Introduction to Forcing Flower Bulbs in Soil
A Hensley's Seed Co. guide
Guide to Forcing Flower Bulbs
- Forcing spring bulbs in soil: the basics
- How to force tulips
- Picking tulips for forcing
Blooming bulbs signal spring almost everywhere. Usually planted in fall, tulips, daffodils, grape hyacinths and hyacinths put on colorful spring shows. With the right conditions, many bulbs can be made to bloom sooner (or later) than they would naturally.
Making bulbs bloom outside their usual season is known as forcing.
Why Force Bulbs?
Commercial cut-flower growers force bulbs to hit periods of strong demand, to beat others to market with spring flowers, or to aim for holiday sales. You can force bulbs at any scale, from a handful for early blooms indoors to thousands for cut-flower sales. This guide concentrates on conditions for commercial cut-flower production, but the same basic principles hold for smaller projects.
The Process of Forcing Bulbs
Tulips, narcissus and other spring bulbs need a cold period to produce good flowers on long stems. They're normally planted in fall, and in the north they get several weeks of mild autumn weather to grow strong roots before the ground freezes. Winter cold supplies the chilling. In spring, as snow melts and soil warms, shoots stretch and the bulbs bloom once they've had enough warmth and light.
Forcing aims to supply everything the bulbs need to bloom, just on a different schedule than nature's.
For earlier flowers, growers use a temperature-controlled space to begin chilling sooner than it would happen outdoors. A good forcing system follows the natural cycle as closely as it can, giving bulbs time to:
- grow a strong root system
- get enough chilling
- receive the warmth and light that trigger growth and bloom
Exact forcing needs differ by species and variety, but in general the process goes like this:
- Plant bulbs in bulb crates or other suitable containers you can move.
- Keep them at a temperature that encourages rooting.
- When roots are well established, lower the temperature and continue chilling.
- After the bulbs have had enough cold, move them into a heated greenhouse to bloom.
Infrastructure Needed for Forcing
To force bulbs successfully, you'll need:
-
Rooting Space
A cool, dark, rodent-proof space that stays around 40–50°F. A basement, garage or a walk-in cooled with a CoolBot® can work; for example, a CoolBot room set to 48°F, or an unheated dark storage space that runs 40–50°F in fall.
-
Chilling Space
A dark, temperature-controlled space that can be held just above freezing, such as a CoolBot room set to 34°F.
-
Housing Space
A greenhouse heated to about 60°F to grow the bulbs on to bloom, with water available so you can irrigate them as they grow.
Materials Needed for Forcing Bulbs in Soil
Bulbs can be forced hydroponically without soil or media, but this guide covers forcing in crates filled with soil or another growing medium. Here's what you'll need:
-
Crates
- Bulb crates are ideal and usually come in two heights. Shorter crates let you stack more in a given space, while taller "deep crates" offer extra depth. Bulbs often start sprouting before they leave cold storage, and deep crates give the shoots more room. Both work; each has its own advantage.
- Planted crates are heavy, so plan your setup to move them as few times as possible.
-
Soil
- A sterile, peat-based potting mix such as Pro-Mix works well.
- Generally, the mix should be free of disease, drain well and have a pH of 5.5–6.5 (Miller 2010). If you steam or heat-treat your mix, let it cool to 55°F or below before planting.
-
Water
- Water crates thoroughly after planting.
-
Flower Bulbs
- A crate usually holds 60–70 bulbs comfortably, depending on bulb size.
Up next: see how to force tulips for step-by-step tulip forcing instructions and results from trial plantings.
