered over the soil around your plants and then watered in.

The process of watering the granules after you have scattered them means they gradually become distributed into the soil and in turn, the plant’s roots will absorb the fertilizer nutrients.

Rhododendrons Fertilizer Spikes

Fertilizer spikes for rhododendrons offer a mess-free, odour-free and slow-release option. Once they have been pushed or hammered onto the soil, they provide a slow release of nutrients for up to 8 weeks.

This means that you only have to fertilize once, maybe twice per season. Spikes do prove to be costly compared to other fertilizing methods and you often need more than one spike per plant depending on their size.

Rhododendrons Soil pH

Acid soil between a pH of 5.0 and 6.0 is best for successful Rhododendron growth. Testing the pH of your soil using a soil test kit before you begin planting or even designing your landscape will provide you with a simple, yet accurate reading of the pH levels in your soil.

A pH of 7.0 indicates that your soil is slightly acidic but if you are planning on successfully growing rhododendrons, you will need to add acidifying materials to lower it sufficiently.

Rhododendrons are ericaceous (lime-hating) plants and any soil pH increase will result in slow and disappointing growth and can also cause leaves to turn yellow.

Regular mulching and composting with nutrient-rich organic matter will serve you well and you can also add sulphur chips for a boost of acid too.

Natural materials (mulch and compost) that add acid to soil can be applied year-round but sulphur will need to be added from late spring to fall as it can take longer to work.  

Rhododendrons Fertilizer N-P-K

For rhododendrons, a nutrient deficiency usually manifests as yellowing of leaves or premature leaf drop, reduction in size or number of flowers and/or stunted growth.

These symptoms may indicate either a lack of nitrogen (N), phosphorus (P) or potassium (K) deficiency and usually mean that your plant needs nursing back to health with the help of a specialist Rhododendrons Fertilizer.


Best Fertilizers for Rhododendrons Reviews

So, if your Rhododendron is anything short of thriving but you are sure you have the right soil conditions, are mulching regularly and your plant is receiving enough water, then the problem may well be that you need to seek out the best fertilizer for Rhododendrons to fix the problem.

Lucky for you I love these beautiful woodland shrubs with their spectacular evergreen foliage and impressive blooms and I’ve been busy scouring the world of fertilizers to bring you five of the best on the market.


1. Jobe’s Azalea, Camellia & Rhododendron Fertilizer Spikes 9-8-7

Jobe's Azalea, Camellia & Rhododendron Fertilizer Spikes 9-8-7

Easy to use, mess-free, no need for measuring and odourless. What’s not to love about the utter convenience of Jobe’s Azalea, Camellia & Rhododendron Fertilizer Spikes? It’s certainly one of the main reasons I’ve placed them at the top of the list of fertilizers for rhododendrons.  

With an N-P-K of 9-8-7, this slow-release option will provide your Rhododendrons with a nice even balance of nutrients for up to 8 weeks without the risk of root burn or over-fertilizing. Click here for Jobe’s Azalea, Camellia & Rhododendron Fertilizer Spikes.

These are also suitable for other acid-loving plants. I have used this with success as a blueberry fertilizer and also on my azaleas too. Expect more lush, green leaves and less yellowing and strong healthy blooms that won’t drop at the first signs of rain or moderate wind.

How to Use: Simply push or hammer into the ground 1ft from the trunk (or around the dripline if this is closer to the trunk). Use 1 spike for each foot of the trunk. For example, a 1ft diameter trunk needs 1 spike and a 3ft diameter trunk needs 3 spikes.

Pros      

  • Convenient, mess-free and odourless
  • Mitigates the risk of over fertilizing

Cons

  • An expensive fertilizing method compared to some others

2. Miracle-Gro for Acid Loving Plants Azalea, Hydrangea, Camellia 30-30-10

Miracle-Gro for Acid Loving Plants Azalea, Hydrangea, Camellia 30-30-10

My next pick is a water-soluble fertilizer from Miracle-Gro. It is rich in nitrogen and also phosphorous so you can expect new shoots and lush foliage and buds and blooms aplenty. Just be sure to dead-head those spent blooms to encourage more to come through.

This fertilizer gets to work straight away as the solution is quickly absorbed through the root system and subsequent applications through the growing season are recommended.

It is also suitable for a whole host of acid-loving plants and can be used as a dogwood fertilizer as well as on camellias, azaleas, as well as Rhododendrons. Compare prices for Miracle-Gro For Acid Loving Plants here.

My top tip for using any water-soluble fertilizer is to follow the manufacturer’s recommendations of application rates and stick to it! Never over-fertilize even if you have an ailing plant that looks like it needs it. Too much fertilizer can lead to root burn and will not do your ailing plant any favours in the long run.

How to use: In a watering can mix 1 tablespoon of fertilizer for every gallon of water. For larger areas, you’ll need a Miracle-Gro feeder jar. Then, simply empty the fertilizer into the feeder jar up to the measuring line and attach it to your garden hose. This can be used every 7-14 days as part of your watering schedule.

Pros      

  • Rich in nitrogen and phosphorous that can improve plant deficiencies, encourage foliage growth and bud development
  • Suitable for use with other acid-loving plants and shrubs

Cons

  • Stick to manufacturer’s application rates to avoid over fertilizing and risk of root burn

3. Dr. Earth Organic Azalea/Camellia/Rhododendron Acid Fertilizer 4-5-4

Dr. Earth Organic Azalea/Camellia/Rhododendron Acid Fertilizer 4-5-4

Next up is an organic and 100% natural pick from Dr Earth. Just like the others, this is suitable for many varieties of Ericaceous plants (lime hating) and can be used as a fertilizer for hydrangea, Azalea and camellia to name but a few.

