I’m in Zone 5. Flower garden is in full sun. The soil was brought in in two distinct batches to make the flower bed. It is all quite clayey, one half worse than the other. It’s awful stuff! Tends to be wet in winter and spring, and turn to concrete in summer. I’ve been adding as much compost as I can – it just seems to disappear. My problem is with perennial flower stems not standing up. They fall over from the base. The stem itself seems normal. The base isn’t chewed or broken, it just falls over. Is there something lacking in the soil?
Thank you for your question. From your description it is hard to know what is happening to cause your plants to fall over. It might have helped to include a picture of an affected plant. Sometimes stems can be too weak if plants are planted too close together. Another cause could be a combination of rain (adding to the weight of the flower head) and wind causing the stems to break. From your description these are mature plants, well past the seedling stage where some causes of weak stems could be ‘leggy’ seedlings (not enough light), root rot and damping off.
You mention that you are worried about poor soil quality. Potassium is an element that supports many plant functions including photosynthesis, strengthening cell walls and water regulation. It is moderately mobile in the soil and rarely considered deficient even under extremes of pH. Potassium levels are affected by crop removal, rainfall, and the soil texture. Potassium fertilizer is sometimes called potash fertilizer. This is because potassium fertilizers often contain a substance called potash. Potash is a naturally occurring substance that occurs when wood is burned away or can be found in mines and the ocean. To add potassium to your soil you can buy a general fertilizer with high potassium or K values. Fertilizer is often labelled with three numbers – always in the order of nitrogen/phosphorus/potassium – NPK, so high potassium fertilizers will have a larger 3rd number. If you wish to add potassium to your soil at home, you can do so in several ways without having to use potash or other commercial potassium fertilizer. Compost made primarily from food byproducts is an excellent source of potassium. In particular, banana peels are very high in potassium. Wood ash can also be used, but make sure that you apply wood ash only lightly, as too much can burn your plants. Greensand, which is available from most nurseries, will also add potassium to you garden. Because potassium deficiency in plants can be hard to spot through looking at the plant, it’s always a good idea to have your soil tested before adding more potassium.
I have included a link to a Fact Sheet on Soil Fertility and Fertilizers from the Missouri Botanical Garden.