Fall Bulb Planting

Selection

The earlier bulbs and corms are purchased in the fall, the better the selection. Select the largest bulbs of a variety, because there is a direct correlation between bulb and flower size.

Generally, a gardener selects the site before purchasing bulbs. This site usually is conspicuously located to show the flowers off to their best advantage. If the bulbs will remain in this location for more than one year, they need adequate sunlight to regenerate strong bulbs. A southern exposure, especially when close to the foundation, induces early emergence that may result in freezing injury. Assure drainage so that the bulbs will not become waterlogged.

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Planting and Growing Fall Bulbs

September and October are the best months for planting bulbs. This will allow ample time for the bulbs to become well rooted before the ground freezes. Bulbs planted after October may not have time to root adequately and therefore may not flower uniformly in the spring.  Consider mulching the bed to maintain a more consistent soil temperature.

bulb-chart-csuPlant the bulbs at a depth consistent with the level indicated on the planting chart. As a general rule, this depth is four times the height of the bulb between the soil surface and the tip of the bulb. Make sure to plant the bulbs with the growing tip up.

After the ground freezes, cover the bed with a 3-inch mulch to prevent alternate freezing and thawing that breaks roots and damages bulbs.  This mulch may be removed in April before the shoots emerge, or left in place if the shoots can penetrate it easily.

Remove flowers as soon as they wither. The developing seed will divert food produced by the foliage that otherwise would be used to produce more vigorous bulbs. Apply fertilizer before the foliage withers. After the foliage has withered completely, the bulb is dormant.

It is possible to interplant annuals among the withering bulb tops. However, do not remove the bulb tops until they are dead. The annuals grow faster and fill in the bed sooner with a fertilizer application.

Soil Preparation
  • Bulbs are planted deeper than seed; therefore, soil preparation methods differ from those used elsewhere in the garden.
  • According to the bulb-planting chart (Figure 1), all bulbs root below 4 inches in depth. For fertilizer to be effective, it must be present in the vicinity of the roots.
  • Excavate the bed to the bulb’s planting depth. Apply the fertilizer and soil amendments at this level, and spade or rototill the soil to a depth of 3 or 4 inches. Take care to not compact the soil beneath the bulbs, which will result in a hard pan with poor drainage. This will promote water logging.
  • Aeration is the most important aspect of soil preparation. Before the soil is shoveled back into the bed, mix it with some type of organic matter. Space the bulbs as desired, refill the bed and water to settle the soil around the bulbs.
  • The flower bud and the food necessary to produce the flower are present inside a bulb when it is planted. Fertilizer is applied to make larger bulbs the following year.
  • Apply phosphorus fertilizer at planting time so it is available to the roots, because it does not translocate in the soil. Bone meal or Triple Super Phosphate may be used for this purpose.
  • To improve the texture of the soil, add compost using up to one-third of the volume of soil removed from the bed as described. Soil amended in this way offers less resistance to the shoot as it emerges and provides better aeration and drainage for root growth. Soils that are high in clay should be heavily amended.

by S.E. Newman ©Colorado State University Extension. 6/97. Reviewed 10/09. www.ext.colostate.edu