To my great surprise Rudbeckia has already germinated! This is what some bottom heat can do. I sowed Rudbeckia so early because it may take one to four months for seeds to germinate. At least so is suggested on the seed sachet. But, being placed on the radiator and having enough light (the days were sunny) the germination was rather quick. It took them only a week to grow three centimeters high and form a pair of leaves. They will be transplanted into individual pots when they develop the next pair of leaves and became large enough to handle. I moved them away from the radiator and placed in a cooler conditions (on the room temperature).
I am going to try tapping the seedlings. They say the seedlings (not only Rudbeckia, but all of them) grow stronger if you lightly tap the top of them with fingers every day.