The Tallgrass Prairie Center Guide to Prairie Restoration in the Upper Midwest

16. Propagating and Transplanting Seedlings

16 16 greg฀houseal Overview Propagating and transplanting seedlings of native species present unique challenges . Most prairie species have seed dormancy mechanisms that must be overcome for good seed germination to occur in a greenhouse environment. Potting containers ideally should accommodate the deep root systems that develop in seedlings. Included in this chapter are tips on greenhouse propagation, potting mediums, containers, light and temperature requirements, and proper watering. The chapter concludes with a discussion of transplanting techniques, including the proper stages of seedling development and the use of a conetainer and dibble transplanting system. Seed฀Dormancy Dormancy is an adaptive trait, allowing germination to occur over time and in the proper season. This vital trait prevents the germination of seed at a time that might be suboptimal, or even lethal, for seedling establishment. Staggered germination over time is normal, even with stratification, and should be expected when propagating native prairie species. Successful propagation of native species in a greenhouse requires an understanding of seed dormancy, including how to remove it so seed will germinate. The benefits of removing dormancy are two-fold. First, more seed germinates in a shorter period of time, which means limited and costly greenhouse space is used more efficiently. Second , increased germination means that more individual plants will potentially establish, flower, and reproduce, contributing their genetic diversity to the next generation. There are two main categories of dormancy: primary and secondary. Primary dormancy occurs when seed is dormant upon dispersal, which is typical of many prairie species. Species with secondary dormancy produce seed with Propagating฀and Transplanting฀Seedlings 240฀ native฀seed฀production the ability to germinate readily upon dispersal (when it is fresh); however, the seed may enter a dormant state if conditions aren’t favorable. Many woodland spring ephemeral species belong to this category of dormancy. Within the two primary categories, there are several types of seed dormancy, and appropriate strategies are required to remove each type (table 16-1). One type is morphological dormancy. The embryo within the seed is underdeveloped upon dispersal, and warm, moist conditions are generally necessary for maturation (55° to 65° F). Species with this type of dormancy are found in the parsley, buttercup, arum, lily, and iris families, among others (Baskin and Baskin 1998). Another type, physical dormancy, is due to a physical characteristic of the seed; for example, the seed coat may be hard or waxy or otherwise impermeable to water and gas exchange, thus inhibiting germination. Species in the sumac, legume, geranium, and buckthorn families have these characteristics. Seed with physical dormancy requires some type of scarification to remove these barriers. Biochemical compounds constitute a third type of dormancy. These compounds may be produced in the seed itself or translocated to the seed from the plant prior to dispersal. Abscisic acid, for example, prevents premature germination of mature seed in the seed head, before dispersal from the parent plant. Concentrations of germination inhibitors, like abscisic acid, decline over time, allowing the seed to germinate. Many seeds may have a combination of dormancy types, which is sometimes called double dormancy. How to remove these developmental, physical, and biochemical inhibitors to germination may not be easily deciphered. If all else fails, gibberellic acid is sometimes effective at inducing germination. Seed฀Treatments scarification Scarification is a technique that simulates the natural disintegration of the seed coat to initiate germination. A hard or waxy coat will not allow the seed to soak up the water needed for germination until the seed coat breaks down. Seed is scarified either through natural processes such as weathering, abrasion, or partial digestion or through artificial techniques. A seed has a natural opening for water uptake, and this weathers or wears away first, especially in a seed with a hard coat, allowing the seed to imbibe water so germination can occur. The trick of scarification, then, is to accelerate the process of weathering this natural opening so the seed can imbibe water, but stopping short of damaging the seed. Some simple scarification techniques are presented here (fig. 16-1). Propagating฀and฀Transplanting฀Seedlings฀ 241 Sandpaper blocks can be constructed using rubber cement to glue a sheet of fine-grain sandpaper to each of two flat plywood boards. Lay one sand block on a tray, and use light pressure and a circular motion to move the other sand block on top of a quantity of seed sandwiched between the two blocks. Another variation of this...