Cosmos flowers grow best in nutrient-poor soils, so fertilizing can be detrimental if not carefully managed, in my experience. Flowers growing in borders won’t need much fertilizer. Those growing in containers will benefit from small doses of fertilizer every month or so. I always avoid fertilizers with too much nitrogen—instead, I use phosphorus fertilizers such as bone meal. Make sure you’re fertilizing your cosmos flowers the right way with this helpful guide.
The Best Fertilizer for Cosmos
I find that fertilizers that are high in phosphorus are the best choice for cosmos. A small dose of high phosphorus fertilizer will help promote flowering for cosmos growing in containers. I always avoid fertilizers containing lots of nitrogen because this supports the growth of bushy foliage rather than flower production.
In my experience, organic fertilizers are a better choice for cosmos flowers than synthetic fertilizers because the latter may contain too many nutrients. Bone meal is the best choice because it’s absorbed quite quickly. Other organic fertilizers that contain lots of phosphorus include blood meal and fish meal.
How to Apply Fertilizer
Different fertilizers need to be applied in different ways. The most common fertilizers are liquid fertilizers. Here’s a quick guide to using liquid fertilizer:
- Choose fertilizers with an NPK ratio with high phosphorus.
- Dilute the fertilizer by adding the recommended dose to an empty watering can before topping it up with water. Aim for a ¼ strength dose.
- Make sure that the fertilizer is thoroughly dissolved in the water.
- Water the plant, as usual, avoiding splashing the flowers or foliage.
Speaking from personal experience, bone meal is one of the best fertilizers for cosmos flowers and needs to be applied differently to a liquid fertilizer. Simply sprinkle a small dose of bone meal around your cosmos and slowly dig it into the soil.
Signs That Your Cosmos Are Lacking Nutrients
Although most cosmos will be fine without fertilizer, there may be times when your cosmos lacks nutrients. If that’s the case, use fertilizer to strengthen your plant.
Flowers Do Not Appear
If you haven’t fertilized your cosmos, but flowers still fail to appear, you may need to give them a helping hand. Most cosmos should flower within 7 to 14 weeks of sowing, starting in early summer.
Use a small dose of high phosphorus fertilizer if your cosmos has plenty of foliage but doesn’t have flowers. This should stimulate the plant to bloom. Bone meal or fertilizers designed for flowering plants are good options.
When and How Often Should You Fertilize?
Cosmos prefer nutrient-poor soils and are well-adapted to thrive in these conditions. In many cases, fertilizing will be detrimental. If you’re growing cosmos in a garden bed or border, avoid fertilizing them unless they aren’t producing flowers.
Cosmos growing in containers require more nutrients because they quickly use the nutrients within the soil. Apply a small dose of fertilizer every 4 to 6 weeks according to the packet instructions.
If you decide to fertilize your cosmos, there are several things to remember. It’s easy to cause more harm than good when fertilizing.
If your cosmos grow in borders or garden beds, they won’t need fertilizing. Your plants should get enough nutrients from the surrounding soil to grow. Only apply a small dose of fertilizer if your cosmos aren’t flowering.
Cosmos need nutrient-poor sandy soils to thrive. Avoid fertilizing if your flowers growing in too fertile soil, such as clay or loam. If you can, work in some sand or grit to reduce fertility.
Too Much Fertilizer
Applying too much fertilizer is highly detrimental to cosmos flowers. If the soil is too fertile, your plants might have trouble flowering. Instead, it’ll focus on producing foliage at the cost of flowers. Only apply small doses of high phosphorus fertilizer when necessary.
The Role of Fertilizers in Plant Health, Growth, and Development
Along with sunlight, water, and carbon dioxide, plants depend on nutrients to help them grow. Plants require four types of nutrients; structural macronutrients, primary macronutrients, secondary macronutrients, and micronutrients.
Carbon (C), hydrogen (H), and oxygen (O) are the structural macronutrients. Plants obtain these vital nutrients from the air and the water sucked up by their roots. Structural macronutrients are used to manufacture glucose and cellulose to create new tissues.
Plants also need three primary macronutrients: nitrogen (N), phosphorus (P), and potassium (K). These nutrients are obtained from the soil and fulfill different functions. Nitrogen promotes bushy foliage and helps produce chlorophyll. Phosphorus is crucial for the development of flowers, while potassium helps the plant circulate water, nutrients, and minerals.
Secondary macronutrients are minerals that also help plants grow and develop. These minerals include calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), and sulfur (S).
Different Types of Fertilizers
Two main fertilizers are available for plants: organic fertilizers and synthetic fertilizers. Both types work in virtually the same way but can have other effects. Fertilizers are usually sold as either liquid feed or slow-release granules.
Organic fertilizers use natural ingredients, making them more environmentally friendly than synthetic fertilizers. Organic fertilizers include compost, blood or bone meal, manure, and worm castings. Organisms in the soil must break down these fertilizers before the plant can use them.
Synthetic fertilizers are made from ingredients such as chemicals and salts. Synthetic fertilizers feed the plant directly rather than being broken down in the soil. While synthetic fertilizers work faster than organic ones, they are less environmentally friendly. Over time, synthetic fertilizers may harm your plants or deteriorate the soil quality.
Choosing the right one for your plants is essential whether you use organic or synthetic fertilizers. All fertilizers use an NPK ratio to indicate how much nitrogen, phosphorus, and potassium they contain.
Depending on the type of plant and what you want to achieve, you’ll need different amounts of these nutrients. Use fertilizers with higher amounts of phosphorus if you want plenty of flowers. Use more nitrogen fertilizers for leafy vegetables, such as kale and other salad leaves.
Cosmos usually don’t need fertilizer to help them produce stunning flowers. If you’re growing cosmos in containers, feed them once every month or so. Use small doses of high phosphorus fertilizers such as bone meal to promote flowering. Avoid fertilizers that contain high amounts of nitrogen, as this encourages foliage growth rather than flower growth.
For more, see our in-depth guide to the amazing uses and benefits of cosmos flowers.