Untreated, or uncooled, tulip bulbs receive their entire cold treatment after planting. Unlike pre-cooled bulbs, which have already received a portion of their chilling requirement, untreated bulbs are planted into trays and then rooted and cooled before being moved into the greenhouse for forcing.

This means that rooting and cooling occur simultaneously. The goal is not simply to expose bulbs to cold temperatures for a specific number of weeks, but to develop a healthy, well-rooted crop that is prepared for rapid growth once moved into the greenhouse.

One of the most important concepts in tulip forcing is that cooling and rooting should be viewed as a single process. A tray that has completed its cold requirement but has poor root development is unlikely to produce the same quality stems as a well-rooted crop.

Tulips are living plants even while dormant and should be handled accordingly.

Upon receiving your bulbs:

• Unpack them promptly.
• Ensure they remain dry and well ventilated.
• Avoid storing bulbs for extended periods whenever possible.

Tulips are highly sensitive to ethylene gas. Exposure may negatively affect crop quality long before visible symptoms appear.

Avoid storing bulbs near ripening fruit, vehicle exhaust, propane exhaust or combustion gases such as gasoline tanks.

Good air circulation and proper storage conditions are essential from the moment bulbs arrive.

Untreated tulips are generally planted from mid-September through mid-December, with the majority of forcing crops planted during October and November.

Planting dates influence both rooting and cooling. Planting too early can result in excessive cooling duration and reduced stem quality, while planting too late can limit root development and reduce overall crop performance.

A minimum rooting period of six weeks is required.

The Ruigrok guide notes that growers may choose to begin cold treatment on dry bulbs before planting rather than planting immediately.

Reasons for doing so may include labor management, greenhouse scheduling, or storage limitations.

Regardless of when bulbs are planted, adequate root development must still occur before forcing begins.

Tray Specifications

Recommended tray dimensions: 60 × 40 cm x Minimum internal depth of 8.5 cm

It's important that trays provide excellent drainage, adequate aeration and at least 5 cm of rooting media beneath the bulbs

During rooting and cooling, stacked trays should allow approximately 7–11 cm between layers.

Growing Media

The recommended growing medium consists of 60% garden peat, 40% peat moss, and 15% coarse sand.

Acceptable media should provide good moisture retention, good drainage, adequate air space, and stable structure throughout production.

Recommended values: pH 6–7, EC 0.5–1.0

Avoid growing media contaminated with soilborne pathogens such as Pythium or Rhizoctonia.

Grower Note

The Ruigrok guide places considerable emphasis on media structure. Tulips perform best when roots have access to both moisture and oxygen. Overly dense media can restrict root growth, while excessively coarse media may dry too quickly.

Planting Procedure

1. Fill trays with a minimum of 5 cm of growing media.
2. Place bulbs upright.
3. Cover with approximately 2 cm of coarse sand.
4. Water thoroughly.

The coarse sand layer serves two purposes. It helps prevent bulbs from lifting during rooting, and it helps keep harvested stems cleaner at harvest.

Planting Density

Recommended densities for a standard 60 × 40 cm tray:

12/+ Bulbs

• Early forcing: 85–110 bulbs
• Late forcing: 75–100 bulbs

11/12 Bulbs

• Early forcing: 100–130 bulbs
• Late forcing: 85–115 bulbs

10/11 Bulbs

• Late forcing: 100–130 bulbs

Bulb size has a direct impact on stem length, flower size, and overall crop performance. Larger bulbs generally produce stronger marketable stems.

Water thoroughly after planting, ensuring that water reaches all portions of the tray.

As a general guideline, approximately 1 litre of water is sufficient for a 40 × 60 cm tray filled with reasonably moist media.

The objective is consistent moisture, not saturated media.

Avoid:

• Standing water
• Waterlogged conditions
• Repeated wet-to-dry cycles

Many production problems that appear later in the crop can often be traced back to poor moisture management during rooting.

Rooting and cooling are the foundation of a successful tulip crop.