This fertilizer is particularly good for new plantings at the time of planting. This is because the inclusion of organic matter plus microbes, probiotics and ecto and endo mycorrhizae helps to enrich the soil and enhance acid levels. Perfect if you are concerned that your soil has become depleted of acid, and where pH levels are too high and need lowering.

It also has a well-balanced N-P-K of 4-5-4 so as not to over-burden with nitrogen but to ensure your rhododendrons produce plenty of buds and subsequent blooms that are bursting with healthy and vibrant colour. Click here for Dr Earth Organic Fertilizer for Acid Loving Plants from Amazon.com

How to Use: Dig a hole big enough for your new rhododendron planting. Add 2 cups of fertilizer for every ½ cubic foot of soil. Gently work this into the soil and water in well. Place planting in the hole and backfill with the fertilizer enriched soil.

This can then also be used bi-monthly by sprinkling to correct dosage around the dripline of your plant, gently working it into the soil and then watering it in. Just be super careful to avoid damaging the roots of your rhododendron that will be just below the surface of the soil.

My top tip is after planting to cover the area with a layer of mulch. This seems to lock in some of the odour and keep my dog off the scent. I’m not going to lie, this one is a bit on the smelly side and I do struggle to keep my dog away from it. But, hey! She’s just a dog trying to do what dogs do!

Pros      

  • 100% natural ingredients
  • Increases acid levels in soil as well as providing nutrients to plants

Cons

  • Very attractive to dogs

4. Schultz Azalea, Camellia, Rhododendron, Slow-Release Plant Food 14-7-7

Schultz Azalea, Camellia, Rhododendron, Slow-Release Plant Food 14-7-7

This is a good value for money option from a brand that is well-known and highly regarded in gardening circles. Schultz Azalea, Camellia, Rhododendron plant food offers a slow-release fertilizer that will continue working for up to 3 months and so will see you good throughout the growing season.

This one calls for the correct pH levels upfront so make sure you have soil with a pH of between 5.0 to 6.0 and keep on top of mulching and adding nutrient-rich compost to your soil on a regular basis.

Often a nitrogen nutrient deficiency in rhododendrons will present as yellowing of leaves and this is where Schultz comes into its own. It has an N-P-K of 14-7-7 so you can be sure of plenty of bushy, lush foliage and new shoots from the growing season get-go. View prices for Schultz Slow-Release Plant Food at Amazon.com

How to use: For established plants, apply in early spring. Scatter 4 cups per 100 sq. foot of soil over the surface of the soil and work it in gently avoiding any damage to roots. No need to water in.

Pros      

  • Slow-relelase fertilizng that continues working for up to 3 months
  • Value for money compared to other more expensive fertilizing options

Cons

  • Assumes that soil is correctly pH balenced to begin with

How to Fertilize Rhododendrons

The best way to naturally fertilize and to ensure optimum soil conditions for your Rhododendrons is to mulch regularly.

Cover the soil surface with a 2inch thick layer of mulch in late fall. Rhododendrons are not overly tolerant to frost and so this will provide a layer of insulation in colder weather. The mulch will also help to add otherwise depleted acid to the soil that has been absorbed by your plant during the growing season.

Fertilizing Rhododendrons for Best Blooms

Plants need phosphorus to assist with the production of buds and subsequent blooms.

If you have noticed a reduction in the size or number of blooms, then choosing a fertilizer with an N-P-K that is higher in phosphorus (P) will help to encourage bud production. This should be applied in early spring and again as the growing season progresses.

Avoiding Fertilizer Burn

Since the roots of Rhododendrons lay just below the surface of the soil they can be prone to fertilizer burn if fed too often or with harsh chemicals. Avoiding fertilizer burn in Rhododendrons can be achieved by applying fertilizer in the correct quantities and watering the product into the soil thoroughly.

When to Fertilize Rhododendrons

For healthy Rhododendrons that have been mulched regularly and have soil that is rich with organic matter then applying fertilizer in early spring before bud set should be sufficient.

How Often to Fertilize Rhododendrons

More regular applications of fertilizer can be made if you are concerned about a lack of blooms, yellowing leaves or if you wish to boost flower production. Always act with caution and avoid over-fertilizing or applying too much as this will lead to fertilizer burn and can cause long term damage.

Verdict: Best Rhododendrons Fertilizer

So there we have it, a full rundown on why rhododendrons need fertilizer, how much, how often and of course, my personal favourites. If you are looking for ease of use, convenience and a slow-release that lasts up to 8 weeks, then Jobe’s Fertilizer Spikes are for your rhododendrons. They offer a balanced N-P-K and are great if your soil is already at the right pH level.

Choose Miracle-Gro for Acid Loving Plants if it’s a blend of nitrogen and phosphorous you need to give your rhododendrons lush foliage and plentiful blooms. My top choice organic fertilizer goes to Dr Earth Organic Azalea/Camellia/Rhododendron Acid Fertilizer. Choose this if you need to acidify your soil as well as increasing the number and size of your blooms.

Fertilizing Rhododendrons FAQ’s