Broccoli is a popular vegetable grown for its immature flower heads. Most varieties produce dome-shaped heads harvested when the the small buds are still tightly closed. However, if you wait too long to harvest, plant root-bound seedlings, or your garden experiences environmental conditions that affect plant growth, you’ll end up with a broccoli flower. Keep reading to learn more about what causes broccoli plants to flower and what you can do to prevent it from happening.
What is a broccoli flower?
It’s not uncommon for broccoli plants to bolt. Bolting means the plant has started to flower which can affect the eating or storage quality of the crop. The first sign of a broccoli flower is that the head becomes loose and the small green buds start to swell. They may even begin to show the yellow color of the flowers. Once this process starts, it only takes a few days for the plants to fully bloom. Bright yellow broccoli flowers are quite beautiful, but that’s not the reason we grow broccoli. We want dark green heads of tightly packed florets. So let’s learn more about why broccoli plants bolt.
Why does broccoli flower?
Now that we know a bit more about what it means for broccoli to bolt, let’s explore the causes for bolting. Below you’ll find 8 reasons why your broccoli plants may start to flower.
1) Planting over-mature seedlings can cause broccoli to flower
There are two ways to plant broccoli in a garden: 1) you can sow seeds or 2) you can transplant seedlings. I typically transplant seedlings in my garden to get a jump start on the harvest. I also find starting with young plants reduces seedling loss from hungry slugs and other critters. To start broccoli seedlings indoors, sow the seeds under grow lights or in a sunny window 4 to 5 weeks before you intend to transplant them into the garden. They’re typically very quick to sprout and you can expect to see the young plants emerge in 7 to 10 days.
If you start broccoli seeds indoors too early, you’ll end up with pot-bound, stressed seedlings by the time you’re ready to move them outdoors. That stress can cause immature broccoli plants to ‘button’. Buttoning occurs when a young plant produces a small head early in the season. The head never grows to a harvestable size and the plants should be pulled and tossed in the compost. To reduce the occurrence of buttoning start the seeds indoors at the right time and harden off and transplant them into the garden while the seedlings are still healthy and vigorous.
If buying broccoli seedlings from a garden centre, carefully pop one out of the cell pack or container to insect the roots. If the root system is very mature and there are a lot of circling roots, don’t buy the seedlings.
2) Reduce flowering by planting broccoli in an ideal site
Like most vegetables, broccoli needs full sun. A garden bed that offers at least 8 hours of direct sunlight each day is perfect. You can grow broccoli in an in-ground garden, raised beds, or even containers. If growing in pots, opt for large pots or fabric planters to minimize plant stress. They should be at least 16 inches in diameter and 12 inches deep. Small pots dry out quickly resulting in stressed out broccoli plants that may flower instead of producing sizeable heads. Well draining soil is also essential. Avoid planting broccoli seeds or seedlings in garden beds where water doesn’t drain quickly as they may rot or die.
3) Excessive cold or hot temperatures can cause broccoli to flower
The ideal temperature for a cool-weather crop like broccoli, a member of the cabbage family, is in the 60 to 70 F (15 to 21 C) range. It’s hard to predict what Mother Nature has in store for us weather-wise, but if temperatures dip below 55 F (13 C) for a prolonged period of time the young broccoli plants can bolt, or produce a broccoli flower. Aim to transplant the seedlings once the soil temperatures and night temperatures are both reliably over 60F.
Cold temperatures can cause a broccoli plant to flower, but so can hot temperatures. A temperature that exceeds 86 F (30 C), impacts broccoli head growth and development. The plants don’t form smooth, rounded heads with uniform bud size. Instead, the broccoli heads grow uneven and bumpy with varied bud sizes. They’re fine to eat, but not as visually appealing and won’t last very long in the garden.
4) Overcrowding broccoli plants can cause them to flower
As noted above, stressed broccoli plants are prone to bolting. Reduce plant stress by spacing broccoli seeds or seedlings at the appropriate distance. Read the seed packet for specific variety spacing recommendations. Generally broccoli seeds are spaced an inch apart and eventually thinned to 12 to 18 inches apart. If transplanting seedlings, space them 12 to 18 inches apart. Space rows 18 to 36 inches apart. Again, plant spacing depends on the variety you’re planting.
