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  • Author or Editor: Vickie Murphy x
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Cycads comprise the most threatened major group of plants on earth and many species require horticultural assistance to ensure their survival. Appropriate container substrate properties, especially relatively high air space content, are crucial to successful cultivation of most cycads from seed. Cycad substrates in common use include substantial portions of organic materials that will decompose over time, reducing aeration. At Montgomery Botanical Center, novel inorganic substrates have improved survival and growth of several very rare and challenging Zamia species, suggesting the need for a rigorous evaluation of different inorganic container substrates. Effects of 1) coarse silica sand (6/20 grade); 2) Fafard (a peat/perlite mix); 3) perlite (expanded volcanic glass); 4) pumice (volcanic rock); 5) Turface (calcined clay); 6) Profile (calcined clay); 7) a 50% sand (6/20): 50% Profile mix; 8) Permatil (calcined slate); or 9) Axis (calcined diatomaceous earth) on growth of Zamia pumila L. seedlings (grown from seed of Dominican Republic provenance) were evaluated. Growth parameters were measured after 18 months. Sand produced significantly higher total dry weight and leaf area than all other substrates. A combination of at least 18% air space combined with little coarse material (sand) or with some coarse material combined with enough smaller particles to fill part of the large pores created by coarse material (Fafard) likely contributed to better growth in these compared with the other seven substrates. The other substrates may have been either too coarse, leading to excessively large pores, which are known to inhibit growth in some plants if the pores are much larger than fine root diameters, or too fine (i.e., too low of an air space percentage). The fine roots of Zamia can be less than 1 mm in diameter, whereas higher proportions of coarse substrate particles over 4 mm in diameter inhibited growth, possibly by creating excessively large pores. In contrast, higher proportions of fine substrate particles of 0.25 to 0.5 mm were beneficial to growth.

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Horticulture is an essential part of plant conservation programs, and botanic gardens are uniquely suited for conservation horticulture work. Here, we present a case study of a successful cycad (Cycadales) propagation program at Montgomery Botanical Center (MBC, Miami, FL), using palma corcho (Microcycas calocoma) as an example. This species is highly sought in the nursery trade, and overcollection of wild plants is one factor leading to imperilment of natural populations. Thus, propagation and distribution of palma corcho can make a strategic contribution to in situ conservation. Provenance history of the living collections is reviewed, and techniques for propagation and establishment are detailed. An innovative botanic garden/industry partnership to provide seed for cultivation is discussed. Finally, we present analysis of market forces with regard to rare plant availability and conservation, using palma corcho as an example. Average price per seed has fallen by over half since offered on public auction. This inversely correlates with seed supply, which has been steadily increasing during the last 15 years and helping meet the high market demand. We project the cost of palma corcho will fall further to a point where collection from the wild has no further economic incentive.

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Palms (Arecaceae) are perhaps the most important tropical plant family for human use, both for utility and ornamental horticulture. The wide diversity of palm species with different seed germination characteristics necessitates tailoring horticultural practices to the needs of each. This is crucial for production and conservation horticulture. In this study, wild-collected seeds of yarey palm (Copernicia berteroana) and buccaneer palm (Pseudophoenix sargentii) were germinated in a variety of organic (standard nursery container mixes) and inorganic substrates. The yarey palm seeds were sown at two different depths, 0.5 inch and at the surface (seed half exposed). Mean maximum germination across all treatments for yarey palm was 79% and for buccaneer palm 60%. The standard nursery mixes generally fostered the best germination and long-term survival. This is likely due to a combination of the lower water availability at the surfaces of the more porous inorganic substrates (sand and perlite) and greater difficulty for coarse palm roots to penetrate the denser inorganic substrates, including fired ceramic, which otherwise had similar water-holding capacity (WHC) and even lower air space than the organic substrates. Difficulty of penetration caused roots of some seedlings to either dry up early in germination as in the surface sown yarey palm, or to “push up” the seed (buccaneer palm) rather than penetrating the substrate and this was often fatal. Thus, inorganic substrates are not recommended for germination and early seedling growth of these palm species and planting the seeds slightly below the surface may be preferable to surface sowing. For conservation horticulture of wild-collected palm seeds, this information can help prevent further genetic bottlenecks while under protective cultivation.

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