Farm Horizons, June 2012

Are you fertilizing perennials too much?

By Christine Schlueter, McLeod CountyMaster Gardener

Most perennials do not need a lot of fertilizing, especially if the soil is healthy and rich and was prepared well at planting time. A spring top-dressing of compost will be just the thing they need. Also, adding a onetime granular application fertilizer in spring should be enough to last all season.

The perennials that do best without any fertilizer include butterfly weed, false indigo, asters, pinks, rock roses, sea holly, bee balm, speedwell, coneflowers, rudbeckia, and all ornamental grasses. As always, there are some exceptions – mums, daylilies, daisies, and hibiscus – which should be fed every 45 to 60 days from March to September.

Many over-fertilized perennials will produce excessive, soft growth and produce very few flowers. Many times, they will not open up if over-fertilized.

As a general rule, unless a soil test indicates otherwise, perennials can benefit from 1 pound of nitrogen per 1,000 square feet. Granular fertilizers with a formulation of 12-12-12, 10-10-10, or 5-10-5 should be adequate. Most really do well with just one application of a high phosphorus fertilizer (5-10-5) by just scratching it around each of the plants.

Personally, I have a wait-and-see approach. If about two or three weeks after planting a perennial, I have not seen it revitalize, I add a very low application of fertilizer or just dig in some homemade compost, and that usually does the trick.

The main object for any garden is to improve the soil and its organic content, so the nutrients are readily available for plants.

It is recommended that weeds be kept to a minimum for the ultimate in perennial performance. Gently pull out, and weed regularly.

You can use a very selective and directed application of glyophosphate (found in Round Up). Mix the material as directed in a small container and use a foam paint brush to paint the weeds with the herbicide. This method will reduce the drifting of the herbicide to other plants you want to keep.

Most perennials prefer a 6.5 pH, which can be determined by a soil test. Of course, some do prefer a more acidic or alkaline soil.

If you are finding that some are not performing the way you think they should be, then it is time for a soil test.

You may not have to add any fertilizer for two to three years, and then I would recommend fertilizing the entire bed and working it in diligently to finish it in one season. Using the minimum amount of fertilizer produces a stockier, tougher plant than one that is heavily-fed. The heavily-fed plant may grow faster, but the growth will be longer and perhaps lanky, which makes them much more prone to breakage.

If your plants look stressed during the growing season, or if you see disease or insect damage, feed your plants with a quick-release organic fertilizer, such as a blend of seaweed and fish emulsion. That might help to perk them up. Give them about 10 days to see if it is really working.

All plants die eventually – some sooner than others – no matter what you do. If a plant is struggling, the first thing to do is try it in a different spot. If it still does not recover, then it is time for the compost pile; or give it to someone who might have the perfect growing conditions for the plant.

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