Growing Snaps in the Field or a High Tunnel? Pinch Them!

Snapdragons for cut flowers have traditionally been raised in greenhouses, using close spacing, approximately 3 x 5 in., and a once-over harvest. For this production technique, the so-called Group 1 varieties were recommended for winter conditions, Group 2 and 3 for spring and fall, and Group 4 varieties for the summer. When growing this crop outdoors or in a high tunnel, it is easier to space the plants farther apart and make repeated harvests over several weeks. With that change in production practice, other factors such as the varieties grown, and the possibility of pinching should be considered. In the last couple years, we have manipulated spacing and pinching, and found some surprising information.
Snapdragons are a cool-season crop, which grows well in spring and again in fall, but stops producing flowers in the heat of summer. In our Zone 5 weather conditions, we can usually transplant a spring crop to the field in mid-May, and a fall crop into a high tunnel by mid-July.  The spring crop will begin flowering in early June, lasting until the hot period (about mid-July in this area); the fall crop will flower when the hot period ends, around mid-September, and should last with the protection of the tunnel, until early to mid-November. We space the plants 9 x 9 in. apart in 4-row beds on black plastic mulch with 2 lines of trickle irrigation under the mulch. One layer of plastic support netting was stretched horizontally over the bed, about 6 in. above the ground.  To test the effect of pinching, we removed the growing points in half the plots when the plants had formed 7 pairs (nodes) of mature leaves. In 2006, we tested 4 varieties covering groups 1,2,3, and 4; in 2007, 5 varieties were included.
In all four trials, pinching increased the number of stems produced per plant from 38 to 50% (Table 1).  The main reason for this increase is that at the wide spacing, the plants that were not pinched developed a number of basal side shoots, which had to be harvested along with the main stem, in order to get adequate stem length (Fig. 1). Pinching allowed these branches to develop and produce marketable flowers.

Figure 1.  A snapdragon plant at time of main stem harvest, showing many basal branches that will be stripped off, and which will not contribute to the yield.

Table 1. Yield (stems per plant) of snapdragons in two trials in each of two years, showing the effect of pinching at node 7.

TreatmentsFlowering groupSpring field trialSpring field trialFall tunnel trialFall tunnel trial
control 121088
Pinched 16131211
Statistical sig. ***********
Statistical sig. ************

Varieties in flowering groups 1 to 3 produced more stems than those of group 4 in all trials (Table 1), and were also 10 to 23 days earlier if not topped (Table 2). The harvested stems on the pinched plants were less than an inch shorter than those of plants that had not been pinched
(Table 2). Pinching consistently delayed the first harvest of flowers by more than 2 weeks, but the group 4 varieties, which naturally flower later, tended to be less delayed by pinching than the earlier flowering varieties.

So pinching snaps grown for cut flowers in a field or high tunnel at  a relatively wide spacing increases yield, but delays flowering. To maximize yield and earliness, pinching only a part of the crop might make the most sense. With regard to choice of variety, I find that a middle-of-the-road approach may be best. Group 1 varieties have long enough stems, but the inflorescence length tends to be short. Group 4 varieties have the potential to produce long stems, but harvesting when the basal florets open leads to wilting of the immature tip in the vase.  Waiting to harvest until the tip florets are older results in pollination and drop of the oldest florets, and detracts from flower appearance. For those reasons, I feel that varieties of Groups 2 and 3 are most suited for field and high tunnels in the Northeast, and should be seriously considered elsewhere.

Figure 2.  The 2007 fall pinching trial in our high tunnel. Chantilly Orange in the foreground, Opus Rose and Potomac Early White in the middle distance.

Table 2.  The influence of pinching and variety on stem length at harvest and relative earliness (DAS = days after sowing) of five varieties of snapdragon grown in the 2007 trials.

TreatmentsStem length, inchesSteam length, inchesFirst harvest date, DASFirst harvest date, DAS
 Spring trialFall trialSpring trial Fall trial
Control 20.924.49167
Statistical significance ********
Statistical Significance************

Chris Wien is Professor of Horticulture at Cornell University. The excellent assistance of Liza White, supervisor, and assistants Martha Gioumousis, Teddy Bucien and Liz Stuprich is gratefully acknowledged. Many thanks to Takii, Gloeckner, Benary and Johnny’s Seed companies for donation of the seeds used in these trials.