Try this with late blooming Asters, returning Hardy Mums, Asteromea, Eupatorium, upright Phlox, Bee Balm, Catmint, and many other vigorous fall bloomers: Using a sharp garden scissors and Osmocote fertilizer, go around to your perennials that always get too tall before they flower. A good time to do this is a couple of days before 4th of July weekend, when your tidying up around the yard anyway. This timing will give the plants a chance to broaden out and fatten up before they flower.
Shear most plants almost in half, but not down to hard woody stems. Then feed each plant with a good handful of Osmocote fertilizer, about 2-4 oz. worked into the top inch of soil for a large perennial. In a month or two, you will see a noticeable difference in the health and flower production of all the fall perennials that you worked on. As always, remember to water once a week in the hot dry time of the summer- a perennial garden needs about 1 inch of water a week at minimum to be successful. Keep notes from year to year on what you tried, and how well it worked in your garden notebook.
The most important factor in chrysanthemum and aster cultivation is abundant water! The number one cause of poor performance is allowing the plants to wilt repeatedly during the last few "strong sun" days of summer. Even when planted, especially in a dry period, the plants should be well watered every other day for about ten days until they start to root out into the bed. Water twice a week, thereafter.
All of the plants grown by Rainbow Mums contain a long-lasting fertilizer built right into the soil, however, will enjoy one or two "medium" doses of Miracle - Gro when they are in full bloom. Only apply liquid fertilizer to plants that have already been watered. Osmocote¨, a 9 - month fertilizer, is excellent at this time, giving a startup burst the following spring, before you will be out in your garden.
Recent research indicates that both hardy mums and asters come back in a strong relationship to how early they are planted. Leaving the tops on after they are dead to catch leaves and create a protective microclimate close to the ground, is now considered the best method of over-wintering. Cutting back plants in the fall, no matter how late, especially within 6 in. of the ground, may actually be harmful, and spoil their chances for next year.
Your course of action the following spring, is to look for signs of growth at or near the base. The dead stems can now be cut away. In late May, the mums or asters are getting tall. Fertilize them with Osmocote, and cut them to half of the height they have made. This is all the pinching back your mums should need. If you desire later blooms for after the 20th of September, pinch only the tips of your mums back, but only about 2 inches, on July 15th.
Planting instructions for all Perennials and Grasses: Rather than to periodically rip out and prepare whole sections of the perennial boarder, save time and energy by doing a simple routine every time you plant or divide. Make a "planting kit", consisting of a small bag of lime, 5-10-5 garden fertilizer, Osmocote, a shovel, and a bucket of real horse or cow manure, not the bagged stuff. Usually, this can be obtained free from local horse owners. Dig a hole, loosen the soil, and mix (right in the hole) 1 or 2 shovels of manure, a large handful of lime for each shovel of manure, a small handful of 5-10-5 granules, and 2 tablespoons of Osmocote per gallon of pot size planted. Plant at the correct height, and firm the soil, standing on it if the plant is 2 gal. or larger. Make a well: A ridge or crater around every new plant, to catch and funnel water to the roots. It is often overlooked as the most important step in any new planting. It brings 10 times as much water to the roots as flat ground. Water in all new plants well, even in the pouring rain. New perennials and Grasses, planted in the hottest part of the summer, must be watered 3 times a week until they are established, usually about 2 weeks. Annual Feeding: Get on a routine of feeding your perennials in mid-summer with 8-9 mo. Osmocote (any Osmocote will do). A few tablespoons at the root area (more for very big plants) will nourish them for the entire year. This practice will end the problem of underfed perennials that seem to be smaller each year until they finally disappear altogether. Now you are a Master Gardener!
A reminder: if you are planting mums only, they do not need any special preparation or soil. Just dig a hole, loosen the soil a little deeper than the pot size, plant the mum, add two tablespoons of Osmocote near to the surface, and make a well to catch the water. Don't make planting the mums too hard! Water is the main ingredient for success and long bloom time.