How to Fertilize Zinnias for Healthy, Vibrant Flowers (2024)

Although zinnias are low-maintenance and easy to grow, they benefit from regular feeding. In my experience, for the best possible flowers, I fertilize zinnias monthly during the blooming season. I choose fertilizers with NPK ratios that provide more significant amounts of phosphorus. Just be sure to dilute your chosen fertilizers with some water.

Signs That Your Zinnias are Lacking Nutrients

Because plants depend so much on getting the proper nutrients, you can often tell if a plant is lacking nutrients. The main signals that your zinnias are lacking adequate nutrition include:

Stunted growth

If your zinnia looks weak and struggling to produce flowers, it’s probably not getting enough nutrients. I find that zinnias need lots of nutrition to produce flowers but won’t flower if they grow in full shade. Check the position of the plant before fertilizing.

Yellowing leaves

A lack of nutrients could be to blame if your zinnia has yellowing leaves. The type of yellowing can tell you exactly which nutrient your zinnia is missing. If new leaves are green but old leaves are yellow, your zinnia needs more nitrogen. In my experience, the plant lacks magnesium if the older leaves are yellow and have green veins.

Yellowing leaves are also a symptom of other problems like overwatering, underwatering, and lack of sunlight. I always check the conditions of my zinnias to eliminate these possibilities before fertilizing.

When and How Often Should You Fertilize Zinnias?

Zinnias have different fertilizer needs at various stages of growth. For established plants, fertilize them once a month during the zinnia flowering season to encourage plenty of flowers. The zinnia flowering season usually starts in early summer and continues until the first fall frosts.

Seedlings can be given a ¼ strength dose of fertilizer once they reach approximately 4 inches tall. I feed them again when they start flowering, and keep feeding monthly.

The Best Fertilizer for Zinnias

Zinnias are grown for their fabulous flowers, which bring bright splashes of color to the garden during the summer. To get the best possible flowers, feed your zinnias with an organic fertilizer high in phosphorus. Choose fertilizers with an NPK ratio similar to 5-10-5.

Avoid fertilizers with a higher nitrogen content, which plants use to produce strong, bushy foliage. Although this benefits some plants, using nitrogen-rich fertilizers for zinnias means more foliage and fewer flowers.

Zinnias need well-draining but fertile soil, so you could work some slow-release organic fertilizer granules into the soil before planting. Alternatively, incorporate some bone meal or organic matter, such as compost, into the soil. Organic matter improves drainage and contains nutrients, while bone meal is high in phosphorus.

If you want to use organic liquid fertilizer, try Epsoma Organic Flower-Tone. An excellent synthetic option is Miracle-Gro All Purpose Plant Food (via Amazon).

How to Apply Fertilizer

Applying fertilizer to your zinnias depends on what type of fertilizer you’re using. Liquid fertilizers must be diluted in water before being applied to your zinnias:

1. Choose a fertilizer with an NPK ratio that favors phosphorus.
2. Follow the packet instructions and put the recommended dose in an empty watering can. In most cases, a ¼ strength dose of fertilizer is sufficient.
3. Fill the can with water and ensure the fertilizer is thoroughly mixed in.
4. Pour the mixture over the soil, avoiding splashing the leaves or flowers.

If you’re using granule fertilizers, follow the packet instructions and mix the correct dose of granules into the soil. For fertilizers like bone meal, sprinkle some over the soil and then mix it in.

The Role of Fertilizers in Plant Health, Growth, and Development

In addition to carbon dioxide, water, and sunlight, plants also need various other nutrients to grow and develop. These nutrients are broken down into four categories; structural macronutrients, primary macronutrients, secondary macronutrients, and micronutrients.

The structural macronutrients are carbon, hydrogen, and oxygen. Plants use these to create sugars like glucose, fuel cellulose growth, and other plant tissues. While structural macronutrients are absorbed from air and water, the other three categories are soil and fertilizer.

Nitrogen (N), phosphorus (P), and potassium (K) are the three primary macronutrients that plants need to thrive. Nitrogen helps the plant to produce bushy foliage and enables the manufacture of chlorophyll, which is vital for photosynthesis. Phosphorus fuels flower growth, creating large, showy blooms, while potassium helps transport water and other nutrients around the plant.

Most fertilizers are focused on providing plants with these primary macronutrients. That’s why these products have an NPK ratio that shows how much nitrogen, phosphorus, and potassium the fertilizer contains. You can use fertilizer to top up major nutrients the plant has absorbed from the soil.

Some fertilizers contain secondary macronutrients like calcium, magnesium, and sulfur. They may also provide micronutrients like copper, iron, and zinc.

Different Types of Fertilizers

With several types of fertilizers on the market, choosing the right one can be confusing. Most fertilizers fall under two main categories: organic fertilizers and synthetic fertilizers. Both types work in the same way, so which type you choose is down to personal preference.

Organic fertilizers are made from natural ingredients and are a more environmentally friendly option than synthetic fertilizers. Organic fertilizers usually need to be broken down by bacteria in the soil before plants can absorb the nutrients. Common organic fertilizers include compost, manure, blood or bone meal, and worm castings.

Synthetic fertilizers are made in labs using chemicals and salts. These fertilizers feed the plant directly, which means the plant can access nutrients faster. However, synthetic fertilizers can cause soil quality to deteriorate over time. Using too much synthetic fertilizer can also damage your plants.

Whether you choose organic or synthetic fertilizer, these products usually come in two main application methods: liquid fertilizers and granules. Liquid fertilizers like liquid seaweed or tomato feed are diluted with water and applied using a watering can. Granules are buried or worked into the soil, releasing nutrients slowly throughout the season.

Key Considerations

Although fertilizer is highly beneficial for zinnias, it’s possible to misuse it or overuse it. Here are a couple of things to consider:

Don’t Apply Too Much Fertilizer

Giving your zinnias too much fertilizer could severely damage them, especially if you’re using synthetic fertilizers. Always follow the recommended dosage on the packet. Start by giving your zinnias small doses to see how they respond.

Soil Type

Depending on the soil your zinnia is growing in, you might not need to use a lot of fertilizer. Zinnias can grow in chalky, sandy, loamy, or clay soils if they’re well-draining. Clay and loam hold nutrients much better than chalk or sand. If your zinnias grow in loam or clay soils, you’ll need less fertilizer.

Wrapping Up

Zinnias are famed for their fabulously showy and colorful flowers that bloom from early summer until the first frost. To get the best zinnia flowers, apply an organic fertilizer high in phosphorus once a month. Apply this during the flowering season and always follow the packet instructions regarding dosage.