Substrate stratification is a new research area in which multiple substrates, or the same substrate with differing physical properties, are layered within a container to accomplish a production goal, such as decreasing water use, nutrient leaching, or potentially reducing weed growth. Previous research using stratification with pine (Pinus sp.) bark screened to ≤1/2 or 3/4 inch reduced the growth of bittercress (Cardamine flexuosa) by 80% to 97%, whereas liverwort (Marchantia polymorpha) coverage was reduced by 95% to 99%. The objective of this study was to evaluate substrate stratification with pine bark screened to remove all fine particles as the top strata of the substrate and determine its effect on common nursery weeds and ornamental plants. Stratified treatments consisted of pine bark screened to either 1/8 to 1/4 inch, 1/4 to 1/2 inch, or 3/8 to 3/4 inch, applied at depths of either 1 or 2 inches on top of a standard ≤1/2-inch pine bark substrate. An industry-standard treatment was also included in which the substrate was not stratified but consisted of only ≤1/2-inch pine bark throughout the container. A controlled-release fertilizer was incorporated at the bottom strata in all stratified treatments (no fertilizer in the top 1 or 2 inches of the container media), whereas the industry standard treatment had fertilizer incorporated throughout. Compared with the nonstratified industry standard, substrate stratification decreased spotted spurge (Euphorbia maculata) counts by 30% to 84% and bittercress counts by 57% to 94% after seeding containers. The shoot dry weight of spotted spurge was reduced by 14% to 55%, and bittercress shoot dry weight was reduced by 71% to 93% in stratified treatments. Liverwort coverage was reduced by nearly 100% in all the stratified substrate treatments. Compared with the industry standard substrate, stratified treatments reduced shoot dry weight of ligustrum (Ligustrum japonicum) by up to 20%, but no differences were observed in growth index, nor were any growth differences observed in blue plumbago (Plumbago auriculata).
Substrate stratification is a method of filling nursery containers with “layers” of different substrates, or different textures of the same substrate. Recently, it has been proposed as a means to improve drainage, substrate moisture dynamics, and optimize nutrient use efficiency. Substrates layered with larger particle bark as the top portion and smaller particle bark as the bottom portion of the container profile would theoretically result in a substrate that dries quickly on the surface, thereby reducing weed germination, but that would also retain adequate moisture for crop growth. The objective of this study was to evaluate the effect of stratified substrates on the growth of common nursery weeds and ornamental crops. This study evaluated the use of coarser bark (<0.5 or 0.75 inches) as the top substrate and finer bark (<0.38 inches) as the bottom substrate with the goal of reducing the water-holding capacity in the top 2 to 3 inches of the substrate to reduce weed germination and growth. Results showed that substrate stratification with more coarse bark on the top decreased the growth of bittercress (Cardamine flexuosa) by 80% to 97%, whereas liverwort (Marchantia polymorpha) coverage was reduced by 95% to 99%. Substrate stratification initially reduced the growth of ligustrum (Ligustrum japonicum) and blue plumbago (Plumbago auriculata), but there was no difference in the shoot or root dry weights of either species in comparison with those of nonstratified industry standard substrates at the end of 24 weeks. The data suggest substrate stratification could be used as an effective weed management strategy for container nursery production.