Tulip trials at Cornell University
For best results, look for ‘naturalizing’ bulbs
Last June, I joined a group of garden communicators for a behind-thescenes tour of the greenhouses and trial gardens at Cornell University in Ithaca, N.Y.
It seemed it apropos to tour such a well-respected institution and learn about some of their ongoing research on the very first day of summer. After visiting the greenhouses and learning about their trials with tomatoes, lettuce and strawberries under glass and with different kinds of artificial light, we settled in a deliciously airconditioned lecture hall. Dr. Bill Miller, professor of horticulture and research director at Cornell, and lecturer extraordinaire, posed a simple question.
“How deep should you plant tulips?”
Suddenly were are all back in school. “Eight inches deep,” was a quick response. Another offered, “Double the height of the bulb.”
“What do the bulb packages recommend?” He pressed on.“What does a Google search turn up?” “Plant tulips eight inches (20 cm) deep.”
We nodded in agreement, but these were loaded questions.
“From a gardener’s perspective, digging an eight inch deep hole is not easy. Depending on the soil, digging an eight inch deep hole for 15 or 20 bulbs is a major task. Many, perhaps most, gardeners will not do this. Can we make bulb planting easier?” he wondered.
Bill has led a series of experiments to see if there’s a way to plant tulips that was not only easier but also might extend their long-term potential in the garden.
“Six experiments, installed in six different years, each maintained for three years to follow the peren nialization (return and persistent flowering) of the tulips.”
Tulips were planted at different depths from one to eight inches deep, some were mulched, others left bare, and the growth and flowering documented. For this study, planting depth means the depth from the soil surface to the bottom of the bulb. In all, nearly 600 plots made up the data of this study, and the results were convincing.
It turns out that deep planting, 20 cm deep, as recommended, is not ideal for tulips to perennialize. The research shows that the best results were obtained when the ground is tilled, and bulbs are planted no deeper than three inches (7.5 cm) deep and then covered with two to three inches (5 to 7 cm) of mulch. Shallower planting is less work for the gardener and the flowers were larger and more plentiful in their second and third years.
He lit up the screen with images of tulips, illustrating the dramatic differences when the bulbs were planted side by side. We were all impressed.
It all sounds pretty simple, preparing a good base is the first step. Use a shovel to prepare holes to depth of 7.5 cm, work a slow release bulb food into the soil, place the bulbs, cover and then mulch with two to four inches (5 to 10 cm) of aged mulch or wellrotted compost. That’s it.
It is interesting to note that other than giving them a good base, the bulbs in the trial were not pampered. Keep in mind that tulips are native to snow-covered gritty soils, moist cool springs and dry and generally hot summers, we were reminded.
After the plants flowered in the spring, they were left to grow on their own. The flowers were allowed to wilt on the plant, and any seed pods were left in place. The leaves were not removed until they were totally dried. No irrigation was used. In the fall, the beds were topped with additional mulch to maintain the original depth (5 to 10 cm). No additional fertilizer was applied.
After seeing the results of this study, I’m ready to give this planting method a shot in my garden. I usually plant tulips in large swaths, rather than digging individual holes for each bulb. As Bill mentioned in his lecture, it’s hard work to dig 15 or 20 (or more) eight-inch deep holes, especially in our heavy clay soil. This year, I’ll give the planting areas a good base and then set the bulbs in 7.5 cm of soil and top them with 5 to 10 cm of mulch. (Read the full report at www.flowerbulbs.cornell.edu/
newsletter.)
Seek out bulbs with a good reputation for naturalizing for this project. The experiments at Cornell used Negrita (purple) and Parade (red) tulips, both known for being good return performers.
When you are shopping for bulbs, look for the words ‘good for naturalizing’ on the label. Some of the most reliable return performers in my home garden include: Shirley (white with purple streaks) paired with Negrita (purple), and early blooming Fosteriana tulips such as Orange Emperor. I’ve also had good results with botanical tulips such as Dasystemon Tarda (yellow and white), Kaufmanniana Waterlily (lemon yellow) and Saxatilis (violet with yellow centre). The botanical tulips often have narrower leaves that are easier to conceal as the foliage begins to fade.
Just to make life interesting, I also attended a lecture by bulb expert Jacqueline van der Kloet this summer. She’s on the traditional side of the fence when it comes to bulb depth, but she’s got some very innovative design ideas — next week we’ll take a closer look at her work.