Why Is My Broccoli Growing Tall and Flowering (Broccoli Bolting)


Let me guess: you have been taking good care of your broccoli plants, and then they start to grow tall and flower.  This can be concerning, but there are ways to address the issue to prevent it from happening.

So, why is your broccoli growing tall and flowering (bolting)?  Broccoli grows tall and starts to flower in order to form seeds and complete its reproductive cycle.  This happens in response to stress, and extreme soil temperatures.  Sometimes broccoli will grow tall and bolt before the broccoli has a chance to form a large head.

Of course, there are steps you can take to prevent broccoli from bolting so that you can increase your harvest.

In this article, we’ll take a closer look at how to prevent bolting broccoli. We’ll also address some other common broccoli problems, such as broccoli plants forming small heads or falling over.

Let’s get going.

Why Is My Broccoli Growing Tall and Flowering? (Broccoli Bolting)

According to the University of Maryland Extension, broccoli will start to grow tall and form flowers in response to:

  • stress
  • high soil temperatures (too far above 70 degrees Fahrenheit)
  • low soil temperatures (too far below 50 degrees Fahrenheit)
broccoli flowers
Broccoli will grow tall and produce flowers in response to stress, or extreme soil temperatures.

This Youtube video I made gives a summary of the causes of tall and flowering broccoli, along with some prevention methods.

Let’s address each of these issues in turn.

Broccoli Growing Tall and Flowering Due To Stress

When a broccoli plant gets stressed, it is more likely to grow tall, produce flowers, and bolt (or “go to seed”). According to the Michigan State University Extension, there are several things that can cause stress to broccoli plants, including:

  • cool temperatures in early stages of growth
  • late transplant
  • increasing day length

Cool Temperatures In Early Stages Of Growth

Before transplant, broccoli seedlings should be kept in warm conditions. Ideally, the temperature should stay above 70 degrees Fahrenheit (21 degrees Celsius).

young broccoli plants
Exposure to cold temperatures in the seedling stage can cause problems for broccoli plants later on.

If broccoli seedlings are exposed to colder temperatures, there is a risk that the plant will not vernalize properly later on. This can lead to eventual stress and bolting.

Late Transplant

In addition, broccoli that is transplanted too late is subject to the stress of transplant shock. One of the most common causes of transplant shock is root damage.

A mature plant has a larger, more extensive root system. The longer you wait to transplant, the more chance of root damage and transplant shock.

broccoli plant
Late transplant makes root damage and stress more likely for broccoli plants.

The University of Maryland Extension suggests transplanting broccoli outside 4 weeks before the last spring frost date, when plants are 5 to 7 weeks old.

This means you should start broccoli seeds 9 to 11 weeks before the last spring frost date. You can find the last spring frost date for your region with this tool from the Old Farmer’s Almanac.

Increasing Day Length

Finally, increasing day length can also cause bolting in broccoli plants. In a sense, the plants “know” how long the days are.

sunrise
As the days become longer, broccoli and other plants will sense it, and may try to produce flowers and seeds.

If planted too late in the season, broccoli will sense that the days are long. It will then rush through its growth stages in an attempt to produce seeds.

Instead of forming a full broccoli head, the plant will simply grow tall to produce flowers and seeds.

Note that disease, drought, and nutrient deficiencies can also cause stress to a broccoli plant. These factors can cause bolting as well.

Broccoli Growing Tall and Flowering Due High Soil Temperatures

Broccoli is a cool-weather crop, so soil that is too warm can also cause broccoli to grow tall and bolt. According to the Ohio State University Extension, the best temperature range fro broccoli growth is 65 to 70 degrees Fahrenheit (18 to 21 degrees Celsius).

The more extreme the temperature, the more risk of a broccoli plant growing tall, flowering, and bolting. When the temperature rises, a broccoli plant will begin the next phase of its life cycle.

This means that the broccoli plant will grow tall, form flowers, and try to produce seeds in order to reproduce. This is known as bolting, and it can affect the flavor of the broccoli.

broccoli flowers
High soil temperatures may cause broccoli to start forming yellow flowers from the green buds on the head.

High temperatures can cause other problems besides bolting in broccoli. According to the University of Massachusetts, there is a period of 10 days when broccoli is more sensitive to heat stress (this period is prior to crown formation).

Exposure to temperatures over 95 degrees Fahrenheit (35 degrees Celsius) for more than 4 days during this 10-day period can cause poorly shaped heads.

