Grow Stars In The Greenhouse

By |

Ornithogalum ‘New Yellow’

Potted sun stars (Ornithogalum dubium) are nearly unknown in the U.S., but in Europe, they are prized and sought-after pot plants, consistently fetching top price at the flower auction.

O. dubium is a geophyte, or bulbous perennial plant. Depending on the cultivar, the bulbs produce five to eight deep green, glossy leaves that are arranged in rosettes. Cultivars suitable for pot culture produce short, linear foliage ranging in length from 3 to 4 inches with a width of less than 1 inch. Strong pot cultivars produce more than one flower shoot per bulb. Each sturdy shoot grows 5 to 7 inches tall, bearing a compact cluster of brilliant flowers. Some clusters may produce up to 20 flowers.

Sun stars come in orange, yellow and white. Potted ornithogalum can be used indoors to brighten up the house. These plants are also ideal for window boxes, patio containers and planters early in the season, as soon as temperatures remain above freezing.

Ornithogalum Hybrids Are Compact And Colorful

Perhaps the best-performing pot cultivar for both bulb sales and crop performance is Ornithogalum ‘Orange Star.’ The majority of these bulbs are sold to the European markets and more recently to the markets in China and throughout Asia. Sales to the U.S. are still marginal compared to the other markets. Ornithogalum ‘Orange Star’ is an improvement over cultivar No. 511, which still is considered a reliable and high-performing plant.

‘Orange Star’ is more compact, making it better suited for smaller pot sizes. The newest additions to this product line are O. ‘Fire Star’ and O. ‘Orange Star Improved.’ Both new cultivars maintain a compact growth habit, but the pigmentation of  ‘Fire Star’ is more pronounced — a sparkling, vivid orange. New yellow-and-white-flowering ornithogalum hybrids will be released soon.

Store Bulbs Properly To Avoid Blue Mold

Check your bulbs for problems upon arrival at your greenhouse. They should be plump and free of mold and other signs of disease. Take the bulbs out of the box and place them into a well-ventilated area. The proper storage temperature is 79°F to 82°F, which is considerably warmer than that at which most other bulbs are stored. The bulbs need to be protected from direct sunlight and relative humidity should be moderate, ranging from 50 to 60 percent.

In my experience, the most common problem is excessive relative humidity causing bacterial soft rot (Erwinia carotovora) and blue mold. A slight infestation with blue mold is unsightly, but does not affect plant growth. Excessive amounts of blue mold, however, can damage the bulb tissue and may lead to deformed foliage and blooms. Comparatively, bacterial soft rot is a very serious problem.

Bulb Preparation Varies With Geographic Location

To control plant development, growers should prepare the bulbs before planting. This preparation or flower induction process takes place in a cooler, temperature-controlled environment and is very similar to that of dry vernalization of tulips. The bulbs are moved into a space with a set temperature point of 55°F and a relative humidity of 60 percent.

Keep the bulbs in this environment for three weeks or until roots start to develop. During this stage air circulation is particularly important. Once the roots start to grow, it’s time to plant — so plan accordingly. Planting time varies depending on geographic location and the grower’s preferred method of production. Suppliers ought to give specific instructions based on production protocol and geographic location.

Prevent Fungal Infections With Adequate Spacing

A number of commercial potting mixes can be used for growing ornithogalum. In general, the media should be well draining with excellent aeration. My preferred bulb mix contains washed sand, peat moss and vermiculite. Apply lime to the media to adjust the pH to a range of 5.5 to 6.

The new cultivars of O. dubium are compact plants that show best in small pots. Plastic pots work fine, but I prefer to grow them in clay pots. The suggested sizes and number of bulbs per pot are as follows: 3-inch pot, 1 bulb; 4-inch pot, 3 bulbs; 5-inch pot, 5 bulbs. The bulbs are planted just below the surface of the potting mix and should be covered by approximately 0.25 inch of media, slightly less is better.

The pots should be watered and moved into the greenhouse. Initially they can be kept on carts or spaced pot-to-pot on the bench. As foliage develops, though, pots need to be moved to final spacing. A spacing of 1 to 1.5 inches between pots is sufficient. Choosing a slightly wider spacing increases airflow, which may prevent fungal infections such as Botrytis.

Water Evenly And Feed Lightly For Successful Growth

Sun stars require firm water management and even moisture during the growing stages until flowering. The potting media needs to be well drained, allowing for excellent aeration. Waterlogging and underwatering are common troubles in the greenhouse.

Intermittent drying out of the media causes significant damage to ornithogalum. When the media becomes too dry, the delicate roots desiccate. Plants usually do not recover from a damaged root system. Fungal infestations start from the damaged roots and slowly kill the affected plants.

Both drip irrigation systems and hand watering work to alleviate this problem. Watering should be limited to the early morning. Otherwise, water might stay in the rosette too long, resulting in rotting of the plant from the base. For most of the production cycle, watering two to three times per week is sufficient.

Ornithogalums are light feeders. Overfeeding quickly becomes a problem that is most commonly expressed by brown tips on the foliage. Using an appropriate water/fertility program results in dark-green foliage, which is an improvement over the old varieties. Yellowing and browning of the tips was a common problem in older varieties. This issue was successfully eliminated through breeding.

A complete 17-10-27 liquid fertilizer is recommended for ornithogalums. I had good experience using Excel 15-05-15 Cal-Mag complete water-soluble fertilizer. A 75 to 100 parts per million nitrogen fertilizer solution is sufficient. Keeping an eye on salts is very important and you should monitor your electrical conductivity (EC) frequently. The optimal EC is between 0.8 to 1.2 micro Siemens.

Use Warm Temperatures And High Light

Ornithogalum flourishes at moderate temperatures and high light conditions. The optimum temperature range is 62°F to 71°F. They grow best at moderately-warm temperatures and high light.

Growing ornithogalum at low temperatures is possible (and an option for conserving fuel), but bench time is significantly increased. Depending
on your growing temperature, the production cycle for the crop is between 100 and 120 days, even longer for low-temperature growing.

Especially in the northern temperate regions, supplemental light is shown to ensure excellent crop development. High light in combination with high temperatures, however, is detrimental to the crop. In southern regions of the U.S., as well as with the late-season flower dates of northern regions, greenhouse shading may be necessary.

The new ornithogalum introductions are genetically dwarf cultivars and do not require plant growth regulators (PGRs) under optimal growing conditions. Under low light, or when speeding up production time, an application of GA3 will produce good results. You can expect to reduce your bench time by two weeks. The PGR is applied as a spray at the four-leaf stage. For the proper concentration, inquire with a supplier or extension agent.

Juergen Steininger is a grower at Longwood Gardens in Kennett Square, Pa. He can be reached at juergensteininger@yahoo.com.

Tags:

    Leave a Reply