Fertilising Bearded Iris
At TenAcres we often purchase new iris from hybridisers and importers and rely on them to produce healthy, well developed and well fed rhizomes and plants. A healthy rhizome is everything in iris growing because it results in good foliage, strong flowers, disease resistance and generous reproduction.
To ensure you feed bearded iris what they need, it is important to understand the nutrient requirements and how they align to the annual growth cycle of the plant.
• Summer dormancy – flowering is finished, and the iris rests.
• Rhizome clump division – typically carried out after summer dormancy. Once divided, the newly separated rhizomes need to establish a strong root system.
• Growth and flowering – as the rhizomes “wake up,” they develop strong foliage and require nutrients to support flowering.
• Post-flowering – the rhizomes gather and store nutrients during this period to support the next season’s growth.
The key nutrients support the following functions:
• Nitrogen (N): Promotes foliage growth. However, excessive nitrogen is undesirable in irises, as it can lead to soft leaves that are more susceptible to pests and diseases, especially Didymellina macrospora (Leaf Spot).
• Phosphorus (P): Promotes strong root development and flower formation.
• Potassium (K): Supports overall plant vigour, disease resistance, and enhances flower colour.
• Trace elements: Critical for overall plant health.
It is also important to recognise that plant health is not solely dependent on nutrients, but also on soil microbiology. Research over the last decade has highlighted the critical role that soil biology and root interaction play in plant health and disease resistance. Because the iris lifecycle often includes division and potential root trimming, enhancing soil microbiology is particularly important.
Recommended Nutrition Program
When planting new rhizomes, apply:
• Compost
• Seaweed solution and/or
• Fish emulsion and/or
• Weed/compost tea and/or
• Any of the above combined with diluted sulphate-free molasses
These amendments are not fertilisers. Their role is to promote soil microbiology, which in turn improves root establishment and facilitates nutrient uptake. Do not apply fertiliser to newly divided irises for at least four weeks, as the new roots will not yet have developed the fine root hairs necessary to interact with soil microbes for optimal nutrient and water uptake. Adequate, manure enriched compost will supply the plant’s nitrogen needs for the season, so additional nitrogen is usually unnecessary.
Four weeks after planting:
At this stage, trace elements and Phosphorus become important for overall plant health. Phospohorus stimulates root growth which is critical when young plants establish. Australian soils are generally low in key micronutrients, so a slow-release, fertiliser containing trace elements (such as copper, magnesium, boron, zinc, and iron) is recommended at this stage. Neutrog Rapid Raiser is cost-effective and works very well; Grow Safe is also suitable. Both are available in easy-to-apply pelletised form. Blood and Bone is also useful, but does lack the trace elements.
Pre-flowering:
Maximising the availability of phosphorus and potassium (potassium sulphate or sulphate of potash) becomes important leading into flowering. Black Marvel Rose and Flower Food is often recommended, but it has a relatively high nitrogen content (12N:2P:10K). To achieve a better balance, try a high-phosphorus fertiliser such as Richgro Organics Phosphorus (0N:12P:0K) combined with a high-potassium fertiliser such as Richgro Potash (0N:0P:41K). Any fertiliser formulated for flower and fruit production can be suitable—check the NPK ratio. A note of caution: apply these fertilisers according application rate instructions because over-application can have a detrimental affect on soil microbes.
Post-flowering:
Assess your plants before deciding to fertilise. If the foliage is strong and the plants appear vigorous, extra fertiliser may not be necessary. Strong rhizomes are formed when excess glucose from photosynthesis is stored as starch, so if foliage looks pale or lacks vitality, a light application of a balanced fertiliser, with trace elements, is appropriate. Deadheading spent blooms is very important, as it directs the plant’s energy into rhizome development rather than seed production.
A Word of Caution
Some gardeners may apply fertiliser diligently and still fail to achieve good results. Soil pH is a critical factor. Extreme acidity or alkalinity can prevent plants from accessing nutrients, regardless of how much fertiliser is applied. The optimal pH range for bearded irises is 7.5–9, while other iris varieties generally prefer the more acidic end of the spectrum. Testing your soil pH is simple and worthwhile; a Manutec soil test kit is a good investment. If your soil is outside this range, there are ways to address this imbalance - but that is for another blog :)
Happy Gardening
