Early to mid-June is when gardeners should fertilize annuals, bedding plants, hanging baskets, perennials and vegetables.
The long days and warmer nights have ignited the growth spurt of your plants and any fertilizer now will act like rocket fuel.
But be careful. Not all fertilizers are the same and there is a risk of overfeeding your plants. To keep it simple, remember that your plants cannot read labels and do not mind if you use a fertilizer made for flowers on your tomatoes and a fertilizer with a picture of a tomato on the box can be used on the roses.
What matters is that you read and follow the instructions about how much fertilizer to use and how often.
What is a complete fertilizer?
An all-purpose plant food with the numbers 12-12-12 or 20-20-20 are examples of complete fertilizers that will benefit both flowers and vegetables.
The three numbers stand for the percentage of nitrogen, phosphorus and potassium, or NPK as it is known in the gardening world.
A very simplified explanation of the three number is that the first number is for foliage, the second promotes flowering and fruiting and the third number helps disease resistance and sturdy growth. By law all fertilizers must state the ratio of these three nutrients.
If a fertilizer contains all three it is considered “complete” as NPK are the three most often needed nutrients. So a plant food that says 20-20-20 gives almost double the amount of nutrients as the 12-12-12 food but is also more likely to “burn” or cause leaf damage to plants that are sensitive.
Hanging baskets and summer annuals are plants that love strong fertilizers but they are best diluted with water. These water-soluble plant foods act very quickly and can even be absorbed almost immediately through their leaves if you sprinkle the foliage. Roses are also heavy feeders and respond well to high-concentrate plant foods.
What if the three numbers are not equal?
Fertilizers made for specific purpose may lean toward a higher ratio of phosphorus (listed as K, the middle number) to promote fruit formation or may have a higher first number (N or nitrogen) to promote more foliage — this is why the first number on a bag of lawn food is the highest number — lawns crave nitrogen and this is the nutrient that greens them up and speeds their growth.
A common fertilizer for roses and flowers will have less nitrogen with a ratio of 5-10-10. There are also special fertilizers made for acid lovers such as blueberry plants, rhododendrons and camellias and for citrus fruits. (Side note: In Western Washington, rhodies and azaleas usually don’t need extra fertilizer as our soil is naturally acidic.)
Organic plant foods improve your soil
Organic or natural plant foods are made from once-living organisms and examples are manure, fish emulsions, bone meal, cottonseed and kelp meal. They are not complete fertilizers but you can use a combination of organic fertilizer to get all three (NPK) into the soil.
The advantage is that organic plant foods improve the soil over time. The nutrients are usually released more slowly over time — but not always. Some organics like blood meal release nitrogen quickly and fresh manures can also burn plants by releasing nitrogen too quickly.
Organic fertilizers must be applied properly just like inorganic or chemical plant foods. Compost is not a fertilizer but compost does improve the soil so nutrients will become available over time.
Inorganic/chemical plant foods
These fertilizers can offer precise formulas with consistent results, and they are less bulky and mix easier with water. They are easier to use and often less expensive to buy. They do not improve the soil, however, and after years of use can harm the living creatures in the soil from a buildup of salt. They are great for container gardens that need a lot of nutrients as the potting soil in pots will need to be replaced every few years anyway.
Inorganic plant foods are made from synthetic substances in factories. They can also contain secondary nutrients (such as calcium (Ca) and magnesium (Mg) plus micronutrients or trace elements such as zinc, iron and manganese. This is one reason why they give such quick and impressive results. Read the label to see if these micronutrients have been added if you want maximum growth from a plant food.
Fast or slow release?
In general, natural or organic plant foods release nutrients slowly over time while chemical plant foods dissolve quickly when the soil is watered.
There are slow-release, inorganic plant foods in pelleted or solid stake form that last all season because a coating releases the fertilizer slowly over time.
You may see these as small round capsules sitting on top of the soil when you purchase plants from a nursery. Some potting soils contain these slow-release plant foods already mixed into the planting medium. You can still use liquid plant foods for a burst of growth while you wait for the slow release plant foods to break down.
Some pelleted plant foods need warm soil to release the nutrients, so in our cool climate these fertilizers are dormant when our plants are dormant.
Are you feeding too much? Or too little?
Most trees, shrubs and established perennials in our area do not need additional fertilizer. They grow just fine in our native soil and an organic mulch of bark chips or fallen leaves keeps the soil healthy. Fallen leaves and debris are nature’s way of feeding plants.
Lawns growing in deep, rich soil can also survive without fertilizer if you leave the clippings on the lawn — but most lawns in Western Washington need at least one application of high-nitrogen plant food each spring or fall to keep the turf thick enough to crowd out weeds.
Thin or weed-filled lawns need more fertilizer — a feeding in spring and fall. Most potted plants, hanging baskets and container gardens are not fertilized enough by the home gardener. They do best if fertilized at least one a month during the growing season using a full strength fertilizer or twice a month using a diluted or half strength plant food. For really impressive hanging baskets and annuals feed every time you water with a very diluted water-soluble plant food.
So there you have it — a simple explanation of a very complex science. To sum it all up — summer is here. Feed your flowers and vegetables.
Marianne Binetti has a degree in horticulture from Washington State University and is the author of several books. Reach her at binettigarden.com.