Forcing Tulip Bulbs | A Comprehensive Tutorial

Make sure bulbs get their full number of cold weeks; otherwise flowers and stems may not form properly. Every variety has its own chilling requirement, which you can usually find online or get from your bulb supplier. Most need 14–18 weeks.

Chilling happens in two phases: a rooting phase at 40–48°F, then the rest of the cold period at about 34°F. Rooting time counts toward the total chilling time provided the temperature doesn't climb above 55°F for more than a day or two (International Flower Bulb Centre, 2022).

About pre-chilled bulbs: For even earlier flowers, many growers buy pre-chilled bulbs, which have already had some or all of their cold weeks. They come in two standard preparations:

• 9C Bulbs: these have had part of their chilling. After arrival they need a rooting period plus more cold.
• 5C Bulbs: these have had all of their chilling and only need a rooting period before moving to the greenhouse.
• All of the trial timings and bloom dates here come from bulbs that were not pre-cooled and got their entire chilling period on the farm.

How to Force Tulip Bulbs: A Step-by-Step Process

Planting

Label each crate with the date in, date out and variety, using a different label color for each removal week.

Put 2–3" of well-moistened soil or potting mix in each crate.

Set bulbs in the crate nearly touching, like eggs in a carton.

1. Bulbs arrive from the supplier. You can begin forcing as soon as bulbs arrive; the earlier you start, the earlier they'll bloom.

   If you can't plant right away, keep bulbs cool and dry until you can. Depending on your target harvest window and varieties, you may plant crates each week for several weeks.

2. Label crates. Locking plant labels work well for recording the date in, date out and variety. Color-coding labels by the week each crate leaves cold storage helps too (for example, green labels come out in week 7, pink in week 8, and so on).

3. Put 2–3" of well-moistened soil or potting mix in each crate. A sterile, soilless mix such as Pro-Mix works well.

   One sheet of newspaper in the bottom of the crate helps keep mix from falling out but isn't required. Don't line crates with heavy, less porous materials such as cardboard or plastic, which can hinder rooting.

4. Set bulbs in the crate nearly touching, like eggs in a carton. A crate usually holds 60–70 bulbs comfortably, depending on size. Bulbs can touch lightly.

5. Add another 3" of soil over the bulbs. No bulb tips should show above the soil.

6. Water thoroughly. The soil should now be saturated, which helps hold the bulbs down.

   Bulbs that aren't buried deeply and weighted enough can push up out of the soil as they root. Tulips can grow a remarkably dense root mass during rooting, and the force of that growth can lift the bulbs out. A heavy layer of wet soil helps prevent it.

   Some growers top crates with wet sand or mix sand into the medium for extra weight. Sand can be hard to keep moist after crates come out of cold storage and tends to spill out of the crates, so a 3" layer of thoroughly soaked potting mix is often enough to stop bulbs from heaving.

Chilling or Cold Period

Crates stacked in cold storage, with 1–2" between stacks for air circulation.

Once you can see roots, lower the temperature to 34°F or move the crates to a 34°F space for the rest of their cold period.

1. Move the well-watered crates into your rooting and/or chilling space. Crates can be stacked during rooting and chilling. Leave 1–2" between stacks so air can move.

   Air movement in the cold room can dry the top crates faster than the bottom ones, so check the top crates for dry soil through the winter and water them if needed.

2. Rooting time: keep crates dark at 40–48°F for 2–3 weeks so roots can form. By the end of this phase you should see roots poking out the sides of the crates.

   • Keep bulbs in the dark, since light triggers shoot growth.
   • Protect bulbs from mice and voles.
   • Water any crates that look dry.
   • High relative humidity (90–95%) helps crates hold moisture.

3. Drop to 34°F for the rest of the chilling period. A variety's total chilling requirement includes the rooting time. So if a variety needs 16 weeks of cold and spent 2 weeks rooting, it needs 14 more weeks at 34°F.

   • When roots are visible, lower the temperature to 34°F or move crates to a 34°F space for the remaining cold period.
   • Varieties root at different speeds. What matters most is giving a general rooting period (cool, dark, moist), not getting identical root growth in every crate and variety before dropping to 34°F.
   • Stay as close to freezing as you can. Bulbs tolerate some variation, but they mustn't freeze, and getting too warm can undermine chilling or push shoots to grow too much before chilling ends.

Forcing

Tulip bulbs at different stages of forcing.

