A bitter end to a sweet crop: why do plants ‘bolt’? by Jeff Pieper

What is bolting?

Bolting is when a plant’s growth suddenly goes from being leaf based, or vegetative growth, to being flower and seed based, or reproductive growth. A tell-tale sign that a vegetable has bolted is the elongation of the stem, or the formation of a central flower stalk in a very short period of time. Once you see this you know that the vegetative growth stage is over and that flowering, or reproductive growth stage, has begun.

Many of the plants that we find bolting here in Eagle County are plants that we are trying to grow out of their preferred growing conditions, think of trying to grow lettuce in the heat of the summer. Often times when plants switch to a reproductive growth stage it consequently leads to declining flavor, i.e., a bitter taste and a woody texture.

Why does bolting occur?

Bolting is a survival mechanism for a plant. The majority of the plants we grow in our gardens are annuals, and an annual plant’s life-cycle is to grow and produce seed in one season. That way the seed is mature and ready to over winter, germinate next season, and start the cycle all over again, perpetuating the species. For most of our cool season crops, we gardeners want the vegetative part of the plant’s life cycle to last as long as possible, but if our plants become stressed, the vegetative cycle halts and the reproductive cycle begins.  Stressors that trigger bolting include too much heat, too much cold, dry soil or overcrowding. These stressors may trigger a growth hormone, gibberellin, which signals to the plant that it is time for the last cycle; seed production. Mountain gardeners have even more of a challenge due to the short growing season and cooler spring temperatures; the stresses come not only from a sudden warm spell, but also from the cold weather the plant experienced early in the season. If plants are planted when the weather is too cool, this could signal the plant to bolt as soon as the weather warms up.

How to prevent bolting?

Bolting cannot be reversed, but there are ways to delay the process.

·         Provide rich soil that facilitates faster growth due to the abundant supply of nutrients.

·         Give plants a head start indoors under lights and place them outside while it is still cool.

• Sow frost resistant varieties before your last spring frost.
• Use mini hoop tunnels and other season extension tools to add a couple of weeks to your spring gardening. This provides a larger window to allow the spring greens to grow a good size before the heat of the summer.
• Practice succession sowing. Sow your vegetables every week or two instead of all at once, to increase your chances of germination and early harvest.
• Plant vegetables that are likely to bolt, in the shade, perhaps on the north side. Temperature fluctuations can be remedied by using a shade cloth to protect your plants on hotter days (75 degrees or more). 
• Grow a mix of cool season and bolt resistant varieties such as Orach (mountain spinach) or Malabar spinach (likes the heat).
• Thin seedlings promptly to eliminate overcrowding.
• Provide ample water throughout the growing season, especially when temperatures rise.

Lettuces can be kept from bolting by regularly picking the outer leaves, keeping them from maturing properly. This ‘cut and come again’ approach to harvesting can extend the time they produce for up to 10 weeks. Crops most likely to bolt are lettuce, radish, radicchio, spinach.  Biennial crops such as onion, beets, cabbage are less likely to bolt since we harvest them in the first year, and they bolt the second year, however, planting too early can cause bolting in biennial plants.

According to Colorado State University (CSU) Extension, cool season vegetables (leafy greens, root vegetables, and herbs) are easiest to grow and most productive for mountain gardeners. Look for varieties that have been bred to be resistant to bolting. Swiss chard and kale are naturally bolt resistant as are Correnta spinach and Spinner spinach. See CSU Extension web site for more information about growing vegetables in mountains gardens: http://www.ext.colostate.edu/pubs/garden/07248.html

Please email jeff.pieper@eaglecounty.us or call the CSU Extension office 970-328-8630 for more information about bolting or any other gardening questions.

A complex signaling system tells plants to stop producing leaves and to start producing flowers. This lettuce plant switched to a reproductive growth stage once temperatures got too hot.

 Onions are biennials so you most likely won’t see them flowering in your garden, however, if you leave them to over winter, or plant them too early, they will bolt the following season.