Broccoli Growing Tall and Flowering Due To Low Soil Temperatures

Cold soil temperatures can also cause broccoli plants to grow tall and flower. According to Clemson University, a mature broccoli plant will bolt:

“if exposed to a prolonged cold period of 10 or more consecutive days of temperatures between 35 and 50 F following a period of favorable growing conditions.”

https://hgic.clemson.edu/factsheet/broccoli/

Even younger transplants are subject to bolting due to cold temperatures. According to the University of Maryland Extension, temperatures below 45 degrees Fahrenheit (7 degrees Celsius) can cause bolting or even stop a broccoli plant from forming a head.

Always check the weather forecast before transplanting. If cold is expected, wait a few days to transplant.

thermometer
Check the weather forecast before transplanting, and avoid extreme cold.

You could also use a cloche to keep young broccoli plants a little warmer. A row cover is an option for more mature broccoli plants.

Will Broccoli Grow After Flowering?

The main head of broccoli will not grow after flowering – but the side shoots will grow if you cut off the main head. Remember that the flavor of the head will become bitter if yellow flowers start to form.

Remember that a broccoli plant wants to form flowers and produce seeds to reproduce. Once it has done so, the plant is satisfied that it has completed its mission in life.

bolting broccoli
This broccoli has bolted (it is starting to flower and produce seeds). Image from: https://commons.wikimedia.org/wiki/File:Broccoli_Flowers.jpg

At that point, it will stop growing the main head. To get a big head of broccoli from your plant, you will need to prevent it from flowering too early (more detail on this later).

When the head of broccoli is large and mature, harvest it by cutting it from the plant.  Make sure to do this while the buds of the broccoli are still green and packed closely and tightly together.

If the buds start to separate and grow taller, cut off the head!

Usually, the head is mature when it has a diameter of 4 to 6 inches (10 to 15 centimeters). However, some heads of broccoli can get up to 8 inches (20 centimeters).

Of course, the size of a broccoli head can vary by variety, so be sure to check the seed catalog. After the main head is cut off, you can harvest side shoots, which may be only 1 to 2 inches (2.5 to 5 centimeters) in diameter.

broccoli head
Mature heads of broccoli can be 4 to 8 inches in diameter.

For more information, check out my article on how big broccoli plants get.

Can You Eat Broccoli That Has Flowered or Bolted?

Yes, you can eat a head of broccoli that has flowered. However, the main broccoli head will not grow well after flowering.

You can eat the flowers themselves, along with and any side shoots that grow. Just remember that they may become bitter, as many plants do after they go to seed.

You can also eat the leaves of a broccoli plant. This means it is not a total loss if the plant flowers early.

You could use broccoli leaves in a salad, or steam them just like spinach.  You can even eat the stems of broccoli if you want!

broccoli plant
Even if you don’t end up getting a large head of broccoli like this one, you can still eat the leaves!

Just be sure to avoid eating the seeds and roots of a broccoli plant, which are poisonous.

How To Prevent Broccoli From Bolting

You may feel powerless to prevent broccoli from bolting.  However, you have more control than you think.

There are several ways to prevent broccoli from bolting, including:

  • choose heat tolerant broccoli varieties
  • choose fast maturing broccoli varieties
  • insulate broccoli plants against extreme temperatures

Choose Heat Tolerant Broccoli Varieties

First, when planning your garden, choose heat-resistant broccoli varieties. These are less likely to bolt when hot weather comes around in the summer.

broccoli
Heat tolerant broccoli can still form large heads, even in high temperatures.

Some heat tolerant broccoli varieties include:

Choose Fast Maturing Broccoli Varieties

Another way to prevent bolting is to choose broccoli that will mature faster. That way, you can completely avoid cold a the start of the season or heat later in the season.

Some fast-maturing broccoli varieties include:

Insulate Broccoli Plants Against Extreme Temperatures

It is worth planting your broccoli in a place where it will not be subject to extreme temperatures. Try to find a spot so that the broccoli is shaded during the hottest part of the day, which is early to mid afternoon.

sunlight through forest
Intense sunlight can raise soil temperatures and cause your broccoli to bolt, so try to keep them shaded for part of the day.

To provide insulation, add layer of mulch or compost over the soil near your broccoli plants.  This will prevent the soil temperature from changing so quickly on a hot day or cold night.

For more information, check out my article on compost and mulch.

Other Problems With Broccoli Plants

There are a few other problems that you may encounter with broccoli plants. Here are a few of them, along with an explanation for why each one occurs.

Why Are My Broccoli Plants Falling Over?

