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Soil Needs of Container Flowers
The Right Soils for Flowers
Flowers grow best in containers filled with loose, fast-draining soil mixes that retain moisture and are equal parts sand, silt, clay, and compost.
Because you’ll control every aspect of life for your flowering plants growing in containers, the soil medium you choose should be as close to the ideal as possible, and it should match the soil needs of each flowering species you plant.
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So, of all the components of container gardening, the right potting soil is the most important.
Most plants need soil to hold them securely in place and provide them locations to grow roots, absorb water and nutrients, and keep them upright. The mixes that do these things best are called soil mediums—a fancy way of saying that they contain more than mineral soil.
The better the soil medium, the healthier the root system your plants will have and the better they will grow.
Plant roots require several things of the soil. They need a soil that provides moisture, air, and nutrients. The mixture you choose must provide all of these things.
Many flowering plants in containers grow best in rich, well-drained soil medium mixed with pumice or perlite as a draining agent.
Plants originally from bogs and stream margins grow best in always-moist conditions, so choose soil mediums for them that contain water-retaining compost, organics, and soil polymers.
When wet, the hard, clear granules of these polymers expand into gellike masses that hold water. As the soil dries, the polymers release water and keep the soil moist, extending the time between waterings.
Polymers are good for plants that evolved to fit environments with constant moisture. They’re not a good choice for most other species that adapted to alternating moist-dry cycles.
You’ll find a wide range of soil mediums and so-called potting soils available in bags at your garden center.
Because different suppliers of potting soil use different formulations, always carefully examine the package contents while considering a specific medium for your particular plants’ needs.
Some specialized flowering plants require their own special mixes. These include African violets and other flowering plants originally from the forests, including bromeliads, ferns, orchids, and epiphytes. Cactus often do best in soils that would be unsuitable for most other flowering plants.
Mixtures containing organic water-holding materials such as shredded bark, compost, or peat moss are good general soil mediums for germinating flower seeds and planting transplants into containers.
Such mixtures keep flowers and buds generally moist and prevent bud drop.
Orchids naturally grow on other plants for support with their roots exposed to the air rather than in soil. Plant them in orchid bark instead of traditional potting soil; such mixes comprise a coarse blend of chips, often fir or pine bark. The bark supports the orchids, holding them upright, and allows plenty of air to get to their roots.
In addition to a soil’s water-retention properties, its degree of acidity and alkalinity—soil pH—is important. Most flowering plants do best in slightly acidic soil. A few, however, prefer soil that is even more acidic. For those species, use a prepared soil medium or mix labeled for use with azaleas and camellias.
Finally, keep in mind that, although your soil medium starts out ideally matched for your plants, it will change over time.
You may have a very alkaline water supply—typically found throughout areas of the U.S. such as the desert southwest, the deep south, and Florida. These waters will change your soil’s pH balance over time.
Fertilizers also can cause soil to become more alkaline, especially those made of animal manure or fish emulsions.
If a plant begins to produce stunted, weak growth despite proper care, check the pH of its soil with a test kit or a meter [see: Needs of Flowering Plants].
If you find soil that tests 6.5 pH or higher (or 5.0 or lower) it’s time to repot the plant, providing it entirely new soil.
Fertilizer and Flowers in Containers
Soil fertility is more important for flowers growing in containers than it is for outdoor flower plantings.
Start most container-grown flowering plants off with rich soil containing balanced nitrogen, phosphorus, and potassium—a 5–5–5 formulation, for instance.
Containers quickly lose their initial load of soil nutrients. Water-soluble compounds such as nitrogen dissolves as we irrigate, leaches, and drains away. Other major nutrients chemically bind to soil minerals over time, making them unavailable for your flowering plants.
Keep your container plants healthy, especially after their shoots emerge from the soil. Applying liquid organic 7–7–7 formulations of fertilizer, diluting it to one-half the package-recommended rate each time you water, usually weekly or semi-weekly.
Foliar fertilizers are another option. Spray them directly onto the foliage and stems of growing plants, but avoid spraying fertilizers on flower buds or flowers. These special fertilizers absorb directly through the plant’s foliage, bypassing the container’s soil and plant’s roots.
Stop fertilizing after flowers begin to open, but keep watering regularly so that the container’s soil remains damp. Withhold water once the foliage begins to yellow and wilt, but before it shrivels and dies.
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