I have the red form, it does grow quite fast nothing like as fast as hornwort. It roots like Egeria D roots from the stem growing down. I only have had it for three months, when you chop the top off it sprouts two or three new smaller shoots and the cuttings grow. Yeah the green form is an invasive weed, but it is a beautiful nonetheless. A book I have says that if you trim it too often it, it damages the plant, not sure what that means. I like the autumn idea, looking forward to the pics. Myaca (not sure of the spelling) now called something else looks conifer shaped. Its light green in colour, not fast growing in my tank much smaller than the myriophyllum its swings in the current so maybe not what you’re looking for.