Broccoli is a member of the Brassica family which includes cabbage, cauliflower, kale, Brussels sprouts, and radishes.  It is a favorite of new and experienced gardeners alike.  As a cold weather crop, it is best suited for late fall and early spring gardens.  While it is relatively easy to grow, sometimes you can run into problems with the heads not forming properly or bolting before they are ready to be harvested. 

If your plant bolts (i.e. flowers) before the head fully forms, it spells an end to your hopes for a harvest.  While all may not be lost–you may be able to save the seed–you won’t be eating homegrown broccoli.  When this happens, it is a sign that the plant has been experiencing stress.  Here are three sources of stress that can cause broccoli plants to bolt prematurely and what you can do to prevent it.

1. Warm Temperatures

Broccoli prefers temperatures between 65°F to 80°F (18° to 27°C).[1]  When the soil temperature becomes too warm, it sends a signal to the plant that it is time to flower so that it can produce seed.  Therefore, it is important to plant broccoli at the right time of year so that it grows in a climate that is conducive to its development.  Sometimes, however, we experience unseasonal temperatures, especially if you live in the south like I do, that can send mixed signals to your broccoli.

 This article contains affiliate links.  If you make a purchase using one of these links, I will receive a very small commission at no additional cost to you, and it will help me maintain this website.  Rest assured, I only recommend products I actually like!

If the forecast for your area predicts warmer than average temperatures, here are a few things you can do to try to prevent the soil from getting too warm and triggering your plant to flower.  First, water frequently so that the soil stays cool and moist, but not soggy.  Second, add a good layer of mulch.  This will help keep the soil temperatures cooler.  Third, install a row cover like the one I use to reduce the amount of solar energy the plant and soil absorb.

Although less common than warm weather, temperatures below 40°F can also cause broccoli to flower if the plant is young.  To prevent this from happening, you can add mulch and a row cover, or start them off in a cold frame or greenhouse.

Note, if you add woodchips as mulch, you will need to keep an eye on the nitrogen levels because woodchips can absorb nitrogen from the soil.  This leads us to the second stressor that can cause premature bolting: nitrogen deficiency.

2.  Nitrogen Deficiency

Broccoli is notorious for being a heavy nitrogen feeder.  If you notice the lower leaves of the plant beginning to yellow, it could be a warning sign that the plant is not getting enough of this important nutrient.  You’ll want to give the plant a dose of nitrogen right away.  If you wait, it can quickly affect the heads.  If you see that the heads are starting to loosen, it may be too, late but try anyway.

To avoid this problem, check the soil to see if the nitrogen levels are acceptable.  This is the test kit I have.  It is easy to use and results are ready in just a few minutes.  You can increase nitrogen levels by adding blood meal to the soil before planting.  When you see the heads beginning to form, add a liquid fertilizer, like fish emulsion or some other type of fertilizer, just make sure it is high in nitrogen and low in phosphorus.  Apply fertilizer again after you have harvested the main head to ensure that the smaller heads or side shoots will form properly.  If you are growing broccoli in containers you may need to fertilizer more often as the soil does not retain the nutrients as well as in-ground gardens do.  When adding fertilizer of any kind, be sure to follow the manufacturer’s directions because excessive nitrogen can also cause the heads to loosen. 

3.  Watering Issues

Another condition that can cause broccoli heads to loosen or flower prematurely is inconsistent watering.  Just like tomatoes and beets, broccoli does not do well with fluctuating soil moisture.  Water the plants thoroughly when the soil feels dry an inch below the surface.  Plants in containers will dry out quicker than plants in the ground so be sure to check more often.

Broccoli is a wonderful addition to any garden, so be sure to take the necessary precautions so that you will be able to harvest and enjoy fully formed heads!

Thank you for reading this article!  If you found it helpful, please consider sharing it with others on social media!   

[1] Many experienced gardeners say the range is more like 65°F to 75°F (18° to 24°C).