Spring Bulbs in a Rock Garden

Planning

• Sketch out your garden using colored pencils – or use an app -- so that you can more easily visualize what the garden will look like once the bulbs are all in bloom. This can be tricky with a rock garden because you must take things like soil depth into account if you're working with rocks inherent to the plot of land, rather than ones that have been trucked in and set in place. Larger bulbs like tulips or daffodils need deeper soil, while shallow nooks and crannies may be able to support grape hyacinths or crocus. Think in three dimensions -- it makes no sense to plant taller flowers in the back and shorter ones in front if your rock garden slopes up at the rear. Keep shade, watering and pH balance in mind when planting different bulb varieties together. Keep blooming times in mind because not all spring bulbs blossom at the same time. With careful planning, you can have flowers constantly blooming in shifts. For example, aconite and snowdrops bloom in early spring. Mid-spring brings chionodoxas, crocus and scillas to life and late spring gives you daffodils, hyacinths and tulips.

Soil Preparation

• Aside from cammassia, which thrives in swampy, wet conditions, most spring bulbs require good drainage. They also need between 8 and 10 hours of full sun to produce their most spectacular blooms, though they can thrive in as little as 5 to 6 hours of sunlight per day. Prepare the soil in the summer so that you can plant your spring bulbs in the fall. Loosen the soil to a depth of between 8 and 12 inches, if there is that much soil available. If there's not, loosen it as much as you can, because any aeration will help the shoots make their way to the surface. It is sometimes possible to add soil to the crevices between rocks, but not always. Before doing so, make sure that it will not be washed away by the first heavy rain. Adjust the pH of the soil where possible by adding lime to increase it, or fertilizer to add the nutrients that will nourish your bulbs. Don’t use fresh manure as a fertilizer because it can harm your bulbs.

Planting

• When to plant your bulbs depends on what climate zone you’re in, so check the USDA plant hardiness zone map to find your zone. Plant spring bulbs in zones 4 and 5 no later than early October. Zone 6 should have your bulbs in by the middle of October, and zone 7 by the beginning of November. Zones 8 to 10 are usually temperate enough to plant spring bulbs almost any time. Check your bulbs to see which end is up. The top is usually more pointed. Don’t panic if you can’t tell for certain which is the top because most stems will find their way to the surface even if you plant them upside down. Bulbs that are smaller than 1 inch in diameter should be planted 2 to 3 inches below the surface of the soil while larger bulbs should be set in 5 inches down. Smaller bulbs do best when clustered, while larger ones should be planted 4 to 6 inches apart. Water the bulbs without soaking them and cover them with a blanket of stone mulch to protect them from cold and critters. Stone mulch is an excellent choice for rock gardens because it does not need to be removed before blooming season starts, and it complements the overall aesthetic of the rocks and blossoms.

Dormancy

• During the summer, many spring bulbs become dormant. Some bulbs, such as species tulips, last for years in the rock garden. Others, such as daffodils, can get tired and crowded. If your bulbs are several seasons old, you can dig them up to refresh them. One method is to divide them from the clumps they have formed and replant them, spacing them several inches apart. Another way is to dig them after the foliage has died back, clean the excess soil off the bulbs, store them in a cool, dry place with lots of air circulation, and replant them in the fall. This should be done with crocus and daffodils at least every five years, so when planting those flowers in your rock garden, make sure the area is easily accessible for digging them back up.