Feeding rhododendrons: when, how & the right fertiliser

Kati
Kati
Kati
Kati

I am a qualified gardener and horticulturalist and love everything that grows! Whether it's a shrub, a tree, a useful plant or a supposed weed: for me, every plant is a little miracle.
In the garden I look after my 13 chickens, grow fruit & vegetables and otherwise observe how nature manages and shapes itself.

Favourite fruit: Blueberry, apple
Favourite vegetables: Braised cucumber, kale, green pepper

Rhododendrons need the right nutrients to fully develop their flowers. We give you tips on feeding rhododendrons.

White flowered rhododendron in a pot
Rhododendrons in pots need to be fertilised regularly [Photo: Katjabakurova/ Shutterstock.com]

Rhododendron (Rhododendron) is a genus of shrub in the heath family (Ericaceaea). In nature, the genus grows mainly on acidic sites with a very permeable substrate. Fertilisation should also be adapted to these special conditions. We reveal how to create the perfect circumstances in the home garden and which fertilisers are particularly suitable for your rhododendron.

Rhododendrons are adapted to an acidic, humus and permeable substrate. They often display deficiency symptoms, particularly yellow leaves. In very many cases, the because is a nutrient deficiency: nitrogen, iron or magnesium deficiency can lead to yellow foliage on rhododendrons. However, it is not always enough to simply distribute an iron-based fertiliser for rhododendrons to remedy this because it is also necessary to take into account the pH value of the soil in which the plant grows. If the value is above 5, i.e. it is no longer nicely acidic, the acid-loving plant is less able to absorb iron efficiently – even if there is sufficient iron in the soil. That is why it is particularly relevant to ensure the acidification of the soil when planting rhododendrons.
If the soil is then acidic enough, you can fertilise your rhododendron so that it thrives. We clarify the key questions about feeding rhododendrons below.

Rhododendron with yellow leaves
Yellow foliage on rhododendrons can be caused by nutrient deficiency or soil that is too alkaline [Photo: streetfotolab/ Shutterstock.com]

When should you fertilise rhododendrons?

Most rhododendrons will thrive if they receive a stock fertiliser once a year; however, it does matter when feeding rhododendrons. In addition, a natural slow-release fertiliser such as our Plantura Flower Food, which releases nutrients over a long period of time, is best suited for this purpose. The best time for fertilisation is late spring between mid-April and mid-May. Rhododendrons should then not be fertilised later than mid-June so as not to jeopardise frost hardiness. If you want a particularly lush growth, you can also fertilise twice at the beginning and end of early summer but one fertilisation is enough for the health of the bush. In special weather conditions or unfavourable site conditions, a second fertilisation in early summer may also make sense. You should consider fertilising especially after rainy spring months. It may be that the nutrients have already been washed out and the soil is depleted.

Flower Food, 1.5kg
Flower Food, 1.5kg
  • Perfect for flowering plants in the garden & on the balcony
  • For healthier plants with beautiful & long-lasting blossoms
  • Long-lasting fertiliser that is free from animal products - child & pet friendly
£12.99

Summary: When to fertilise rhododendrons?

  • Annual fertilisation with a slow release fertiliser provides sufficient nutrients
  • The ideal time is between April and May
  • In early summer, a second fertilisation may be applied
  • After mid-June fertilisation is no longer useful
A rhododendron bud
Rhododendrons flower early in the year and are fertilised at about the same time [Photo: Yulia YasPe/ Shutterstock.com]

The right fertiliser for rhododendrons

Ericaceous plants like the rhododendron have shallow roots and love moderately nutrient-rich, humus-rich, well-drained soil with an acidic pH between 5 and 6. This is because rhododendrons originated in the acidic granite rocks of the Himalayas. To mimic these conditions, rhododendrons in this country are almost always planted in acid peat. This is very suitable for the soil preferences of rhododendrons but peat is very poor in iron and magnesium and generally low in nutrients. A good fertiliser for rhododendrons therefore provides the plant with the necessary nutrients and particularly provides the two micronutrients of magnesium and iron. In addition, the plants are mostly shallow rooted, which is especially advantageous on slightly calcareous soils. However, if you do not find the optimal conditions in your garden, you can help a little with targeted fertilisation. You need to do this only in the upper layers of the soil, rooted by the rhododendron. Below we tell you how and with what you can best fertilise your rhododendron.

Purple and pink flowered rhododendron
With the right fertiliser you will not only ensure a healthy plant, but also abundant flowering [Photo: Nataliya Schmidt/ Shutterstock.com]

Feeding rhododendrons organically

Organic fertilisation offers – not only for rhododendrons – some advantages over the mineral variant. While it does not work as quickly in the short term, it is the more sustainable choice in the long term. For rhododendrons, you should use pH-lowering fertilisers, or at least those that do not affect the pH. Fertilisation with compost is not necessarily recommended because compost is very variable in its properties and often even has a rather high pH. Fertilising rhododendrons with horse manure is a better option because horse manure is rich in magnesium and iron. However, composted horse manure should be used here as fresh horse manure is far too rich in nutrients.

