October is the ideal month for planting spring flowering bulbs including daffodils and tulips as well as scilla, grape hyacinth and snow drops. These bulbs vary greatly in size, even within the same species so its always a question how deep to plant them.
Because some of these bulbs are up to two inches in diameter, instructions that say to plant them four inches deep, for example, are confusing. Does that mean the hole is four inches deep or the bulb is buried under four inches of soil?
The rule of thumb I find the easiest to follow is to always try to cover the bulb with twice as much soil as the bulb is high. When in doubt, go a little deeper. A tulip bulb that is two inches tall needs four inches of soil over it, so it would be put in a hole six inches deep in order to have room for the two-inch bulb and four-inch column of soil. A little one inch crocus would go in a hole three inches deep so it can be covered with two inches of soil.
I am often asked about adding fertilizer to each bulb planting hole. Remember that spring bulbs come all ready to go. The time they need fertilizer is next spring after they have finished flowering, in order to store enough food for the following years bloom.
Rather than adding a spoonful of fertilizer or bone meal to each hole, you have a couple of options. Either dig up the entire planting area so you can work organic matter throughout the future rooting zone or if the bulbs are going in between established perennials in a flower bed, treat them the same way you treat the rest of this garden for fertilizer. Add the fertilizer just to the planting hole doesnt put the food where its really needed since the roots are going to quickly grow out beyond that space anyway. Its not harmful to add fertilizer to each planting hole (as long as you dont use too much) but its not that effective.