Efficacy of Cork Granulates as a Top Coat Substrate Component for Seed Germination as Compared to Vermiculite

A top coat is a lightweight substrate component used in seed germination. The seeds are typically placed on a substrate such as peat and then the seeds are covered with a layer of the top coating substrate. The top coat serves to maintain adequate moisture around the seeds and to exclude light. Vermiculite and cork granulates (1 mm) were used as top coat substrates for seed germination to determine if cork granulates could be successfully used as an alternative to vermiculite. The cork granulates had a bulk density of 0.16 g·cm −3 , which was higher than that of vermiculite that had a bulk density of 0.12 g·cm −3 . Cork granulates had an air-filled pore space of 22.7% (v/v), which was higher than vermiculite which was 13.2%. The water-holding capacity of vermiculite was 63.4% (v/v), which was higher than that of cork granulates that was 35.1%. Seeds of ‘Rutgers Select’ tomato ( Solanum lycopersicum ), ‘Dazzler Lilac Splash’ impatiens ( Impatiens walleriana ), ‘Orbital Cardinal Red’ geranium ( Pelargonium × hortorum) , ‘Better Belle’ pepper ( Capsicum annuum ), and ‘Cooler Grape’ vinca ( Catharanthus roseus ) were placed on top of peat and covered with a 4-mm top coating of either vermiculite or cork granulates. For tomato, impatiens, and vinca, days to germination were similar between seeds germinated using vermiculite and granulated cork as a top coat. Days to germination of geranium and pepper were significantly different with geranium and pepper seeds coated with cork granulates germinating 0.7 and 1.5 days earlier than those coated with vermiculite. For tomato, impatiens, and geranium, the number of seeds germinating per plug tray was similar between the top coats. Number of seeds germinating per tray for pepper and vinca were significantly different. Pepper had an average of 2.8 more seeds germinating per tray, and vinca had an average of 2.4 more seeds germinating per tray if seeds were germinated using granulated cork vs. vermiculite. For all species, dry shoot and dry root weights were similar for seedlings germinated using cork and vermiculite top coats.