If you do not have access to such natural fertilisers or want to avoid animal substances in the fertiliser, you can use slow-release organic fertilisers. This includes our primarily organic Plantura Hydrangea Food. This not only has the advantage of a long-lasting effect, but also provides rhododendrons and other bog plants with exactly what they often lack: iron and magnesium. In addition, it is also much better for the soil life of your garden. Hydrangeas, blueberries, cranberries, skimmias, heathers and lavender heather, for example, can also be fertilised as needed with our hydrangea fertiliser. Of course, our iron fertiliser for rhododendrons is safe for children, pets and garden animals. Below we have compiled a guide for you on how to optimally fertilise your rhododendron organically.

Hydrangea Food, 1.5kg
Hydrangea Food, 1.5kg
star-placeholder star-placeholder star-placeholder star-placeholder star-placeholder
star-rating star-rating star-rating star-rating star-rating
(5/5)
  • For beautiful hydrangeas with lush blooms in pots & flower beds
  • Prevents common deficiency symptoms & supports healthy plant growth
  • Long-lasting fertiliser that is free from animal products - child & pet friendly
£12.99
The pink blossom of a rhododendron
Our Plantura Flower Fertiliser can help your rhododendron bloom beautifully [Photo: zzz555zzz/ Shutterstock.com]

Slow release fertiliser: Application recommendation for rhododendron

  • Before planting, work 100 – 150 g/m² (about a large drinking glass full) of our Plantura Flower Food into the top layer of soil.
  • Dig a wide planting hole and loosen deeply. Water the soil and freshly planted rhododendron well so that the granules can dissolve well.
  • During maintenance fertilisation in spring and summer, you should fertilise it again with 80 – 120 g/m² (0.2-litre jar) per plant.
  • To do this, push aside the layer of mulch on the roots of the rhododendron, apply the fertiliser and water well. However, do not work this in because the shallow roots of the rhododendron should not be damaged.
  • As the rhododendron grows larger, the amount of fertiliser can be gradually increased to about 400 grams of our Plantura Flower Food to maintain the vitality of the plant at an older age.

Tip: Feel free to be generous with the surface distribution of fertiliser for the rhododendron. Due to its shallow sprawling roots, the rhododendron can absorb nutrients even far away from the trunk. Therefore, distribute the fertiliser in a radius of one and a half to two metres around your plant.

Fertilising rhododendrons with mineral fertilisers

In many areas, mineral fertilisation offers a good way to quickly intervene in the nutrient supply of our garden plants, especially when there are acute nutrient deficiencies. While mineral rhododendron fertilisers are good for short-term intervention, they often do not work as a long-term depot fertiliser. This means that blue grain and other mineral fertilisers must be applied at shorter intervals than organic fertilisers. Stock fertilisation is not possible unless special and expensive depot fertilisers are used. Mineral fertilisers can actually harm your rhododendron because incorrectly dosed mineral fertiliser application easily leads to over-fertilisation, which in the best case only ensures susceptibility to pests and frost. In the worst case, even more severe damage to the plant occurs due to the so-called exosmosis at the root. It is especially important to always water sufficiently when using mineral rhododendron fertilisers to dilute the high concentrations of nutrients. However, iron fertilisers for rhododendrons are generally mineral since iron is hardly present in most organic materials – even our primarily organic Plantura Hydrangea Food is supplemented with iron sulfate. It dissolves well in the soil solution, slightly acidifying it in the process to improve absorption.

It is best to choose a gentle but effective option for the plant and the environment, and use an organic fertiliser that replenishes the soil’s iron and magnesium supply either naturally or through supplementation.

Tip: Even rhododendrons planted in containers can be fertilised organically or minerally. However, due to the limited soil volume, nutrients cannot be stored well, so two fertilisations per year are recommended. Organic fertilisation is very useful for container plants because mineral fertilisers do not provide all the essential nutrients and also because the planting soil to slump in the long term. Organic fertilisers are simply spread under the mulch layer, watered and then covered again. You should administer these nutrient-packed fertilisers in small doses from April to mid-June. In this regard, 70 grams per square meter per season is recommended when fertilising your rhododendron. Alternatively, you can dissolve five grams of blue grain per ten litres of water and fertilise your potted rhododendron with it once a week. If you do use mineral fertilisers, you should always administer them with enough water to avoid harmful salt concentrations in the small container, so that the nutrients can be absorbed and avoid burning. Mineral-fertilised rhododendrons should also be repotted regularly to replenish important trace nutrients through new soil. However, over-watering immediately after fertilisation can also flush the nutrients out of the root zone of shallow-rooted rhododendrons.

Pink rhododendron bush in a pot
For rhododendrons grown in pots, you can also – with caution – reach for mineral fertiliser [Photo: Kev Gregory/ Shutterstock.com]

Feeding rhododendrons with coffee grounds

Coffee grounds are produced daily in almost every household. However, with a pH of around 6.5, it does not tend to meet the needs of rhododendrons because it is not acidic enough. However, few know that after infusing it can also be used to fertilise plants such as rhododendrons. The slightly acidic properties of coffee grounds can be used to lower soil pH. However, coffee grounds in small quantities provide useful nutrients that are released over a longer period of time when incorporated on the surface, and also increase the organic matter content of the soil, keeping it loose. Proceed as follows to fertilise with coffee grounds:

  • Dry the coffee grounds
  • Spread coffee grounds in the root zone under the mulch cover
  • However, do not incorporate them
  • Water well
  • Alternatively, you can add the coffee grounds directly into the irrigation water and distribute. For rhododendrons in a flower bed, you can use coffee grounds up to four times a year. For pot culture it is best to fertilise once in April and mid-June
Subscribe to the Plantura newsletter