Bolting Explained And How To Prevent It

Have you ever seen lettuce start randomly growing up? Or did you notice your spinach start making flowers? If you’ve ever tried to make a salad from these oddly-shaped greens, you’ll notice bitter leaves and woody stems. What happened?

Bolting occurs when a plant is triggered to produce seed, easily recognizable by the sudden vertical growth of a central flower-bearing stalk. While a natural part of the growing cycle, premature bolting makes cole crops, root vegetables, leafy greens, and herbs taste bitter. Fortunately, premature bolting can be prevented by minimizing plant stress and transplant shock. 

Keep reading to understand the science behind bolting and how to delay flowering so that you can get the most out of your garden this year. 

What is bolting?

Bolting is a term that describes what happens when plants flower and go to seed. While bolting is a normal part of any plant’s life cycle, plants in this stage of life aren’t all that great for eating. Once plants have bolted they can’t go back, so prevention is the best cure.

Which plants are prone to bolting

While all plants are capable of bolting, bolting is most undesirable in brassicas, root vegetables, leafy greens, and herbs. Cabbage, broccoli, and kale are just a few cool-season plants that will bolt and ruin in weather that is too warm. Onions, carrots, radishes, and beets are some root crops that are especially susceptible to bolting. Leafy greens like lettuce and spinach, as well as herbs like cilantro and dill, are especially likely to go to flower and set seed.

How bolting affects harvests

Bolting directs a plant’s attention away from vegetative growth toward flower and seed production. The flowering process typically renders the rest of the plant inedible–stems and leaves will taste bitter after a plant has bolted.

Signs of bolting

There are a couple telltale signs of bolting. Generally, plants will develop two main features:

• Tall single stalks

When your low-growing plants seem to shoot up overnight, it’s typically a sign of bolting. Plants will send up a long, single stem with flower buds. Bushy plants like lettuce and spinach will start to take on an upright growth habit.

• Flowers and seeds

If a plant not typically grown for its fruit starts to produce flowers and seeds, it’s over. Flowering is the next-to-last stage of an annual plant’s life cycle and not a step that can be reversed. Once you notice your plants making flowers and seeds, it's a good indicator the plant has bolted.

7 ways to prevent bolting

While bolting is not totally preventable, there are some steps you can take to delay the process to enjoy your vegetables for far longer.  

1. Perfect your timing

   Knowing when to plant your crops is the best way to prevent bolting. Cool-season veggies like cabbage and spinach don’t handle heat well, while some crops will bolt if exposed to frost prematurely. 

   Plant cool-season crops in late spring or early fall so that they’ll be doing the majority of their growth in the cooler months. Wait to plant heat-loving veggies until the air and soil are warm enough to accommodate sensitive plants. Know your crops, know your climate, and plant accordingly. 

2. Minimize transplant shock 

   Plants that experience transplant shock are more likely to bolt than other plants. If you’re going to start your own seeds, there are a few ways to ensure a seamless transition from seed tray to field:

   • Start seeds at the right time

   Starting seeds too early is more damaging than starting seeds too late. Most annual crops can be started four weeks before they need to be transplanted outside. Some perennial plants can sit in pots a little longer. Reference your frost dates and have a backup plan (like using a row cover or a cold frame to protect tender seedlings from cold snaps). 

   • Don’t let seedlings get rootbound 

   Start seedlings in the correct size flat. Small seeds like lettuce can be started in 72-cell trays, but you’ll want to start your sunflowers in 50-cells or four-inch pots. Keep an eye on your seedlings as they grow, and if you see roots coming through the bottom of the pot, bump the seedling into a bigger container if it still has weeks until it can be planted outside.

   • Properly harden off seedlings

   Hardening off seedlings involves acclimating indoor-grown plants to outdoor temperatures and is an essential step in the transplanting process. To harden off seedlings, leave the flats outside for a few hours a day, gradually increasing their time outside over the course of a week or so. Have the plants spend a few full nights outside before transplanting. 

   • Water before and after transplanting 

   Watering before and immediately after transplanting reduces transplant shock. 

   • Transplant on a cloudy day

   If you can, check the weather forecast and opt to transplant seedlings on a cloudy day. While plants do need sunlight to thrive, overcast days allow plants to put energy towards developing healthy root systems rather than photosynthesizing sunlight. 

3. Avoid stressing your plants

   Heat stress is the number-one cause of bolting, but water stress and pest pressure can also cause plants to flower prematurely. Even too much sun or wind can cause plants to make the switch to set seed. Stress of any kind signals accelerated growth so that plants will have a chance to reproduce before they die.

   Consider automating your irrigation system with drip tape or sprinklers to ensure your garden is even and consistently watered. Check your plants once weekly for pests, and if you do see pests, start treatment as soon as possible. 

4. Use a shade cloth and row cover 

   If your cool-season veggies seem to be wilting in the sweltering heat, use a shade cloth to protect them from the harsh sun and heat. If your plants are in a row, place metal hoops or posts on either side of the bed and drape the shade cloth over the row. Shade cloths vary by the percentage of sunlight allowed through, so you can choose the perfect weight for your garden and your plants. Shade cloths drastically reduce temperatures underneath them and prevent young growth from sunburn. 

   Similarly, warm-season veggies might need a little protection from late-spring cold snaps or harsh winds. Drape remay or frost cloth over your rows and weight the edges with sandbags or soil, making sure that no foliage makes contact with the cloth. Frost cloth raises temperatures underneath it as much as five degrees!  

5. Prune plants frequently

   Regular pruning keeps plants from going to seed. The more you harvest leafy greens and herbs, the more the plants are prompted to produce new growth, which ultimately delays flowering. Just be sure to never cut back a plant more than a third of its height. 

6. Succession plant quick-bolting varieties

   Instead of trying to delay the inevitable, you could just plant multiple successions of bolt-susceptible varieties so that you always have a younger crop to harvest. Root vegetables like radishes and leafy greens like lettuce are perfect candidates for succession planting. Easy and quick to grow, you can sow most vegetables every three weeks. 

7. Plant bolt-resistant varieties

   Do yourself a favor and opt for bolt-resistant varieties, especially if you live in a warmer climate. Jericho is a slow-to-bolt lettuce variety bred to tolerate extreme heat. Southern Giant Curled Mustard is particularly well-equipped to handle drastic temperature swings, from cold snaps to heat waves. 

In summary

No gardener looks forward to their vegetables bolting, but it isn’t the end of the world. Bolting is to be expected, but there are ways to delay the process so that you get the most out of your cool-season vegetables and herbs.