5) Reduce stress from weeds, insects, and other pests to stop bolting
Competition from weeds or damage from insects and other pests may stress plants and result in bolting broccoli. Young broccoli seedlings don’t compete well with aggressive or dense garden weeds. If they have to fight for water, nutrients, and sunlight, they may lose out to the weeds. Try to pull weeds often and use a mulch like straw or shredded leaves to reduce weed growth around broccoli plants. My go-to short handled weeding tool is a Cobrahead and my favorite long-handled weeding tool is a collinear hoe. They make quick work of weeding.
Common broccoli pests include slugs, aphids, imported cabbageworm, white grubs, wireworms, and cutworms. An easy way to prevent pests like imported cabbage worms or aphids from nibbling on your broccoli plants is to float insect netting or lightweight row cover on hoops overtop the bed. Make sure to bury the edges under the soil, use garden staples, or weigh them down with rocks or other heavy materials. This prevents a pest from sneaking underneath.
6) Infertile or poor soil can cause broccoli to flower
A fertile soil with a soil pH in the 6.0 to 6.8 range is ideal for broccoli. I test my garden soil every 2 to 3 years using a soil test kit or sending a sample off to my local extension office. The results tell me whether I need to lime my naturally acidic soil or add fertilizers to correct a nutrient imbalance.
My strategy is simple. I amend the soil with an inch of compost or well rotted manure before planting. I also incorporate a slow-release organic vegetable fertilizer into the soil to ensure a good supply of nitrogen, phosphorous, potassium, and other essential nutrients. Side-dress broccoli plants 6 weeks after transplanting with more granular fertilizer, or use a liquid vegetable fertilizer every 2 to 3 weeks during the growing season to promote healthy growth and big heads.
7) Water stress can prompt broccoli flowers
Broccoli, as well as other cabbage family vegetables like Brussels sprouts and cauliflower, have shallow root systems. That means they need regular irrigation to form vigorous plants. I use a long handled watering wand to deliver an inch of water to the base of each plant on a weekly basis. If you’re not sure whether you should water, stick your finger into the soil of the garden bed. If it’s dry an inch down, water. Using a layer of mulch on the soil surface not only reduces weed growth but it also helps retain soil moisture. Apply 2 to 3 inches of straw, shredded leaves, or untreated weed-free grass clippings around the plants.
8) Harvest broccoli heads at the right time to stop flowering
One of the biggest advantages of having a vegetable garden is harvesting your crops when they’ve reached peak flavor, quality, and nutrition. Knowing approximately when your broccoli crop will be ready to harvest starts with reading the ‘days to maturity’ information listed on the seed packet. Every variety of broccoli has a certain range of time it needs to go from seed to harvest. Most broccoli varieties require 60 to 70 days to mature.
Pay close attention to your crop as the days to maturity date nears so you can harvest when the dome-shaped heads are full of tightly packed buds. If you wait too long, the broccoli flowers will develop and the florets start to separate, the individual buds swell, and the yellow color of the blooms become visible.
Can you eat a broccoli flower?
If you find a broccoli flowering in your garden you may wonder whether it’s still edible. It is, although bolted vegetables often become more bitter tasting. Ideally, aim to cut your broccoli heads at the tight bud stage, when the head is firm. If you spot a plant beginning to bolt, harvest the head immediately. Don’t forget that broccoli stalks, stems, and leaves are also edible. Once the central head has been harvested, side shoots develop. These smaller broccoli heads produce a secondary crop, so all is not necessarily lost if you find the main head of your broccoli is flowering.
If you happened to be away from your garden for a few days and come home to broccoli plants in full bloom, you can pick some of the small flowers and sprinkle them on salads or other dishes. Or, you can let the plants bloom for the bees, pollinators, and beneficial insects. They love the yellow flowers!
For more info on growing broccoli and related vegetables, be sure to check out these detailed articles:
- When to harvest broccoli
- Growing broccoli from seed
- How to grow broccoli sprouts
- How to grow cabbage from seed to harvest
- Learn how to grow brussels sprouts
Have you had your broccoli flower on you?
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