How to Force Spring-Flowering Bulbs Indoors
Flowering bulbs are a popular way to add color and cheer to our indoor spaces, especially during the doldrums of winter in regions that experience below-freezing temperatures. In these regions, spring-flowering bulbs like daffodils, hyacinths and tulips are normally purchased in the fall and planted outdoors where they’ll be exposed to several weeks of cold winter temperatures. This cold period is necessary to trigger the bulbs’ internal biological process of flower development; a process known as vernalization. Without it, the bulbs would grow leaves but no flowers.
Spring-flowering bulbs can also be purchased potted up and ready to bloom indoors anytime from December through March, when gardeners in cold regions can only dream about these flowers in their outdoor gardens. The reason these potted spring-flowering bulbs bloom indoors is that they’ve been exposed to cold under artificial conditions, and “forced” to flower out of season.
This article will take you through the steps to force spring-flowering bulbs at home so you can enjoy some flowering beauty in your home indoors in winter.
Learning Objectives
- What are flowering bulbs?
- What it means to “force bulbs” indoors
- What types of bulbs can be forced for flowering indoors
- How to force bulbs to flower indoors for an out-of-season display
- Choosing bulbs for forcing indoors
Table of Contents
What Is A Flowering Bulb?
In the gardening world, the word “bulb” is used as a general term for the round or tuberous plant structures that get buried in an outdoor garden to grow into flowering plants. Once these bulbs have flowered in the garden, their top growth dies back and they go through a period of dormancy. Root growth continues underground but top growth only begins again when triggered by environmental factors like a change in temperature, day length or soil moisture levels.
There are many different types of flowering bulbs, and they bloom at different times of year. Some bloom in early spring, others in mid or late summer. The growth cycle is triggered by natural environmental factors such as changing temperatures (heat or cold), light levels (i.e. day length), or an increase in soil moisture.
Most spring-flowering bulbs (e.g. tulips, daffodils) are triggered to regrow and bloom following several weeks exposed to cold conditions outdoors. This period of cold is necessary for flower bud development, which is why these bulbs need to be planted in the fall. The required cold exposure over the winter is a natural process known as vernalization.
Not all flowering bulbs require vernalization to bloom. Bulbs native to regions that don’t experience freezing temperatures aren’t adapted to extreme cold temperatures. They still go dormant after flowering, but their regrowth and bloom cycle doesn’t need an extended period of cold temperatures.
Different Types of Bulbs
The term “bulb” is used in a general sense to refer to a variety of different plant parts that get buried in soil to grow into flowering plants. There are actually several different forms of “bulb” and they differ in terms of their botanical structure, how they grow and how they store nutrients to support each season’s new growth.
True bulb
Most of what gardeners purchase for planting are considered true bulbs. They have the classic oval or pear-shape, with one end pointed (the growth tip) and the other flattish (where the roots will grow). A true bulb is like a whole plant in miniature; it contains the flower embryo, an undeveloped shoot and structures called scales that will grow into leaves. The scales – held together around a base – are usually tightly packed together (e.g. hyacinth) or sometimes loose and fleshy (e.g. Lily). The scales store nutrients for future growth. Most true bulbs have a protective papery covering. Examples of true bulbs include tulips, daffodils, and snowdrops
Corm
Corms are often shaped like true bulbs, with one flat side and the other where one or more growth tips will emerge. Corms are the thickened base of an underground stem that stores the nutrients. Corms are covered with a smooth or fibrous protective outer layer which is made up of the dry leaf bases from the previous season’s growth. Unlike true bulbs that regrow from year to year, corms last just one season; once they start to grow, they gradually shrivel. At the same time, new corms will develop on top of, or around the sides of, the old one. Common corms include crocus, gladiolus and freesia.
Tuber
A tuber is a swollen undergound stem. It has no fibrous protective coating and the growing points aren’t neatly arranged; there’s no top and bottom to a tuber. The growing points are spread randomly across the surface. Potatoes are good examples of a tuber. Examples of tubers planted for their flowers include cyclamen, eranthis and begonia.
Rhizome
A rhizome is a thickened underground stem that grows horizontally beneath the soil. The main growing tip is at the end of the rhizome but other buds will form along the upper surface as the rhizome grows longer. Roots grow from the bottom surface. Examples of rhizomes grown for their flowers include lily-of-the-valley, canna and calla lily.
Tuberous Root
This bulb type is a swollen root. Nutrients are stored in a cluster of swollen roots produced at the base of old stems. Once growth begins, fibrous roots develop across the lower surface of the tubers to absorb water and additional nutrients from the soil. The most familiar tuber to gardeners is probably the dahlia; others include alstroemeria and ranunculus.
What Does It Mean to “Force” A Bulb Indoors?
