Why to Fertilize Petunias

Petunias are fast-growing plants that produce mounds of foliage and abundant blooms. In a garden bed, a petunia can continue expanding below ground, sending roots out in search of nutrients to fuel all that growth. But in a hanging basket or container, crowded with a few other plants, resources are limited and quickly depleted.

Nutrient leaching when water drains out of containers and baskets adds to the problem. Nitrogen, one of the main nutrients needed for strong plant growth, is especially susceptible to being carried away like this.

Before long, the nutrients in the potting mix are depleted enough that your petunias start showing it with slower growth and less blooming. To keep your plants in tip-top shape and blooming strong all summer long, you'll need to add fertilizer regularly.

Fertilizer is commonly called plant food, but that isn't quite true because plants make their own "food" via photosynthesis. Fertilizer can't substitute for that process, only supplement it.

When to Fertilize

Exact fertilizer needs will vary based on plant size, potting mix, and watering frequency. Petunias purchased as starter plants from a garden center were grown in a greenhouse where their every need was efficiently met, including fertilization. When we bring them home, there's naturally more variability in their care so it's common to see these plants slow down a bit in response.

Begin regular fertilizer applications for container-grown petunias 2 to 4 weeks after bringing them home, depending on the potting soil you used. If the mix contains slow-release fertilizer, you can safely wait 4 weeks before beginning. Many organic potting mixes contain no fertilizer, or a mix of organic fertilizers that have a slow release rate. If you used one of these potting mixes, start fertilizing a week or two after transplanting.

For all fertilizer products, follow the directions on the label to avoid overfertilizing. Too much of a good thing is just as problematic as too little. A label recommendation to apply one tablespoon of fertilizer diluted in a gallon of water every two weeks can also be done at half that strength weekly if that's easier to remember to do.

Liquid fertilizers diluted in water and applied when watering are quick to supply nutrients to plants, but need to be reapplied more often. Slow-release (also called timed-release) products are granular, and can typically be applied once in early summer and again about six weeks later.

Best Fertilizer to Use

Petunias are heavy feeders, and do well with a complete fertilizer, meaning one that has all three main nutrients, nitrogen, phosphorus, and potassium. You'll see these represented by three numbers separated by dashes, like 10-10-10. Look for labels where N is not dramatically higher than P and K. Skip products focused on only one nutrient, like 0-5-0, for your petunias.

A balanced fertilizer is one that has equal amounts of NPK, like a 5-5-5. These work well for petunias, and the rest of the garden too. While you may see bloom booster products heavy in phosphorus (the middle number) advertised for flowering plants, petunias still need the other two nutrients in large numbers. A petunia that is short on nitrogen won't bloom well no matter how much phosphorus is given.

While a balanced fertilizer is fine, avoid nitrogen-heavy products like a 10-3-3, which can cause excessive green leafy growth and slow blooming as lots of nitrogen will encourage the plant to focus more on producing foliage.

Organic fertilizers are fine for container plants, but remember they are more of a slow-release product. Dry products like blood meal will take a while to be mineralized into plant-available forms because the microbes required to break them down aren't as plentiful in commercial potting mixes as they are in garden soil. To combat that, add finished compost (to boost those microbe levels) or use a liquid product like fish emulsion that is more immediately usable by plants.

Signs of Fertilizer Problems

Fertilizer can be easy to overdo, building up to toxic levels in the soil. Too much fertilizer can burn roots, reducing their functionality and can eventually kill the plant. Wilting when soil moisture seems adequate, browning brittle leaves, and stunted growth can all be signs of too much fertilizer.

Too few nutrients, or not enough fertilizer, for petunias looks like leggy, spindly growth, pale color, few or no blooms, and slow growth. If the petunias are getting enough moisture and at least 5 hours of direct sunlight per day but look weak, the problem might be low nutrients. A petunia that is getting enough water, sun, and nutrients will grow vigorously, with healthy green foliage and frequent new blooms.

What about Petunias in the Garden?

Petunias grown in garden beds typically have fewer issues with nutrients, because their roots can spread out more to find what they need. However, they still respond well to fertilizer; they just won't typically need it as often to bloom well.

Compost will provide much of what petunias need, but by blooming time, a supplement of a balanced fertilizer will provide a welcome boost. As with containers, apply a granular fertilizer once or twice per summer, or a liquid product with watering at the frequency recommended on the label.