I grow all of my annual flowers in big pots on my back patio. At last count, there were 10 and replacing the potting mix each year would bankrupt me. Instead of buying new potting mix each spring, I refresh and reuse the old.
I start by digging out the dead roots and stems from last year, if I didn't remove them in the fall. Potting mixes tend to compact throughout the course of the growing season, so I use a trowel and garden knife to break apart residual roots and loosen the mix to a depth of at least 8 inches. Along with loosening the mix each spring, I also add some controlled-release fertilizer and work it into those top eight inches.
After refreshing the mix, I add some new potting mix if the level in the pot has declined because of decomposition or from removing the old plant roots. After several years, I may replace the old mix in the top half of the pot with new because it is not draining well due to the break down of organic matter over time. When I remove old mix, I don't throw it away. Instead, I mix it into my sandy garden soil.
I recommend investing in a quality potting mix when starting a new container. I prefer a mix that consists of peat moss or coconut coir, perlite or pumice, earthworm castings and some compost. I also like the ones that contain controlled-release fertilizer that the label indicates will last for several months.
I mentioned earlier adding fertilizer to potting mix that is being reused. This is necessary because last year's plants probably used up most of the available nutrients and whatever they did not use was likely lost through leaching with the frequent watering necessitated by hot weather. The addition of fertilizer to reused potting mix is important for the good growth of the annuals, flower or vegetables, planted in containers. Just imagine the fertilizer needs of a vigorous growing sweet potato vine, trailing petunia or tomato vine.
I prefer controlled-release or "time-release" fertilizers. They are more expensive than traditional water soluble granular fertilizers, but I like the convenience of not needing to reapply them frequently. When I select a controlled-release fertilizer, I look for one that is a balanced fertilizer, one that contains nitrogen (N), phosphorus (P) and potassium (K). The percentage of these by total weight is indicated somewhere on the product label in that order: N, P and K. Because the amount of nutrients vary with types and brands of fertilizer, I follow the recommendation on the label.
Product labels also indicate the length of time that the nutrients should last. However, because hot summer and fall weather in our region dictates frequent watering, it may not last that long. Consider applying the same fertilizer again in midsummer. If not, you can use a water soluble liquid or crystallized fertilizer if the plants are showing signs of nutrient deficiency, such as yellowing leaves or poor growth.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.