Good morning Susan! Yes, pots require a different method of fertilizing than in-ground roses. The soil in pots suffers greater temperature swings than the ground, both colder and hotter and that puts a great stress on the plants at the extremes. Because it remains warmer during warmer weather, it can also mean the plants grow more quickly and vigorously because warmer soil not only has greater soil bacteria action, but also warmer sap pushes growth in the plant.
If you've ever grown standards, you may have noticed how the warm sap in the trunk can stimulate top growth faster than the same cultivar may produce on a traditional bush. The same happens in a pot. You do want to shade them enough to prevent direct, intense sun from overheating the pots to prevent the roots close to the exposure from literally cooking.
Pots also suffer from greater water stress for obvious reasons. Combine that increase in heat and water stress and you can imagine what some of the more serious fertilizing issues for pots are.
Organics work wonderfully in pots precisely because of the greater soil temperatures. It gets warm enough to get the bacteria cooking, even in areas where the soil temperatures remain on the cusp, or lower, for maximum effect. The size and type of pot affect fertilizing and watering greatly. I guess you're using plastic pots? Black plastic nursery cans will actually work better for your use than terra cotta or ceramic. Yes, black plastic heats up when the sun shines on it, but it begins releasing that heat once the heat source is removed. Terra cotta and ceramic retain and radiate the heat for a long time. Both materials are used to manufacture cookware because of their heat retention and radiation. Long after the sun moves from them, touch them and feel it for yourself.
Of course, smaller pots heat up much faster than larger ones, just like small pools and small sauce pans do. There is a smaller soil ball inside and that heats up more thoroughly than larger ones do. All of these factors will vary the type and amount of fertilizer you can safely use.
Time release types often give a range of time they are effective. Heat and quantity of water are usually what causes that variance. Hotter conditions cause the timing agent to break down faster, releasing more of the salts they contain. The instructions should give the quantity needed based upon the size of the pot. In most circumstances, that is safe to follow. If the weather is significantly hotter, using a smaller amount is generally safer because the potential for burning the plant in extreme conditions is lower. You can always add more later, but it's nearly impossible to remove it once it has begun releasing those salts.
More water also dissolves the coatings faster, releasing the salts faster and flushing more of the nitrogen, which follows water, out the drain holes.
Pelletized, water soluble types can be used with increased care. Most often, their instructions for use in ground state they are effective for a month. In hotter pots with increased water than would normally be used in the ground, they break down much faster and release their salt contents quickly.
You can use water soluble types safely, but they will require more frequent applications and should be used at reduced strengths. All inorganic fertilizers should most safely only be used in pots after they have been thoroughly watered and the plants absorb the moisture.
Nature requires things to be in balance. The water inside the plant becomes the same concentration of the nutrients as that of the soil. The salt form of the nutrient is what the plant absorbs. Organics are "digested" by the soil bacteria and fungi in to their salt forms. Inorganics begin as salts so they can burn more easily, but they are also immediately available for the plant's use, providing a much faster, more often, larger bang for the buck.
The only way for the salts to get into the plant is for water to be removed from the plant into the soil and replaced in the plant by the salts. If the plant is stressed for water, applying any kind of salt, whether it be a fertilizer or "tonic" can pull out enough water and replace it with enough salt to 'burn' the plant. Depending on how stressed the plant was initially and how much salt was put into it, it may just look awful for a while, or it can down right kill it.
If you want the fastest, greatest bang for your buck and time, begin with a good, organic filled potting soil. Once the plants begin to show growth, indicating they are settled in and actively growing, water them well, perhaps the night before you intend to fertilize, or the morning of the afternoon you want to do it. Waiting a few hours will allow the plant to take up all the water it requires. Once the plant is turgid, full of water, applying the fertilizer will pull out water and replace it with the salts without damaging the plant. Using a weaker solution of water soluble type (doesn't matter if it's Miracle Grow, Peters or whomever) of the correct guaranteed analysis, will give the plant a fast feeding. Using smaller doses more frequently provides a more continuous feeding and gets them growing steadily.
Nitrogen is required, along with proper temperature and soil moisture, for the bacteria to begin digesting the organics. If there is no source of nitrogen outside of the plant once they begin, they rob it from the plant to start the process. Often, you can see the plant appear more chlorotic after their application. Once things get working and they provide more nitrogen than they are requiring, things balance out and the plants appear "nourished", alleviating the chlorosis. That's why organics used to correct chlorosis work so slowly. Feeding with an inorganic source, after properly watering to prevent burn, corrects the problem much more quickly.
With that said, if you want faster results, use the inorganic water soluble types per the label directions, or if it's extremely hot, dry and windy, a little less than they suggest. As with the pellet types, you can always add more, but trying to fix a burned plant can be nearly impossible. And, it's far easier to flush too much dissolved salt from a soil ball than it is soggy, partially broken down pellets. You have a great deal more control over the concentration of salts in the soil using the weaker water soluble solution. And, the available nitrogen in that solution will begin feeding the soil bacteria so it won't need to rob it from the plant to begin working.
Of course, the choices are yours to make. Some consider MG "Satan's spawn", and that's their right. If organics are your only choice, that's fine, too. But, now you understand why they work as they do and why so many suggest using inorganics to correct the problems which concern you. They work faster than organics, and if used correctly, will not damage plants nor destroy the soil from salt build up. Proper drainage and water amounts and frequency are necessary.
Does that help? Kim
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Mme Isaac Pereire
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