The cold period encourages stem elongation, flower development, and uniform flowering, while root development supports water uptake and stem quality.

Most cultivars require approximately 15–16 weeks of cooling, although some may require longer.

Cooling Schedule

Temperature and Timing

1. 9°C (48.2°F) until October 20
2. 7°C (44.6°F) from October 20 – November 10
3. 5°C (41°F) starting November 10
4. 5 to 2°C (41°F to 35.6°F) starting December 1
5. 2 to 0°C (35.6°F to 32°F) depending on shoot development

Avoid temperatures below approximately -1.5 to -2°C (29.3°F to 28.4°F) after planting.

Rooting Room Conditions

Rooting rooms should provide:

• Darkness
• Stable temperatures
• Good air circulation
• 90–95% relative humidity

Fresh air movement is important throughout rooting and cooling. Air circulation promotes more uniform temperatures, reduces disease pressure, and supports consistent crop development.

Grower Note: Cooling Is Not Just a Calendar Event

One of the most important concepts in tulip forcing is that cooling should not be viewed simply as a countdown of weeks.

Before moving trays into the greenhouse, evaluate:

• Root development
• Shoot development
• Moisture levels
• Overall crop condition

A crop that has completed its cold requirement but has poor root development may produce weaker stems and reduced quality.

Monitoring Shoot Growth

The Ruigrok guide recommends monitoring shoot growth throughout the cooling period. Excessive shoot elongation during cooling can reduce final crop quality and may indicate that temperatures are too warm.

Move trays into the greenhouse once:

1. The cold requirement has been satisfied.
2. Root development is adequate.
3. Greenhouse scheduling allows.

For the first several days after transfer:

12–14°C (54–57°F)

After establishment:

16–18°C (61–64°F)

Grower Note: Quality vs. Speed

Higher greenhouse temperatures will accelerate crop development but may also reduce stem quality and increase the risk of excessive elongation. The Ruigrok guide consistently favors moderate forcing temperatures to produce stronger, more uniform stems.

Maintain consistent moisture throughout forcing.

Avoid:

• Saturated conditions
• Severe dry-downs
• Standing water

The goal is a healthy root system capable of supporting rapid stem elongation and flower development.

Most disease issues encountered during tulip forcing are associated with:

• Excessive moisture
• Poor drainage
• Stagnant air
• Contaminated growing media
• Plant stress

The Ruigrok guide specifically highlights concerns around soilborne pathogens such as Pythium and Rhizoctonia, both of which can negatively affect root development and crop performance.

Successful disease management begins with:

• Healthy bulbs
• Clean trays
• Clean growing media
• Consistent moisture management
• Good air circulation

When to Investigate Further

If you observe:

• Poor rooting
• Uneven growth
• Weak stems
• Declining bulbs
• Abnormal flowers
• Significant crop variability

Consult the disease and disorder sections of the full Ruigrok forcing guide for detailed descriptions and diagnostic information.

Harvest tulips when flower buds have developed color but before they have fully opened.

Tulips continue developing after harvest, making proper harvest timing critical for:

• Vase life
• Storage quality
• Shipping performance

Crops that have received adequate cooling and developed strong root systems generally produce the most uniform harvest.

Harvest Methods

Tulips may be harvested:

• With the bulb attached
• With the stem removed from the bulb

The appropriate method depends on your market and sales channel.

Postharvest Handling

Immediately after harvest:

• Keep stems cool.
• Avoid physical damage.
• Process promptly.

Temperature management remains one of the most important tools for preserving quality after harvest.

Short Stems

Potential causes:

• Insufficient cooling
• Poor root development

Weak Stems

Potential causes:

• Excessive cooling
• Excessive greenhouse temperatures
• Poor rooting

Uneven Crop

Potential causes:

• Uneven cooling
• Uneven moisture
• Poor airflow
• Variable rooting

Poor Flower Quality

Potential causes:

• Improper cooling
• Root stress
• Temperature stress
• Ethylene exposure