When bulbs have had their full cold period, take them out of cold storage into a warm, bright space; a heated greenhouse is best. During this phase, called housing, watch these conditions:

• Light: in a greenhouse, natural day length usually supplies enough light for forcing tulips.
• Heat: Aim for about 58–65°F (International Flower Bulb Centre, 2022).
  • Cooler conditions slow growth. Too cool, and flowering can be delayed, plants can develop problems, and flower quality and stem length can suffer.
  • Warmer conditions speed growth and bring bloom sooner. Too warm, and some varieties shoot up too fast and flower on long, thin stems, while others rush into bloom on very short stems.
• Water: water heavily as soon as crates leave cold storage, since the bulbs haven't been watered for months. Make sure water reaches the bottom of the crate where the roots are. Keep crates well watered throughout forcing; the mix should stay close to saturated and never dry out. Hand watering takes time, and once buds form, overhead watering can damage flowers, so drip irrigation is best.
• Fertilizer: no extra fertilizer is needed; the bulb holds enough reserves to produce a flower on its own.
• Ventilation: vent the greenhouse during the day to keep temperatures as even as possible and control humidity.

Harvesting

For longer storage, harvest tulips with the bulb attached and store them dry.

In a greenhouse kept at about 60°F, most varieties bloom roughly 4 weeks after leaving cold storage. Bloom timing varies a lot with variety, weather and forcing conditions. Cool, cloudy weather slows flowering; hot, sunny days heat the greenhouse and speed it up.

1. Harvest as you would field or in-ground tulips. For longer storage, cut them with the bulb attached and keep them dry.
2. Compost the spent bulbs and soil.
3. Wash and disinfect crates before reusing them to help prevent diseases such as Rhizoctonia and Pythium.
4. Keep records of bloom dates and how each variety performs. Some varieties do better forced early in the season than late. Because performance varies so much between varieties and over the season, good notes on favorite varieties and gaps in bloom will help with next year's plan.
5. Order early for next year, since popular varieties sell out quickly.

Common Problems & Their Causes

Figuring out what went wrong in forcing can be tricky. Bulbs handle some deviation from ideal conditions, but too much at any stage can hurt flower and stem quality. Pinpointing the cause can be hard, because quality reflects conditions throughout the bulb's life. Problems also vary by season; varieties forced in late winter and early spring may show different issues than those forced later. Here are common problems and likely causes.

• Bulbs heaving; roots growing too vigorously
  • Getting root growth just right can be tricky. Overly vigorous roots can lift bulbs out of the soil, and a very dense root mat may be more prone to Trichoderma infection. On the other hand, if roots are underdeveloped when crates come into the greenhouse, the bulb has to grow roots and shoots at once.
  • Generally, warmer temperatures (40–48°F) encourage root growth and cooler ones slow it. If roots are growing too much or bulbs are heaving, lowering the temperature toward 32°F slows them down.
• Shoots growing too early or too fast
  • Shoots will probably poke through the soil during chilling. Ideally they stay 1–2" tall and below the crate rim.
  • Warmth encourages shoot growth too, so if shoots are developing too fast, lower the cold room temperature.
  • After you lower the temperature at the end of rooting, don't raise it again before housing, since warming can trigger shoot growth.
• Short stems
  • Variety choice: some varieties don't force well or fit better in a different part of the season.
  • Too little chilling: cold is what allows the stem to develop and reach full length. Chilling at cooler temperatures (35–38°F) is usually recommended for cut tulips because it gives longer stems, while pot tulips are often chilled warmer (45–48°F) for slightly shorter stems (Miller, 2010).
  • Too little water: plants short of moisture during greenhouse forcing can end up stunted.
• Stems too long
  • Variety choice: varieties that grow tall and lanky in the field tend to do the same when forced.
  • Too much chilling: bulbs given more cold weeks than needed may stretch quickly and form tall, weak stems. This happens more with late-season forcing, when bulbs have usually had longer chilling and warmer conditions.
• Flower abortion
  • Failure to produce a sellable flower can stem from several conditions at different points in the bulb's life (De Hertogh, 1996). If flowers abort, consider these causes:
    • Excess heat during shipping, dry storage before planting, or greenhouse forcing.
    • Ethylene: tulips are highly sensitive to it, so exposure of bulbs or plants to ethylene from ripening fruit, other flowers or other sources can harm flower development.
    • Lack of water: bulbs that dry out during chilling or forcing may abort flowers.
    • Poorly developed roots.
    • Inadequate ventilation.
• Stem Topple
  • This disorder happens when growing plants can't get enough calcium. The shortage weakens the neck or stem, and flowers may flop or snap just below the bloom (De Hertogh, 1996). Possible causes:
    • Variety choice: some varieties are more prone to it.
    • Too many cold weeks: overchilled bulbs grow fast once forced, and the plant may not deliver enough calcium to the quickly growing stem, leaving it weak.
    • Hot greenhouse (above 68°F): this also drives rapid growth that can outrun calcium movement up the stem.
      • Calcium travels with the transpiration stream, so supporting transpiration helps. Ventilate well and keep humidity from running too high.

Up next: see picking tulips for forcing for advice on variety choice and succession planting.