In the Garden: Before using fertilizer, understand what your plants need

I’ve been receiving a lot of questions about fertilizer: Which kinds do I use, how much and how often? I wish these had straightforward answers, but there are a lot of things to take into consideration.

If your plants are growing normally, don’t give them any extra nutrients. I’ve found that if the soil is fertile, plants rarely require anything of us other than regular watering and weeding.

Let’s talk about what those three numbers on fertilizer packages mean and what they do for a plant. The first number represents the amount of nitrogen, which promotes leafy green growth. The middle number is phosphorus, an important nutrient needed for blooming, setting fruit and developing good root systems. The third number represents potassium, which also helps with root growth and moisture retention.

A reader recently told me her bean and tomato plants are looking great but haven’t bloomed or set any fruit. When I asked if she’d been using a fertilizer that is high in nitrogen, it turns out she was. Now that she’s stopped using it, I am hopeful the plants will get back on track.

It’s important to get young seedlings off to a good start by feeding them with a nitrogen fertilizer, but once they’ve been growing in the garden for about three weeks, they should get everything they need from the soil – if it’s warm enough, that is.

Did your plants look pale this spring? That was due to our extended period of chilly temperatures. If the soil is too cold for microorganisms to make nitrogen available to the roots, plants grow slowly and look pathetic. Once the temperatures warm up, they grow by leaps and bounds.

When growing plants in containers filled with sterile potting mix, you need to provide them with nutrients unless the mix already contains some type of fertilizer.

Give ornamental plants such as flowering annuals a slow-release fertilizer at planting time and again in mid-July so they’ll continue blooming until fall.

If you’re growing vegetables in pots, use an organic fertilizer that is appropriate for your crops – such as tomato fertilizer – and follow the label directions. You can tell a product is organic from the “OMRI” certification on the label. This stands for Organic Materials Review Institute, an independent, nonprofit organization that reviews products for use in organic agriculture.

I choose organic fertilizers over synthetic (inorganic) ones because the latter kill soil organisms and provide more nutrients than plants are able to take up. This results in excess nutrients going into our groundwater, which is not a good thing.

There are a few types of plants that benefit from supplemental fertilization. In the Inland Northwest, roses typically bloom profusely during June and July. Once that first flush of bloom has subsided, feed them with a rose fertilizer so they’ll bloom again through the remainder of the growing season.

Plants that require an acidic soil also need a little extra help since the soils in this region aren’t very acidic. Examples include rhododendrons, azaleas and blueberries. Look for acid fertilizers and apply as directed.

This brings me to an important point. We gardeners are often a generous lot when feeding our plants by employing the “more is better” approach. When a fertilizer label states a specific amount to use and how often, follow those label directions.

If your plants look good and are growing well, they rarely need extra food. If they’re really struggling for no apparent reason, consider getting your soil tested.

Contact Susan Mulvihill at susan@susansinthegarden.com. Watch this week’s “Everyone Can Grow a Garden” video at youtube.com/susansinthegarden.