Spring-flowering bulbs planted in the outdoor garden require a cold period – vernalization – to trigger flower development and regrowth. These cold conditions can be simulated artificially so the bulbs bloom indoors much earlier than they would under natural conditions outdoors.
What is “Forcing”
Potted spring-flowering bulbs sold to bloom indoors during the winter have undergone their required vernalization under artificial conditions. As a result, they bloom indoors at a different time of year than they otherwise would naturally when planted outdoors. The process of treating bulbs with cold artificially so they bloom indoors is known as “forcing.”
There are some flowering bulbs, such as amaryllis, that don’t need vernalization, but they do need a dormant period. This dormant period requires several weeks in dark and cool – but above freezing – conditions to trigger flower development and regrowth. Simulating these dark conditions is another process of forcing bulbs.
What Types of Bulbs Can Be Forced Indoors
By definition, the process of forcing bulbs means making them flower at a time outside their natural cycle by imitating the environmental conditions they’d experience outdoors. So, bulbs that require vernalization are the ones that are most often forced indoors. Bulbs that don’t need vernalization can be planted in pots at any time for indoor blooms, as long as the required growth conditions (i.e., light and temperature) are provided.
Thinking of it another way, bulbs that are cold-hardy in the outdoor garden (i.e., those that can survive near or below freezing temperatures) are the ones that have adapted with vernalization as part of their natural growth cycle. Bulbs that are more tender and don’t tolerate freezing temperatures don’t need vernalization.
Most of the bulbs used for forcing indoors are ones we refer to as spring-flowering bulbs, such as tulips, daffodils and crocuses. There are other bulbs, such as amaryllis, that also need specific conditions for dormancy that rely less on extended cold conditions and more on light levels to trigger flower development.
Some bulbs sold in late fall or nearing the Christmas season are labelled “prepared bulbs”. These are bulbs that have already been given a cold treatment and are ready for forcing. The most common forced bulbs are the Paperwhites, a type of narcissus. The other common prepared bulbs are hyacinths. These prepared bulbs can be forced in water or potted in soil.
How to Force Spring-flowering Bulbs Indoors
Forcing spring-flowering bulbs for a cheery out-of-season display indoors isn’t complicated, but you need a cool dark space and patience. The process needs to imitate the natural cycle a bulb would go through in an outdoor garden where they grow roots, undergoes a chilling period, then blooms again.
The process of forcing spring-flowering bulbs indoors is a matter of creating conditions as close as possible to those in the winter garden outdoors where the soil is cold, dark and slightly moist for several weeks.
Forced bulbs can be grown one of two ways: in soil or rooted in water.
Regardless of which method is used, the bulbs will need to undergo a cold treatment. Without this treatment, the bulbs won’t flower. They need to chill in the dark at about 40 F (4 C) for several weeks. An unheated garage is suitable for the cold treatment if the temperature doesn’t go below freezing. Inside a refrigerator also works well.
Choose healthy bulbs for forcing; they should be firm, not mushy, with no signs of mold or disease.
Forcing Bulbs in Water
Forcing bulbs in water is less complicated than planting bulbs in soil but not all bulb varieties can be forced this way. Also, the bulbs tend to grow weaker foliage and stems when forced in water and usually need support so they don’t flop over.
1. Chill the bulbs
Place the bulbs in a paper bag with the top edge folded over and put the bag in the refrigerator. Be sure the bulbs aren’t near any ripening fruits or vegetables like apples or bell peppers. As fruit ripens, it releases ethylene gas, which will prevent the bulbs from developing a viable flower bud (i.e., the bulbs won’t flower).
The bulbs need to stay in the fridge for several weeks; how long will depend on the kind of bulb (see below). If they’re not chilled for long enough, the bulbs may not flower.
2. Place bulbs in a container of water to root
When the bulbs have had sufficient chilling, they are ready to root in containers with water.
For individual bulbs, you can use a specially-designed glass container called a bulb jar or bulb vase. These jars are pear-shaped with a narrow section in the neck to hold the bottom of a bulb just above the water in the jar below. The bottom of the bulb stays dry and prevents the bulb from rotting.
For groupings of bulbs, a wide shallow bowl or tall straight-sided glass vase can be used. Fill the bottom of the container with several inches of pea gravel or decorative glass beads and set the bulbs flat end down – pointed growth tips up – onto the gravel with their base slightly buried to hold them upright. The bulbs can be placed close together but not touching each other or the side of the container.
3. Add water
Fill the container with water almost to a level of, but not touching, the bottom of the bulbs. The bottom of the bulb will remain dry, and roots will grow down into the water.
Tip for choosing a container
Bulbs forced in water generally grow weaker stems than those grown in soil. These stems and foliage tend to flop over. Try a single bulb or grouping of bulbs grown in a tall glass vase – with gravel in the bottom – so the vase supports the foliage and stems.
