Recommendations for planting bulbs using bulb augers or planters bother me. Most instructions advise planting large bulbs six inches deep and small bulbs and corms three inches deep.
If you want your bulbs, especially tulip bulbs, to flourish year after year, ignore those recommendations and take the shovel to the garden — as well as a bag or two of compost and some agricultural-grade limestone.
Planting bulbs early in the fall enables them to grow an abundance of roots while the ground is still warm, before their cooling process begins. Cooling occurs naturally step-by-step when bulbs are planted in early fall and results in large flowers and long-lasting bulbs. The flower buds in the bulbs do not fully develop until the bulbs have cooled.
If you delay planting the bulbs until the ground is already cold, they will not be able to generate adequate roots, the flowers in the spring will be smaller and the bulbs will last only one or two years. Florists purchase pre-cooled bulbs for forcing with the understanding that the bulbs will be planted and forced into flower, then discarded.
The topsoil layer is no more than six inches deep, down to three if you are living in a house less than 30 years old. This means that if you use a bulb auger or bulb planter, you are placing the base of that bulb on sub-soil that is most likely compacted and deficient in essential plant nutrients. Spring-flowering bulbs planted under those conditions generally last a year and decline rapidly. The roots of bulbs have the same nutrient needs as do all other plants in the landscape.
To do it right, use a shovel to double-dig the area to be planted with bulbs. Double-digging means digging down the full depth of the shovel, about six inches, and removing that layer of soil. Then dig again in the same area and remove another six inches of soil.
If you are planting the bulbs in a bed, remove the top six inches of soil, pile it to one side, then spade the sub-soil to a depth of six inches. Since most of the sub-soils in Southern Maryland are quite acidic, spread a cup of agricultural limestone per square yard of area and cover with two inches of compost. Thoroughly mix the sub-soil with the lime and compost by either spading or with a rototiller.
If you are planting the bulbs in groups, dig 12- to 18-inch-diameter holes 12 inches deep and blend the sub-soil with equal amounts of compost by volume and one to two tablespoons of agricultural limestone. Place four to six inches of amended soil in the bottom of each hole.
If you want your tulips to last for several years, plant the bulbs so that the top of each will be eight inches below the surface. Bulbs planted this deep are less likely to form daughter bulbs, because of a lower oxygen level, and the foliage will remain longer after the flowers have wilted, thus replenishing the bulb with adequate levels of food for next year’s flowers.
The growth of daughter bulbs from the mother bulb is based on the oxygen levels in the soil. The deeper the mother bulbs are planted, the fewer daughter bulbs produced. Also the deeper you penetrate, the cooler the soil temperature. Planting deep promotes the growth of large bulbs, and flower size is directly related to bulb size.
Hyacinths, daffodils and narcissus will perform well when planted at a depth of six inches. However, for maximum growth and flowering they too should be planted in a bed of well-prepared soil.