Lilies are a striking feature in any garden, but their bulbs naturally multiply underground, a process known as naturalization. Over three to five years, this multiplication creates an overcrowded cluster of bulbs competing for finite resources like moisture and nutrients. Thinning, or dividing, these lily bulbs is a necessary practice that maintains the plant’s vigor, ensuring robust health and consistent, spectacular blooms. Separating the offsets and replanting them with proper spacing prevents the decline in flower size and quantity caused by congestion.
Identifying the Need and Optimal Timing for Thinning
Lilies provide clear visual signals when the bulb cluster is too dense and requires division. A primary indicator is a noticeable reduction in the size and number of flowers produced compared to previous seasons. Stems may also appear weaker, thinner, or shorter than their typical height, signaling the bulbs are struggling for space and energy.
It is best to undertake division during the early fall, after the plant has completed its annual flowering cycle. The foliage must be allowed to yellow and die back naturally, which ensures the bulb has stored maximum energy reserves for the following year. Waiting until the foliage has withered, but before the ground freezes, allows the bulbs to enter a semi-dormant state, minimizing the shock of being moved. While division can be done in very early spring, moving the bulbs during the active growing or blooming season is highly detrimental and can lead to plant loss.
The Process of Lifting and Dividing Lily Bulbs
Before lifting the bulbs, the dead or dying stem should be cut down to approximately six inches above the soil line to provide a handle and mark the plant’s location. The goal is to excavate the entire cluster without causing any physical damage to the delicate, scale-covered bulbs. A garden fork or pitchfork is the preferred tool over a spade, as its tines are less likely to slice through the bulbs.
Carefully insert the fork into the soil about 12 inches away from the stem and gently loosen the earth all around the clump. Once the soil is sufficiently disturbed, use the fork to lever the entire root mass out of the ground. After removing the cluster, lightly shake or rinse away the excess soil to clearly expose the bulbs and the attached offsets.
The cluster will reveal the large, original mother bulbs surrounded by smaller, immature bulblets. These bulblets can be gently twisted or pulled away from the main bulb, separating the cluster into individual planting units. Any remaining dead stem material should be carefully twisted off the top of the bulb, leaving a clean surface.
Lily bulbs must not be allowed to dry out, as they do not possess the protective tunic found on bulbs like tulips. Inspect all bulbs for signs of rot, disease, or insect damage, which appear as soft, discolored, or mushy spots. Trim any excessively long or broken roots with clean shears, leaving only the healthy, firm root structure intact before preparing the bulbs for immediate replanting.
Replanting and Establishing New Lily Clumps
The separated bulbs must be replanted promptly to ensure successful re-establishment. Lilies flourish in a location that receives full sun or light afternoon shade and features soil with excellent drainage. If the native soil is heavy clay, amending the area with organic matter like compost or grit is highly recommended to improve aeration and prevent the bulbs from rotting.
The general rule for planting depth is to bury the bulb two to three times its vertical height, measuring from the base to the tip. For a standard large lily bulb, this often translates to a planting depth of five to six inches. Smaller bulblets should be planted less deeply, perhaps two to three inches down. Deeper planting encourages the stem to produce stabilizing roots and helps anchor the tall, mature plant.
Proper spacing is necessary to delay future overcrowding and ensure each plant has sufficient access to resources. Individual bulbs should be spaced at least eight to twelve inches apart, allowing ample room for the new bulbs to multiply over the next few seasons. After positioning the bulb with the pointed growing tip facing upward, backfill the hole with soil and tamp it down gently to remove large air pockets.
Immediately after planting, the area should be watered thoroughly to settle the soil around the bulbs and initiate root growth. If the division was performed in the fall, a layer of mulch applied over the planting site will help insulate the bulbs from extreme temperature fluctuations over winter. While the larger, mature bulbs are likely to produce flowers the following season, the smaller bulblets may take one to four seasons before they reach flowering size.