Collard greens are part of the "leafy greens" category of vegetable crops, along with mustard greens, kale, bok choi, turnip green and others, since the leaf itself is the part of the plant that is eaten. If you live in a climate that is favorable to collard green growth, growing collards at home can save you lots of money over purchasing them from the grocery store as needed. Just as is the case when growing many other types of plants at home, you may need to use fertilizers to supplement nutrients that the collards cannot get from the soil. Knowing what type of fertilizer and how to use it is one of the keys to growing healthy collard greens.
Collard Nutrient Needs
Collards are considered "medium feeders," meaning that they have moderate nutritional needs but that growers often still need to use fertilizer to ensure collard green health. Collards require more nitrogen than any other growth nutrient. Have your soil fertility tested by a local university extension office to determine the concentrations of nitrogen and other growth nutrients in your soil. If your soil is seriously deficient in nutrients, adding manure, compost or garden soil at a rate of 2 lbs. per 100 square feet can bring the soil to proper fertility.
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Once planted, collard greens have marginal fertilizer needs. A "side-dress" fertilizer application is recommended when the plants are about 1/3 of the way to full grown. Use a high-nitrogen fertilizer, such as nitrate of soda (15-0-0) or calcium nitrate (16-0-0), or a garden fertilizer with high concentrations of nitrogen and less phosphorus, such as 27-3-3, 24-0-15 or similar formulation. If the collards are light green or new growth is stunted, and if the discoloration and stunting are not the result of insect damage, increase fertilizer applications, since the dark green color of collards is primarily a result of the amount of nitrogen that the collards can take up.
The best way to make sure that fertilizers are used to their maximum effectiveness is to follow the printed application instructions exactly and to customize your fertilizer applications to the unique fertility conditions of your soil as indicated by soil fertility tests. Also, avoid purchasing any fertilizers that contain herbicides, even if weeds are a common problem for collard greens in your area. Avoid constant use of fertilizers with a high concentration of phosphorus, as phosphorus buildup in soil can become a major source of water and soil pollution.
Don't make the mistake, as some gardeners do, of conflating fertilizer and plant food. Some gardeners haphazardly add more fertilizer than is needed by their plants, erroneously thinking that overfertilization will simply produce larger, more productive plants. Not only is this not the case, but overfertilization can actually cause more health problems for your collard green plants than if you had not fertilized at all. Use only the minimum amount of fertilizer needed to bring soil conditions closer to the ideal for healthy collard green growth.