Diversified produce growers and u-pick operations are increasingly exploring cut-flower production as a way to broaden their markets and increase revenue. To support this interest, our team of Extension specialists and educators is establishing demonstration sites in Lafayette, Indianapolis, and Vincennes, spanning north-south Indiana. In this article, we share lessons we’ve learned from our vegetable-growing experience about starting cut-flower transplants from seed. We started with cool-season cut flower species; those we have explored include dianthus, stock, centaurea, snapdragon, lisianthus, ornamental cabbage, and delphinium. Seed germination begins in winter. Since a few small-scale produce growers operate heated greenhouses during this period, most rely on indoor setups with shelving units and LED lights to start seedlings—an approach we used at two of our three demonstration sites (Figure 1). The third site utilized a growth chamber with a more advanced controlled environment.
Figure 1. An indoor setup with shelves and LED lights for growing cut flower seedlings (Photo by: Jayde Marie Grisham).
We quickly realized that growing the above cut flower seedlings is not the same as growing vegetable transplants. For most of these cut-flower species, seed germination and seedling growth take longer compared to growing vegetable transplants. As a result, the same substrate, irrigation water, and management practices used to grow vegetable transplants may not be suitable for growing some cut-flower seedlings. We have seen that seed germination and plant growth are more sensitive to environmental conditions than many vegetable seedlings; thus, temperature, moisture, and light need to be adjusted accordingly.
Seed germination
Among the species we explored, ornamental cabbage and centaurea germinated the fastest across all locations. Stock, dianthus, and snapdragon had generally acceptable germination. In contrast, lisianthus and delphinium had the lowest germination at all sites.
Several common cut-flower species are light-sensitive during germination, whereas light is seldom a limiting factor for vegetable seeds except lettuce. Seeds that require light to germinate should be surface-sown or covered only very lightly so that light can reach the seed. At our three demonstration sites, seeds were surface-sown without covering at one location, while at the other two, they were covered with a thin layer of vermiculite or germination mix. Because lisianthus requires light to germinate, we suspect that excessive covering may have reduced germination.
Delphinium germination was slow and uneven, and no germination was observed at one site. Delphinium performs best at cooler germination temperatures (64°F to 71°F). At the site where temperatures were maintained at 75-85 °F, germination was particularly poor. In addition, pre-chilling seeds at 35 to 40 °F for several weeks prior to sowing can improve germination rate and reduce germination time, a process mimicking natural environmental conditions, known as stratification.
High EC and damping off
At one location, we used soil blocks made with peat, black cow compost, perlite, vermiculite and soluble fertilizers. This recipe has been used successfully in growing several vegetable transplants. However, we observed significant challenges when using it for lisianthus and dianthus.
Because lisianthus germinates very slowly, the soil blocks remained under high moisture conditions for an extended period. This led to the development of algal and saprophytic fungal growth on the surface, which likely inhibited germination. Among the seeds that did germinate, lisianthus and dianthus seedlings showed poor growth, and many failed to survive. We suspect that elevated electrical conductivity (EC) in the mix may have suppressed seedling growth and made the slow-growing seedlings more susceptible to damping-off pathogens.
A common concern when incorporating compost into potting mixes is an increase in EC. Plant species vary in their tolerance to EC, and highly sensitive crops such as lisianthus and dianthus are more likely to be negatively affected by high-EC media. At the other two locations, commercial potting mixes with main ingredients including peat, perlite, vermiculite, or bark were used. No saprophytic fungal growth was observed, and dianthus performed well.
Ensuring vigorous seedling growth is essential for reducing the risk of damping-off. Careful moisture management is particularly important. To better balance moisture needs during germination and early growth, we found that using smaller soil blocks or germinating seeds in flat trays followed by transplanting into larger soil blocks or cell trays produced better results, despite the additional labor required. This approach allows growers to use a medium with high water-holding capacity during germination and switch to a better-drained medium for seedling growth. It also avoids maintaining the same medium under prolonged wet conditions. Some growers may also incorporate biological products containing beneficial microorganisms, such as Bacillus spp. or Trichoderma, to suppress damping-off pathogens and support plant growth. These products were not used at our three locations.
Among the cut-flower species we evaluated, lisianthus was the most challenging to grow from seed. Due to these difficulties, many growers choose to purchase plugs instead. Ordering larger quantities is typically more cost-effective, so groups of growers may benefit from placing a combined order and sharing the cost.
Nutrient deficiency
Growing cut-flower seedlings generally requires more time than producing vegetable transplants. Because of this longer production period, the interaction between irrigation water and substrate becomes more influential, making the chemical characteristics of both especially important.
At the trial location, where we used a commercial bark-based potting soil and 72-cell trays, the substrate initially measured a pH of 5.7, 172 ppm nitrate, and 40.6 ppm phosphorus, values considered acceptable for a general potting soil. However, the irrigation water had a pH of 7.6 and an alkalinity of 240 mg/L. Without acidification, this water is likely to raise substrate pH over time. Although we typically do not have issues when growing vegetable transplants with potting soil and irrigation water, stock and centaurea seedlings at this site developed symptoms consistent with micronutrient deficiencies (Figure 2). At another location, the substrate contains very low initial nutrient levels, and nutrient deficiencies were observed across multiple species. In both situations, supplemental fertilization should be added to produce healthy seedlings. The optimal fertilizer solution depends on the potting soil’s starting nutrient content, irrigation frequency, and the quality of the irrigation water. No nutrient deficiency symptoms were observed in plants grown in the compost-containing medium.
Figure 2. Symptoms consistent with micro nutrient deficiency was observed on stock seedlings (Photo by: Wenjing Guan).
Stem elongation
Stem elongation was observed across multiple species at one of the three locations (Figure 3). Seeds were initially germinated under full-spectrum LED tube lights. After elongation was noted, supplemental blue LED lighting was introduced 12 days after seeding. Under the combined lighting, intensity ranged from 300- 700 µmol·m⁻²·s⁻¹ and 16-hr photoperiod, and temperatures were maintained between 77-85°F. While inadequate light quality and intensity may contribute to plant stretching, elevated moisture levels and high temperatures may also have played a role. Additional studies are planned to better understand how indoor light quality and its interactions with other environmental factors influence stem elongation in low-cost production systems commonly used by small-scale growers.
Figure 3. Stem elongation of centaurea seedlings (Photo by: Laura Ingwell)
The authors thank Emily Evers, Macon Ann Beck, and Cesar Escalante for their contributions to this article. The Purdue AgSeed program provides financial support for the project.
References:
Germination requirements for annuals and vegetables. Iowa State University Extension and Outreach.
Delphinium cut flower production in Utah. Utah State University Extension.