Effective scarification of honeylocust (Gleditsia triacanthos) seeds using direct heat and flame

MATERIALS AND METHODS

Seeds were collected from planted specimens on Eastern Connecticut State University campus in Willimantic, Connecticut, USA, during the fall of 2024. Seeds were removed from fruits and stored in sealed plastic bags or glass jars; all seeds used for these experiments were less than 1 y old.

We had 8 experimental treatments, a negative control of no treatment, and a positive control of nicking the seeds. All treatments or controls had 20 replicates; each replicate consisted of 5 seeds planted in 8.9 cm (3.5 in) pots filled with Promix HP soil. Each of the 5 seeds after treatment was soaked for 24 h and then planted about 1 cm (0.4 in) deep; seeds were spaced out evenly within the pot. The experiments took place within the Biology department greenhouse at Eastern Connecticut State University during fall 2024 and winter 2025. The greenhouse was kept at 26 to 30 °C (79–86 °F) with natural light conditions. Seeds were watered every other day or as needed to keep the soil moist. We observed the seedlings for germination 2 wk after sowing.

Treatments and controls were as follows:

• Control: no treatment.

• Nicked (positive control): Small section of seedcoat clipped off with pruning shears.

  • 1) 2 ml isopropyl: 2 ml (0.07 oz) of 70% isopropyl alcohol placed in a 142 g (5 oz) tuna tin can with 5 seeds, lit on fire, and left to burn off.

  • 2) 5 ml isopropyl: 5 ml (0.17 oz) isopropyl of 70% isopropyl alcohol placed in a 142 g (5 oz) tuna tin can with 5 seeds lit on fire and left to burn off.

  • 3) Lighter non-radicle end: Seeds were carefully held by hand between the thumb and index finger with the non-radicle end of the seed (end opposite from where radicle emerges) sticking out and burned with a simple butane lighter for about 3 sec or until the end appears charred (Figure 1).

  • 4) Lighter radicle end: Seeds were carefully held by hand between the thumb and index finger with the radicle end of the seed sticking out and burned with a simple butane lighter for about 3 sec or until the end appeared charred.

  • 5) Torch non-radicle end: A butane culinary torch, the type used for caramelizing sugar or browning meringue, for example, was used to char the non-radicle end of the seeds; treatment was less than 1 second per seed.

  • 6) Torch radicle end: A butane culinary torch was used to char the radicle end of the seeds; treatment was less than 1 second per seed.

  • 7) Torch brief: A butane culinary torch was passed over the non-radicle end of the seeds, without charring the seed; treatment was extremely brief, just a fraction of a second per seed and did not char the seedcoat.

  • 8) Soldering iron: A small soldering iron was set to 400 °C (752 °F); the tip was placed in the center of one of the flat sides of the seed (Figure 2) and held there for 5 sec or until the tip of the iron melted and charred a small section of seedcoat (Figure 3).

RESULTS

Several treatments had low germination rates or damaged seedlings and were not viable options for seed scarification. The low germination group included: 2 ml (0.07 oz) isopropyl, 0.15 (average out of 5 seeds) or 3%; 5 ml (0.17 oz) isopropyl 0.25 (average out of 5 seeds) or 5%; torch brief 0.25 (average out of 5 seeds) or 5%; lighter radicle end germination was 3.8 (average out of 5 seeds) or 76% but seedlings had damaged radicles and delayed root formation; torch radicle end 1.6 (average out of 5 seeds) or 31%, seedlings highly heat damaged.

Three treatments had high germination rates with healthy seedlings: lighter non-radicle end, torch non-radicle end, and soldering iron (see Table 1). These methods were statistically significantly different from the negative control of no treatment and not statistically different from the positive control of nicking.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1.

Using a simple butane lighter to scarify the non-radicle end of a honeylocust (Gleditsia triacanthos) seed.

• Download figure
• Open in new tab
• Download powerpoint

Figure 2.

Utilizing a soldering iron to scarify a honeylocust seed.

The ANOVA had a F = 104.864 and a P <0.001, and for the pairwise Tukey mean separation the significantly different comparisons were as follows. Lighter non-radicle end: control Q = 24.96 (P <0.001); torch non-radicle end: control Q = 23.33 (P <0.001); soldering iron: control Q = 20.31 (P <0.001); and soldering iron: lighter non-radicle end Q = 4.64 (P = 0.012).

The nonsignificant relationships were lighter non-radicle end: nicked Q = 1.4 (P = 0.733); torch non-radicle end: nicked Q = 1.45 (P = 0.842); soldering iron: nicked Q = 2.90 (P = 0.249). See Table 1 for summary of mean separations.

• Download figure
• Open in new tab
• Download powerpoint

Figure 3.

Charred mark left by a soldering iron on a honeylocust seed, effectively scarifying it.

DISCUSSION

To our knowledge, direct heat or flame has not been previously demonstrated as an effective method for scarifying honeylocust seed. We originally thought that a variation on alcohol flaming as in Sugii (2018) would work but found that was not the case for this species and sought out other ways of using heat. Using a butane lighter to char the radicle end of the seed did have a high germination rate, but the seedlings emerged with radicles lacking their root apical meristems and often fell over in their pots. Surprisingly, many did survive and produced secondary roots, but we thought this type of damage was too severe to consider this a worthwhile technique. Additionally, using the torch on the radicle end did have some effectiveness on increasing germination, but the seedlings that did emerge had brown sections of cotyledon or a warped and wavy appearance.

Our results indicate lighter non-radicle end, torch non-radicle end, and soldering iron treatments are reasonable alternative methods to induce germination in honeylocust and have a similar germination rate to nicking the seedcoat. The lighter method had a slightly better result. As far as time and handling are concerned, the lighter non-radicle treatment and using the soldering iron were both slower than nicking the seeds. The culinary torch on the non-radicle end was effective and faster than nicking the seed, but heat damage is still a concern. Also, the torch must be pointed just at the end of the seed, and the direction of the flame should be straight down or away from the radicle end. Additionally, the surface the seeds are placed on while using the torch must be carefully selected. The extreme heat of the torch will scorch wood, melt plastic, possibly buckle or discolor stainless steel counters, and may cause stone or concrete surfaces to chip or crack. Using the torch on sand or bare ground may be the best option.

This small-scale study illustrates useful techniques to germinate seeds that need physical scarification for germination.