In the last Gardening in the Sky post I discussed the horticulture team's motivation for starting a small propagation program. In addition to resolving some of the difficulties we've had with sourcing our plants locally, propagating plants helps us understand them better. Each plant has different requirements. Some, like stonecrops (Sedum) and willow (Salix), can easily be propagated vegetatively by clipping a stem and putting it directly in the soil. Others can only be grown from seed and are particular about their growing conditions. The seeds of many of our local species need to experience winter before they will germinate, often demanding one to three months of in cold, moist conditions. These conditions can be created by keeping seeds in a moist paper towel in the refrigerator. However, nurseryman Roy Diblik, who grows many native prairie plants at his nursery outside Milwaukee, recommends letting the seeds experience the local winter climate. Each individual seed has unique genetics and those that germinate under natural conditions may be better suited to the local climate.
Late last December the gardeners sowed 20 flats of seeds and placed them outdoors under screens to keep out rodents and birds. We had mixed results. Some of the seeds began to germinate during the warm spell in February, when temperatures crept up toward the 70s. These seedlings were wiped out by the cold snap in March. We did, however, get some healthy seedlings later in the spring. Over eighty sweet everlasting plants (Pseudognaphlium obtusifolium) are now growing in the gardens.
We also had success with showy goldenrod (Solidago speciosa) seeds collected on the High Line last October. To prepare the seeds, gardener Taryn Cunha had to separate the pappas (tufts of hair that help the seed disperse by wind) from the seeds by rubbing them across a sieve. She then stored them in homemade seed packets in the refrigerator and planted them in early spring.
The butterfly milkweed (Asclepias tuberosa) collected in the park did not even germinate and only a handful of the purple prairie clover (Dalea purpurea) survived. We found that having a shade cloth over the newly emerged seedlings, prevented them from getting battered by heavy rain storms. Pink evening primrose (Oenothera speciosa) had a high rate of germination, but the seedlings that were not under the shade cloth were destroyed by a recent downpour.
Propagating plants provides endless opportunities for learning. We are still tweaking our nursery irrigation system, which is supplied by rainwater collected in barrels. We are also adjusting our potting mix recipe to find the right balance of nutrition, water retention, organic matter and drainage. As we develop our best practices, we are reaching out to other organizations to learn about their operations. We have volunteered at Long Island Native Plant Initiative's greenhouse and will tour Greenbelt Native Plant Center on Staten Island this summer.
From seed collection, through storage, treatment, sowing, and transplanting, there are countless variables to consider when propagating plants. Paying attention to these details gives us a deeper understanding of the life cycles of our plants and helps us become better gardeners. We look forward to growing more onsite and, maybe one day we'll even develop our own High Line variety from one of the plants in the park.