In the Garden:
Northern California Coastal & Inland Valleys
Tuberous begonias can be encouraged to bloom with the right fertilizer.
I have received many questions over the years about fertilizer, which is understandable considering there are hundreds, if not thousands, of different kinds of fertilizer on the shelves. The type you select depends on what you are trying to grow and what time of year you are applying it.
There are always three numbers on the front of every bag or bottle of inorganic fertilizer, and some organic ones as well, but I'll talk more about the latter in a minute. The numbers are always in the same order and stand for N-P-K, or nitrogen, phosphorus, and potassium. Each of these elements is necessary for plant growth and development. A bag of fertilizer on the shelf at your local nursery might carry these numbers: 22-14-14. Even though the numbers are not all the same, this type of fertilizer is considered "balanced" because the product contains all three elements.
The first number indicates nitrogen, which is necessary for growing lush, green foliage. A fertilizer high in nitrogen is a good choice for lawns, turf grass, and bamboo, since none of these plants are intended to bloom. A common ratio of N-P-K for lawn fertilizer is 36-0-0. Fertilizers high in nitrogen are also good choices for young annuals, since you want them to grow to full size as quickly as possible.
Keep in mind that products high in nitrogen can burn foliage, and plants should be rinsed with fresh water directly after fertilizing. Another warning: Once vegetable plants, such as chilis and tomatoes, begin blooming, cut back on fertilizers high in nitrogen because they can actually cause bud drop!
The second number refers to phosphorus, which is necessary for flower and bud development, as well as root growth. Blooming plants should be switched from a high-nitrogen fertilizer when they are ready to set buds. For example, a tuberous begonia could be fertilized with 22-14-14 at the beginning of the season while the foliage is developing from the tuber, then switched to 0-10-10 when the leaves are fully developed to promote blooming.
Superphosphate is a type of fertilizer commonly added at planting time to spring-blooming bulbs so they develop a magnificent root system.
The third number refers to potassium, which is necessary for root growth and development. Potassium is used especially on root crops such as carrots, beets, and potatoes, but also on young trees in the fall to encourage root growth during the winter months.
Using organic fertilizers, such as liquid fish or cottonseed meal, will not burn foliage or cause bud drop, however they do work more slowly. Do not expect the quick results common with chemical fertilizers. Although organic products work at a slower pace, they have the benefit of improving soil texture and lasting over a longer period of time.
There are fertilizers targeted for specific plant groups, such as citrus and rhododendrons. I recommend using these products because they are formulated with a particular plant in mind. Just make sure to follow the directions carefully on whatever products you use.
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