Effect of Plug Flat on Plant Growth and Prevention of Post-transplant Stnting.

Abstract

Plugs should be transplanted at the pullable plug stage (PPS), which is when the root ball holds together after removal from the plug flat. If seedlings are held in plug flats for too long after PPS, they may not return to a normal growth rate after transplanting. This may be due to several factors including nutrient deficiency, hormone deficiency, low oxygen availability, water stress, altered light quality and quantity, the mechanism of the root hitting the edge of the container and the inability of rootballs to expand once transplanted. The objectives of this study were to: 1) determine which floriculture crops are sensitive to stunting caused by plug flats; 2) determine possible causes of post-transplant stunting; and 3) examine methods of overcoming plug stunting and the inability of plants to return to a normal growth rate after transplanting.

Effect of plug flat on plant growth. In the fall of 2003 and the spring of 2004, Antirrhinum L. 'Floral Showers Coral Bicolor', Begonia L. 'Harmony Pink', Brassica L. 'Red Peacock', Callistephus L. (Nees) 'Matsumoto Rose', Celosia L. 'Century Red', Consolida (L.) P.W. Ball & Hey. 'Pink Fantasy', Dianthus L. 'Telstar Picotee', Eustoma (Raf.) Shinn. 'Balboa Purple, Gazania L. 'Daybreak Mix', Impatiens Hook. F. 'Dazzler Red', Lycopersicon Mill. 'Heartland', Matthiola (L.) R. 'Christmas Ruby' and 'Harmony Cherry Blossom; Tagetes L. 'Little Devil Fire', and Viola L. 'Starlet Rose with Blotch' were grown in 200 or 288 plug flats until PPS. Species that exhibited stunting included Brassica, Callistephus, Celosia, Consolida, Dianthus, and Tagetes. The remaining species were not affected by the amount of time held in the plug flat after PPS. These species allow the industry more flexibility in terms of transplanting time. Further research needs to be conducted examining other species and their reactions to being held in plug flats after the optimal transplanting time.

Prevention of post-transplant stunting. Beginning in the fall of 2004, seed of Catharanthus L. 'Pacifica Lilac' and/or Celosia L. 'Century Red' or 'Century Fire' was sown into 288 size (7.3 ml) plug flats and subjected to such treatments as pre-transplant nitrogen application, pre-transplant gibberellic acid application, root obstruction, pre-transplant root ball disturbance, and longer drainage columns. In addition, four of the experiments included a control with seeds sown directly into 17 cm (1.66 L) pots. Plants directly sown into 17 cm (1.66 L) pots were significantly larger than both control and treated plugs. Only two treatments made consistent differences in post-transplant growth. Growing plugs on a longer drainage column led to a significantly larger final diameter in Celosia 'Century Fire' plugs transplanted on time. This leads us to believe that low oxygen availability could be one cause of plug stunting and the inability to regain a normal growth rate after transplanting. Also, root obstructed plugs were 5.1 cm smaller than control plants at transplanting, but there was no difference in the height of root obstructed plants and control plants after eight weeks. This suggests that root obstruction may be a contributing, but temporary factor to plug stunting and may allow for plugs to be held longer in the plug flat without sacrificing final plant quality.

In summary, our study confirms that post-transplant stunting due to excessive holding in plug flats is a problem in some species. Two of the methods evaluated for overcoming this problem proved to be successful. This information will allow plug users to better manage the growth of plugs after transplanting.