When to fertilize, how often, and which fertilizer for which plant
Fertilizing is like cooking: too little is dull, too much ruins everything. The right dose makes the difference between a struggling plant and one that grows beautifully.
Houseplants live in a limited volume of soil. The nutrients in potting soil are used up after about six to eight weeks. After that, you need to help from the outside - with fertilizer. Without fertilizer, the plant grows more slowly, the leaves become paler, and flowers fail to appear.
But: overfertilizing is just as harmful as not fertilizing at all. Too much fertilizer burns the roots and can cause lasting damage to the plant. Less is usually more.
When should you fertilize?
The golden rule: Only fertilize during the growing season - that means from March to October. In winter (November to February), most plants are dormant and cannot process nutrients. Winter fertilizing leads to salt build-up in the soil, which damages the roots.
Start in spring with half the dosage, then switch to the full dosage from April/May, reduce again from September, and stop completely from November onward.
Overview of fertilizer types
Liquid fertilizer: Mixed into the watering water. Quickly available and easy to dose. Apply every 1-2 weeks. The most popular method for indoor plants.
Slow-release fertilizer (sticks/cones): Insert into the soil; works for 2-3 months. Practical, but difficult to dose and often distributed unevenly.
Granules: Sprinkle over the soil surface. Slow-release and great for balcony and garden plants.
Organic fertilizer: Compost, horn shavings, worm compost. Slow-acting and improves soil structure. Ideal for the garden, but less practical for indoor plants (odor).
Which fertilizer for which plant?
- Green plants (Monstera, Ficus, pothos): Universal houseplant fertilizer with a balanced NPK ratio. Every 2 weeks from April to September.
- Orchids: Special orchid fertilizer (more lightly diluted). Add it to the soaking water every 2-3 weeks. Never fertilize dry roots.
- Cacti and succulents: Cactus fertilizer (phosphorus-rich, low in nitrogen). Every 4-6 weeks in summer. Not at all in winter.
- Flowering plants: Flower fertilizer (high in potassium). Potassium supports flower formation.
- Hydrangeas: Rhododendron fertilizer (acidic). Blue hydrangeas also need alum for their color.
- Balcony flowers (geraniums, petunias): Balcony flower fertilizer, weekly. Balcony flowers are heavy feeders and need plenty of nutrients.
Warning: Never fertilize dry soil. Water first, then fertilize (or mix the fertilizer into the watering can). Fertilizer on dry roots can cause burns. And: less is more - use half as much as the package says. Manufacturers tend to recommend generous doses.
NPK - what do the numbers mean?
Every fertilizer lists three numbers, e.g. 7-3-5. They stand for:
- N (Nitrogen): Promotes leaf growth and rich green color.
- P (Phosphorus): Promotes root growth and flowering.
- K (Potassium): Strengthens the plant overall and promotes flowering and fruit production.
Green plants need more N, flowering plants need more P and K. All-purpose fertilizer has a balanced ratio and works well for most indoor plants.
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Frequently Asked Questions About Fertilizing
The leaves develop brown edges, the plant wilts despite moist soil, and white salt deposits form on the surface of the soil. Immediate action: flush the soil thoroughly (water several times and let it drain) or repot into fresh soil.
Yes, coffee grounds contain nitrogen, phosphorus and potassium. They lower the pH slightly, which is good for hydrangeas, azaleas and rhododendrons. But: only sprinkle a thin layer on the soil and let it dry, otherwise it can go moldy. For most indoor plants, regular fertilizer is more reliable.
No, fresh potting soil contains enough nutrients for six to eight weeks. Only start fertilizing after that. Fertilizing too early after repotting can burn the freshly damaged roots.
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