Growing Broccoli Successfully: How to Prevent Bolting and Get Big Heads

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Watching your homegrown broccoli heads develop from little buttons into lush florets is pretty exciting, mainly because broccoli plants are one of the more fickle among vegetables. A lot of gardeners have seen many a broccoli crop grow into big, gorgeous plants, only to go straight to flowering instead of forming heads.

This isn’t just bad luck. Okay, well, sometimes it is. Unseasonable, unfavorable weather situations come out of nowhere during certain years, and there’s only so much you can do to soften the blow. That aside, there are some essential steps you can take to give your broccoli a much better shot at heading up and yielding a nice harvest.

Variety, timing, good maintenance, and protection from the elements all come into play to avoid flowering broccoli and other broccoli growing problems.

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What Causes Broccoli to Bolt?

Broccoli plants are notorious for getting in a hurry on flowering, or bolting. They’ll form small, loose heads (or skip the heading stage entirely) and start producing tall flower stalks instead of the tight bunch of florets you were hoping for. Soon, those stalks will be covered in little yellow flowers. Once bolting begins, there’s no way to stop or reverse the process. Sadly, bolted broccoli will never yield an edible harvest.

Bolting in broccoli plants happens for one of two main reasons: temperature and plant stress.

Broccoli is technically a biennial, meaning it should produce vegetative growth in its first year, overwinter, and then flower and set seeds the next season. However, it’s a bit sensitive and easily fooled into thinking it’s already completed its first season of growth. Temperatures that climb too high or drop too low can both trigger early flowering, depending on where the plants are in their life cycle.

First off, a prolonged period of cold weather when your plants are seedlings often triggers bolting when they start to mature. All it takes is a cold snap where temperatures stay below 45 for three or four days to convince broccoli seedlings they’ve experienced winter. One night with temperatures in the low 20s may have the same effect (especially if the seedlings experience freeze damage). Hot weather, especially if it comes on suddenly, causes bolting in mature broccoli plants.  

Environmental stress is the other major culprit for bolting broccoli. Insufficient water, high pest pressure, root damage, or anything else that makes the plant think its survival is in jeopardy can flip the switch.     

Now, let’s get on to what you can do to keep broccoli plants happy and prevent situations that trigger bolting and harvest loss.

Choosing the Right Broccoli Varieties

Doing your homework before settling on a broccoli variety to try is important, since some varieties will grow better than others in your area. Your weather and the season you plan to grow in are both factors to consider.

Certain broccoli varieties are slower to pull the bolt trigger when late spring temperatures jump, making them good for spring growing in warmer climates. Other broccoli types can tough out hard frosts and colder days. These are best for a fall crop in areas where temperatures will drop before the plants reach maturity.

Second to climate tolerance, you’ll also want to consider what kind of harvest you’re after. Some varieties produce one large central head before winding down for the season. Others will produce a smaller main head, and, once it’s cut, will continue to yield lots of little side shoots for multiple harvests.

Below are a few good broccoli varieties to try:     

Heat Tolerant Broccoli

Green Magic- Produces large and uniform heads with a lovely blue-green color. Will produce side shoots after main head is harvested. This variety is also quite compact for broccoli, making it perfect for smaller spaces.

Sun King- Similar to Green Magic, Sun King takes a bit longer to mature and produces slightly larger heads.

Belstar- Another compact broccoli variety that yields uniform heads and plentiful side shoots, Belstar is also known for good disease resistance.

Cold Hardy Broccoli

Waltham 29- A slower-maturing variety that thrives in cooler temperatures, this heirloom is perfect for growing out into the winter months and has a pronounced sweet flavor.

Marathon- Yields large, tight heads with notably small “beads.” This is a go-to fall crop variety for market gardeners further north because of its cold tolerance, perfect heads, and reliability.  

Multiple Harvest Varieties

Destiny– A heat tolerant, fast-maturing variety that yields medium heads followed by many side shoots for “cut and come again” harvests throughout the summer.

De Cicco– This heirloom is another variety you’ll be harvesting from for months. Though the main heads are small (three to four inches), it’s one of the top varieties for side shoot production. De Cicco doesn’t like heat and will do best in the fall in warmer climates.

Note: When it comes to broccoli, heat tolerant doesn’t mean heat proof. It’s still important to time your planting so that your broccoli isn’t trying to mature during the hottest part of summer.

Starting Broccoli From Seed

While you’ll see broccoli seedlings available at garden centers, I strongly recommend starting your own transplants from seed. If you have control over their environment when they’re small, you can avoid stressors that could cause bolting later.

It’s possible to direct sow broccoli in the garden, but you’ll almost always get better results from starting seeds indoors. Broccoli shouldn’t be planted until soil temperatures are above 50. If you have seedlings ready to go by that time instead of seeds, you’ll have around a month’s head start. That gives your plants more time to head up before the summer heat becomes risky.

Fall broccoli crops do best when started indoors as well, since you’ll need to start them during the summer when it’s still hot out.

When to Sow

For spring: Broccoli seedlings are cold hardy and can be planted out several weeks to a month before your average last frost date. Sow your seeds around 6 to 8 weeks before that date, or roughly 4 to 6 weeks before you intend to plant them out.

For fall: Start broccoli seeds for fall about 10 to 12 weeks before your first fall frost date, aiming to plant them 6 weeks or so before then. You can start later if you don’t get a lot of freezing temperatures over the winter, but don’t wait TOO late. Lower sunlight intensity in the dead of winter can hinder growth even in warmer climates.

