Comprehensive Guide to Plant Bolting

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It seems to happen overnight.  You go out to the garden to harvest some greens for a salad, and there’s a tall flower stalk rising from the lettuce.  The flowers will go to seed in a few days, and the leave are small and taste bitter. Your lettuce becomes inedible, so you dig up the plants and sadly toss them in the compost.

What went wrong?

Well, nothing went “wrong.”  Your lettuce was doing what plants do – reproducing. 

All plants have a life cycle.  Most of our garden vegetable plants are annuals.  They grow leaves, make flowers and fruit, set seeds, and then die all in one growing season.  

We’re happy to see them flower when it’s vegetables grown for their flowers, fruits, and seeds – like tomatoes, cucumbers, and squash   We know we’ll have some tasty homegrown veggies very soon.  

But when it’s leafy vegetables that go to seed, it can mean doom for your continued harvest.  It’s called bolting.  And it makes the part of the plant we like to eat, the leaves, tough and bitter. 

Bolting helps plants ultimately create seeds for reproduction.

What is Bolting?

Bolting is the rapid development of a flower stalk on leafy vegetables.  The plant has been signaled to set seed and complete its life cycle.

When your leafy vegetables grow flowers, that means the juvenile period is over.  It is time to create the next generation, and the plant is responding to natural signals telling it is time to do so.

And that bitter taste?  That comes from compounds the plant produces to protect it from pests that might eat it during this critical reproductive time.  To a plant, pests include you!

Why Do Plants Do It?

Bolting is not bad.  It’s just the plant doing what a plant does. 

Plants do this because they are completing their life cycle.  Plants can bolt when they are physiologically signaled by day length, a phenomenon called photoperiodism – or by stress.

There is not much a gardener can do when plants bolt due to photoperiodism.  As days get longer, spring crops like lettuce, spinach, and some radishes end their juvenile stage and begin reproduction.  

Other plants are signaled to go to seed as the days get shorter.  To ensure you are getting the full harvest before this happens, make sure you are planting at the right time.  

But there are steps a gardener can take when plants are triggered by stress.  Stressors include too much or too little water, too much or too little fertilizer, too hot or too cold temperatures, insect infestations, and disease problems. Basically, any conditions that are not optimal can trigger bolting for a plant prone to do so.

Warm weather and other stressors encourage plants to start bolting

Which Plants Do It?

When planning your vegetable garden, think about more than what to plant.  Also take into consideration what time of year you are planting certain vegetables.  Plants prone to bolt need to be planted at the proper time.

Annual Vegetables

These vegetable crops complete their life cycle in one growing season.  They can bolt when the late spring/early summer days heat up:  

  • Lettuces, arugula, and fennel
  • Basil and cilantro
  • Bok choy and spinach
  • Broccoli
An example of a small floret in the middle of lettuce signifying the beginning of this process.

Biennial Vegetables

Biennial plants are those that take two growing seasons to complete their life cycle.  They produce vegetative growth the first year and then flower and set seed the second year.  

Many vegetables we grow are biennial, but we harvest them in their first year of growth.  These include:

  • Root vegetables like beets, turnips, carrots, parsnips, and rutabaga
  • Onions, leeks, and shallots (shallots won’t form a good bulb once it flowers)
  • Brussel sprouts, cauliflower, celery, and leeks
  • Kale, endive, swiss chard, and collards
  • Cabbage, kohlrabi, and parsley

Cold weather conditions can “convince” these plants that the second year of their lives has arrived, and that can cause them to bolt.  So be sure you are planting them at the proper time for your area. 

Your county extension service can help you determine the right time to plant for a longer, tastier harvest. 

Minimize stress whenever possible and plant earlier in the season.

How to Prevent It?

Once a plant has been triggered to set seed, it can’t be stopped.  Cutting the flower stalk won’t stop the plant from trying to go to seed.  But there are steps a gardener can take to delay the process. 

  • Plant slow to bolt varieties.  Plant descriptions will include phrases like “bolt resistant,” “long standing,” and “holds well.”
  • Keep plant roots cool with mulch.  Plants respond more to soil temperatures than to air temperatures.
  • Use row covers, cloches, and shade cloth to protect annuals from heat and biennials from cold.  Row covers can also offer some late afternoon shade.
  • Feed your leafy vegetables a high nitrogen fertilizer, which encourages vegetative growth.  Look for a fertilizer with a higher first number.  Don’t use a fertilizer with high middle and third numbers – those fertilizers encourage flowering and fruiting.
  • Keep up with your harvesting.  If you continually harvest the outer leaves, your plants will stay in the juvenile vegetative stage for a longer time.
Protecting from the heat of the sun can delay the process from starting.

It Has Its Benefits!

A plant gone to flower invites pollinators.  Their presence is beneficial to your garden as a whole.  

And a plant gone to seed means free seeds for your garden next season.  

The bitter leaves won’t be tasty in a salad anymore, but they can be added to soups and stews where their strong flavor might be appreciated.  

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About the author: Alaine has been working way too hard in horticulture since 1992, beautifying golf courses, resorts, and hotels. She is a part time landscape designer who works full time caring for a 28,000 square foot public garden. At home, she maintains her own 400 square feet plot. Alaine lives in northern Illinois - zone 5b.

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