Cultivated for thousands of years, with countless varieties, versatile grape vines (Vitis spp.) are grown for their fruit, leaves and often simply as ornamentation. Grapes require no fertilization if soil is at least moderately fertile. (The Fruit Gardeners Bible, pg 109, "If your soils are reasonable fertile you may not need to add fertilizer). If fertilizer is needed, apply low-concentration fertilizers with a balanced mineral content to raise grape vines that produce lush vegetation and juicy, sweet grapes. Grapes grow well, depending on variety, in U.S. Department of Agriculture plant hardiness zones 3 through 7.
When to Fertilize
When fertilization is needed, fertilize prior to mid-summer. Late fertilization can lead to late-season growth when the plant should be heading into a dormant stage. Late growth may weaken the grape vine's ability to survive the winter and affect its growth the following year. Grapes planted in balanced, healthy soil may not require fertilization.
To determine the quality of your soil have it tested at a local garden center. Garden centers test the ratio of acidity and alkalinity in soils (their pH). Grapes grow best in soils with a pH balance of 5.5-7.0 ( The Fruit Gardeners Bible, pg 108).
Selecting a Fertilizer
Commercial fertilizers are a combination of nitrogen (N), phosphorus (P), potassium (K) and other less-critical supporting micro-nutrients and fillers. A commercial label will indicate the N-P-K ratio, such as 24-8-16. This particular N-P-K ratio indicates that the product is 24 percent nitrogen, 8 percent phosphorus, 16 percent potassium and 52 percent micro-nutrients and fillers, by volume.
Grapes prefer well-balanced formulas with relatively low percentages. Look for a label with an equal amount of each major mineral in percentages from 10 to 18 percent. For example, look for a label reading 10-10-10 or 15-15-15.
Applying the Fertilizer
Fertilizers come in liquid and dry forms. If your chosen fertilizer does not recommend exactly how much to apply for grapes, err on the side of caution and use half the recommended amount for other fruit-bearing plants. (The Fruit Gardener's Bible, pg 109).
- Manufacturer Lilly Millers's Ultragreen is an appropriate dry fertilizer for grapes. Apply 1/3 a cup of Ultragreen for every 25 feet of grape vines. After scattering fertilizer, gently rake it into soil and water thoroughly (see label: http://www.lillymiller.com/labels/Ultragreen/UG_All-Purpose_10lb.pdf). Follow manufacturer's specific directions for other types of fertilizers.
- Avoid getting liquid and dry fertilizers on vines and apply it approximately 18 inches away from the base of the plants.
- Fertilizers containing iron may stain surfaces other than soil; avoid spilling them on concrete, driveways and wood.