Your mature broccoli plants may fall over if they bolt due and become very tall.  This is especially likely after the plant flowers and goes to seed.

Younger broccoli plants in the garden may fall over due to pest or disease damage.  For example, cutworms love to chew through the stems of young, tender plants before they become too tough.

cutworm
A cutworm can chew through the base of young broccoli plants and sever them at soil level.

A cutworm collar is one way to prevent damage from these pests. For more information, check out my article on how to get rid of cutworms.

Broccoli seedlings may fall over if they become “leggy”, or long and spindly, due to stretching as they grow to reach up higher for a limited light supply.

Why Are My Broccoli Plants Producing Small Heads? (Broccoli Buttoning)

Broccoli plants will button, or produce small heads, for several possible reasons.  One of the main reasons is exposure to cold while they are young and still developing.

Broccoli Buttoning Due To Cold Exposure

Although broccoli prefers cool weather, they do not necessarily like extreme cold.  Exposure to cold temperatures of less than 40 degrees Fahrenheit (4 degrees Celsius) for a week or more can cause broccoli plants to form small heads.

frost
Cold and frost can damage broccoli plants so that they will never produce large heads.

Buttoning is more likely if the broccoli plant is exposed to frost.  According to the Texas A&M Extension, temperatures of 26 to 31 degrees Fahrenheit (-3 to -1 degrees Celsius) will burn the leaves of broccoli plants, but may not kill them.

Your broccoli plants are more likely to be exposed to cold temperatures if you plant too early in the spring. However, cold exposure is also a risk if you plant too late in the fall for a 2nd harvest.

To protect your plants from cold outside, use cloches or row covers.  For more information, check out my article on how to protect plants from cold and frost.

You can also start your plants early, growing them from seeds indoors to protect them from the worst of the cold.  Be sure to check frost dates before transplanting broccoli outside.

For more information, check out this frost date calculator from the Old Farmer’s Almanac.

Broccoli Buttoning Due To Lack Of Water Or Nutrients

Broccoli may also button due to stress caused by a lack of water or nutrients.  Especially in the hot, dry summer months, you will want to make sure your plants get enough water (lack of water can also lead to brown heads of broccoli).

If you have problems with dry soil, you can try mixing compost into your garden. Adding a layer of mulch over your topsoil will also help.

The compost will improve the soil structure, which will help to retain water. The mulch will insulate the soil and prevent the sun from evaporating water so rapidly.

For more information, check out my article on how to treat dry soil.

garden hose
Be sure to water your broccoli adequately – but not too much.

Keep in mind that it is also possible to over water plants. For more information, check out my article on the signs of over watering plants.

Another possible source of stress for broccoli plants is soil pH.  Broccoli plants prefer a soil pH of 6.0 to 6.8 (slightly acidic to neutral).

If your soil pH is far outside of this range, then your broccoli plant may suffer a nutrient deficiency.  For more information, check out this article from Research Gate on the effect of soil pH on nutrient availability.

A soil test is really the only way to find out exactly what your soil pH is.  You can buy a do-it-yourself test kit online or at a garden center.

However, you can also send a soil sample to your local agricultural extension office for testing.  If you send information about what you are growing, the lab can also send back recommendations on how to treat your soil.

For more information, check out my article on how to do a soil test.

If your soil pH is fine, then you should consider the possibility of a nutrient deficiency in your soil.  Generally, a soil test will also tell you about such problems.

Broccoli plants are considered heavy feeders, which means they use up lots of nitrogen.  Adding compost to your soil and using a balanced fertilizer that provides nitrogen, phosphorus, and potassium should provide your broccoli plant with the nutrition it needs.

Still, you should be careful with stronger fertilizers.  It is possible to burn your plants with fertilizer by adding too much at once, especially during dry weather.

For more information, check out my article on over fertilizing your plants.

Why Are My Broccoli Plants Not Producing or Forming Heads?

Broccoli plants can become damaged during handling when they are young and vulnerable.  This is more likely if you transplanted them from your house or a garden center instead of growing them from seed.

In this case, the broccoli plant may never form a head at all.  Instead, you may need to settle for using the leaves of the plant for a salad or steaming them like spinach.

As mentioned earlier, extreme cold or heat can also prevent broccoli from forming heads of the proper shape and size.

broccoli
Exposure to extreme heat or cold can prevent broccoli plants from forming full heads.

Conclusion

Now you have a much better idea of why your broccoli plant is growing tall and flowering.  You also know about some methods to prevent the problem in the future.

I hope you found this article helpful – if so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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