How to Force Dutch Iris Bulbs for Professional Cut Flower Production
Forcing Dutch iris (Iris × hollandica) bulbs to bloom out of their natural season is an excellent way to ensure a steady supply of beautiful, elegant flowers for professional cut flower production. Dutch iris are prized for their rich colors and dramatic form, making them a valuable addition to any florist’s collection. Here’s a step-by-step guide to successfully force these bulbs for commercial flower production.
1. Understanding the Basics of Forcing Bulbs
Forcing refers to the process of manipulating the environment to encourage bulbs to bloom outside their typical flowering period. With Dutch iris, you mimic the natural conditions of winter and spring to prompt the bulbs into early growth and flowering. For professional flower growers, precise timing is crucial to ensure blooms coincide with demand, such as during the winter or early spring market.
We recommend that you read the instructions on this page to get started. For additional, in depth information, see this .pdf guide.
2. Choosing the Right Dutch Iris Varieties
Different varieties of Dutch iris bloom at slightly different times, so selecting the right cultivar for forcing is important. Early-blooming varieties are typically better suited for forcing. Some popular choices for cut flower production include:
'Blue Magic': A striking blue/violet iris.
‘Nofa Blue’: A gorgeous two tone softer violet/blue iris.**
‘Park Avenue’: A richly colored, deep blue variety with good height.**
'White Wedgewood': Known for its crisp white flowers.
‘Nofa White’: tall white variety with sweet yellow falls.**
'Apollo': A yellow variety with excellent form.**
'Casablanca': A strong white variety.
**Offered by Ampersand Bulb Co. Login to reserve some for your collection.
3. Proper Bulb Selection
For professional cut flower production, quality is paramount. Start with top-size bulbs (10+ cm in circumference) from a reputable supplier. Larger bulbs typically produce stronger plants with more flowers per stem. Inspect the bulbs carefully for signs of disease, mold, or damage. High-quality, healthy bulbs are key to a successful forcing program.
4. Pre-Chilling the Bulbs
Dutch iris bulbs naturally require a period of cold dormancy to trigger blooming. To mimic winter conditions, the bulbs must be pre-chilled before planting. Here’s how:
Temperature: Plant the bulbs in a soilless mix, water them in lightly and place them in a cool, dark area where the temperature is consistently between 35-41°F (2-5°C). For our bulbs, this cooling process should begin upon receipt of the bulbs or when you’d like to begin the forcing process. Avoid storing the bulbs near fruits like apples, as they emit ethylene gas, which can harm the bulbs. Warning: Dry storing unplanted Dutch iris bulbs is difficult and can lead to mold, fusarium, and crop failure if good conditions are not maintained.
Chilling duration: The bulbs need about 8 to 12 weeks of chilling in their planting medium to simulate winter dormancy. This chilling period ensures proper development and prepares the bulbs for growth once planted.
5. Planting the Bulbs
Follow these steps for optimal results:
Growing medium: Use a well-draining, sterile medium such as a mix of peat, sand, and perlite to prevent rot and disease. We recommend a product like ProMix. We offer this product at a wholesale rate for pick up or freight shipping. If planting in the field, ensure the soil is well-draining and nutrient-rich.
Planting depth: Plant the bulbs about 3-4 inches deep, with the pointed end facing upward.
Spacing: For dense, commercial cut flower field production, plant bulbs 4-6 inches apart in rows spaced 12 inches apart. For forcing applications, 256 bulbs per square meter can be planted.
Location: Choose a location with full sunlight, or ensure adequate lighting if growing indoors. Dutch iris need plenty of light to develop strong stems and vibrant blooms. If you are growing them indoors, we recommend full spectrum LED lights (not limited spectrum red/purple grow lights.
6. Managing Temperature and Light
After planting, it’s critical to control the environment to ensure strong growth and timely blooming.
Temperature: For the first few weeks, keep the bulbs at temperatures between 50-55°F (10-13°C) to encourage root growth. Once shoots appear, increase the temperature to 60-65°F (15-18°C) day temperature and cooler night time temps around 55F, if possible. The maximum day time temperature for Dutch iris is 68F. Do not grow them warmer than 68F.
Light: Provide at least 10-12 hours of light per day, especially if growing indoors under artificial lights. Dutch iris bulbs respond well to full-spectrum grow lights.
7. Watering and Fertilization
Watering: Keep the soil consistently moist but not waterlogged. Dutch iris bulbs are susceptible to rot, so proper drainage is essential. Water the bulbs lightly until you see shoots, then increase the watering frequency as the plants grow.
Fertilization: Apply a balanced fertilizer (10-10-10 or 5-10-10) once shoots appear. Fertilize every two weeks during the growth phase to promote healthy foliage and strong flower stems. Alternatively, fertilize weekly with calcium nitrate.
8. Ventilation
Dutch iris are sensitive to excessive humidity build up. As with tulips, ensure that your growing space is well ventilated. Use a dehumidifier if you are growing inside and cannot allow fresh air exchange.
9. Timing the Bloom
The goal of forcing is to produce blooms at specific times, usually to meet market demand. Once the bulbs are planted, expect them to bloom in 10-12 weeks. Adjust your planting schedule based on when you need the flowers to be ready.
Dutch iris forcing in North America is typically from Mid December to June. For beginners, we recommend aiming for Valentine’s Day to mid March. Do not expect to be able to get the earliest possible flowers during your very first season of forcing any flower. Bulb forcing takes time. -Time to figure our how your chilling, rooting, and growing spaces will support your work. -Time to learn the habits of the flowers.
As your flowers grow, you may need to add support. So think about this from the start. If you are growing in crates or pots, you’ll may want to add some netting or cages.
9. Harvesting Dutch Iris Flowers
Harvest timing: Cut the flowers when the buds are well-formed and starting to show color but before they fully open. This ensures they have a longer vase life and will continue to open after harvesting. Properly grown Dutch Iris should be 20-30 inches tall.
Cutting technique: Use clean, sharp scissors or pruning shears to cut the stems at an angle. Leave some foliage on the plant to allow continued photosynthesis if growing for multiple harvests.
Post-harvest care: Immediately place the cut stems in cool water, and store them in a cool, dark room until ready for sale or transport.
10. Troubleshooting Common Issues
Weak stems: This can occur if the plants don’t receive enough light or if the bulbs were planted too shallow. Ensure proper lighting and correct planting depth.
Poor blooms: If blooms are small or underdeveloped, it may be due to insufficient chilling or poor soil conditions. Ensure bulbs are pre-chilled for the recommended time and that the growing medium is fertile.
Rotting bulbs: Overwatering or poorly draining soil can cause bulb rot. Make sure the soil or medium drains well and avoid overwatering, especially before the shoots appear.
Forcing Dutch iris bulbs is a rewarding process that can provide a steady supply of high-quality flowers for professional cut flower production. By carefully controlling the environment, pre-chilling the bulbs, and maintaining proper growing conditions, you can produce spectacular blooms that meet market demands and delight your customers. With attention to detail, Dutch iris can be a profitable and stunning addition to your flower production business.