4. Place container in a cool, dark location
Once the bulbs are set in a container with water, place the container in a cool dark space to allow roots to develop. Check on the bulbs every few days.
5. Move container to warm bright location
When the bulbs have grown roots into the water, move the container to a warm location in bright indirect light. This will trigger the bulbs to start growing leaves and flower shoots. As the bulbs grow, be sure to keep the water level filled to cover all the roots but never touching the base of the bulb.
Forcing Bulbs in Soil
Forced bulbs purchased from florists in winter have been potted in soil to root. This is the preferred method to force bulbs indoors because it is more like the natural setting in a garden bed. The growing bulbs can also obtain additional nutrients from the soil without relying only on the nutrients stored in the bulb. As a result, the flowering plants grow sturdy stems and foliage that won’t flop over as much as bulbs forced in water.
As with bulbs forced in water, the bulbs in soil must undergo a chilling period. However, the bulbs are planted into containers of soil BEFORE they receive their cold treatment. This allows the bulbs to begin growing roots during the chilling period (as they would do in the natural outdoor setting). When the chilling period is over, the bulbs have begun rooting, resulting in sturdier upper growth.
1. Choose a container
Begin with a container that’s 6-8 inches (15-20 cm) deep and has at least one drainage hole. Once you start watering your growing bulbs, excess water needs to drain away. Without proper drainage, soil becomes waterlogged and will rot the bulbs.
2. Plant bulbs
Using an all-purpose potting mix, fill the container a third to half full. This is the soil that will be below the base of the bulb; it needs to be deep enough to allow for adequate root growth. The bigger the bulbs being planted, the more soil should be added.
Place bulbs flat end down – pointed growth tips up – onto the soil. Don’t force them down into the soil. The bulbs can be placed close together but not touching each other or the side of the container. If planting tulips, face the flat side of the bulb facing the rim of the pot; the largest leaf will emerge from that side, giving the pot of bulbs an attractive balanced shape.
Carefully add more potting mix around the bulbs, making sure they stay upright. Tap the bottom of the container gently on the tabletop to settle the soil. Top up the container to about a quarter inch from the rim. The bulb tips should be just above the soil surface.
3. Water in the bulbs
Once the bulbs are planted, gently add water over the soil surface to soak it well and settle the soil around the bulbs. Don’t worry if the tips of the bulbs are sticking out a bit above the soil. Be sure to label each container of bulbs to remember which is which.
4. Chill bulbs
Pots of planted bulbs can be chilled in an unheated attic or garage, or in the refrigerator. Check the soil periodically and add water if needed to prevent soil from drying out.
If placed in the refrigerator for cooling, first cover each bulb pot with a plastic bag with a few breathing holes poked in it.
5. Place the container in a cool, low-light location
When the bulbs are being chilled, they will start rooting in the soil. Then the top shoots will begin to grow. Following the required chilling period, the shoots will be 1-2 inches (2.5 -5 cm) tall and pale green or white.
Once the bulbs have been chilled for the required time and small shoots have developed, move the container into a slightly warmer location, about 50 F (10 C) in low light. The temperature change will trigger the growth of flower stalks and foliage.
Leave the containers in low light for a few days, until the shoots have turned from white to deep green.
6. Move the container to a warmer, brighter location
When the shoots have turned green, move the container to a warmer spot with bright light to come into bloom. Ideal room temperature is about 65 F (18 C). Avoid locations close to heat sources or cold droughts. If the container is on a bright windowsill, rotate the container every few days to give the bulbs even light exposure. This will prevent the bulbs from leaning towards the light as they grow.
Choosing Flowering Bulbs For Forcing Indoors
Many types of spring-flowering bulbs can be forced to bloom indoors. Each variety – tulip, crocus, daffodil – needs a specific length of time for a cold treatment. The time even varies from one species to another within the same genus (e.g., Narcissus)
The following table lists some of the common spring-flowering bulbs by genus, and gives the approximate number of weeks each genus requires for chilling.
| Type of Bulb | Required Chilling Period(in weeks) |
|---|---|
| Snow drops (Galanthus) | 10-12 |
| Crocus (Crocus sp) | 8-10 |
| Hyacinth (Hyacinthus) | 12-14 |
| Dwarf iris; Dutch (Iris reticulata) | 10-12 |
| Grape hyacinth (Muscari sp) | 6-8 |
| Daffodils (Narcissus sp) | 14-18 |
| Tulip (Tulipa sp) | 14-16 |
| Summer snowflake (Leucojum) | 8-10 |
| Windflower (Anemone) | 8-10 |
| Glory of the snow (Chionodoxa) | 10-12 |
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