How to Sow Broccoli Seeds Indoors

Since broccoli seeds are quite tiny, you’ll need a light, fine-textured seed starting mix to get good germination. Peat moss or coco coir-based seed starting blends work best and are inexpensive. Avoid potting soil or anything with compost and/or lots of woody bits. Compost (and garden soil) contains germs that can cause problems for tiny seedlings in a contained environment indoors.    

Soil Temperature: Broccoli seeds will sprout fastest in soil temperatures between 70 and 80 F. To get germination within two or three days, you can use a seedling heat mat to warm the soil up. This isn’t a requirement, though; they’ll just take a couple days longer to emerge at room temperature. If you do use a heat mat, remove the tray from the heat as soon as the seedlings sprout.

Overseeding: Broccoli seeds have pretty solid germination rates. However, it’s not a bad idea to sow two or three seeds in each container. Then, you can thin them and keep only the strongest seedlings (one per cell) once they start to fill out.

Moisten your seedling mix well before sowing and plant broccoli seeds ¼ inch deep. I like to make little furrows with a plastic knife, drop the seeds in, and carefully push the soil back in place. After sowing, water your seeds in well. A spray bottle works best to avoid displacing the tiny, shallow seeds or washing the soil away from them.

Preventing Damping Off and Leggy Seedlings

Like other cole crops, broccoli sprouts are prone to damping off or root rot if the soil stays soggy for too long when they’re small. Starting them in smaller containers (3” deep or less) and potting them up later helps prevent this. Smaller seedling cells don’t stay saturated for as long after watering.

You’ll need to keep the soil surface good and damp with that spray bottle while you wait for germination. Once the seedlings emerge, though, be careful not to overwater them. Wait until the soil looks mostly dry on top between each watering.   

Grow lights are also a must for growing healthy broccoli seedlings. They get “leggy” easily, so window light definitely won’t be enough. Put your seedling trays under the lights right after sowing, since “leggy-ness” can happen in no time after they sprout. Unless you’re using expensive, high-powered bulbs, LED grow lights should be only a few inches from the tops of your seedlings. Watch for any “stretching” and move the lights closer if needed.

Potting Up and Hardening Off Broccoli Seedlings

To prevent seedling stress that may cause bolting later on, it’s essential not to let your broccoli seedlings get rootbound at any point. I like to play it safe and pot mine up into bigger containers as soon as they’ve developed a full set of “true” leaves. At this point, the roots are well developed and no longer vulnerable to rot.

It’s also important to thoroughly harden off broccoli seedlings before transplanting. The more time you take introducing them to outside conditions, the less likely they’ll experience major transplant stress. Read this post to learn more about properly hardening off your transplants: How to Harden Off Seedlings the Right Way (and Prevent Transplant Shock).

Soil Needs for Broccoli

It may be finicky about temperatures, but broccoli is adaptable when it comes to soil. It’ll grow well in raised beds or in-ground, as long as the soil drains properly and isn’t too acidic. It also doesn’t mind clay-heavy soil. That makes broccoli a good candidate for planting in-ground so you can save raised bed or container space for less-tolerant crops.

Broccoli is a heavy feeder and really likes some compost for extra nutrients. If your soil is lower in organic matter, mix some good compost into the top few inches of soil where you’ll plant your seedlings.

When to Transplant Seedlings

Check your weekly forecast before planting broccoli seedlings and wait until the threat of hard freezing temperatures (below 25) has passed and daytime highs look to be mostly above 45 F. Again, too many cold days in a row can trigger bolting later on.

The seedlings can handle some frost, but it’s best to transplant them when you’re expecting a few days of milder conditions. That way, they can adjust before having to cope with serious cold.  

Space broccoli seedlings about 1.5 to 2 feet apart; the plants get BIG! If you’re planting in ground, broccoli likes being planted on small mounds or raised rows for good drainage. When planting, bury the stems up to the first little seedling leaves (cotyledons) to encourage a deep, stable root system.

Note: Broccoli seedlings raised during the summer for fall planting can get a little tall, due to the warm temperatures. They may get top-heavy and lean over. Staking lanky seedlings while young and prone to flopping will help them develop upright. Broccoli really doesn’t mind growing lopsided, but leaves touching the dirt increases the chances of disease.

Watering and Fertilizing Broccoli

Watering

Broccoli plants need plenty of moisture to support their vigorous growth, and they don’t enjoy dry conditions. Allowing the soil to dry out too much between waterings can cause water stress and trigger early bolting.

To keep your plants happy and hydrated, aim for consistently damp (but not constantly flooded) soil. Installing a soaker hose in your broccoli bed at planting time makes it a whole lot easier to maintain that deep, even moisture the plants need to thrive.    

Mature, thirsty broccoli plants can drink the soil dry quicker than other crops do. Check the soil every few days during warm and dry weather, and water the plants well if it starts to feel dry at the surface. A good layer of mulch will also help a lot to retain soil moisture and slow drying.

Fertilizing

Broccoli needs plenty of nutrients to grow into large plants and produce full heads. Since the plants require a lot of nitrogen, compost alone usually isn’t enough unless your soil is very rich.

Slow-release nitrogen fertilizers are best for broccoli, since they’ll continue supplying nutrients as the plants grow. Good organic options include blood meal and feather meal. Side dress young plants at or shortly after transplanting time so the nutrients will be available as they start to grow. For an added boost, water in the dry fertilizer with liquid fish emulsion.

Broccoli can use some extra phosphorous and potassium, too, if your soil is low in either. We like to have our soil tested at our county extension office every few years to see what’s needed. Basic soil tests usually cost less than $20 through an extension office, and targeted fertilizing can save you a good bit of money.

Read this post to learn more about using organic fertilizers: Fertilizing Your Garden Organically: The Basics + Some Natural Fertilizer Options.

How to Protect Broccoli Plants From Pests

Broccoli, like other brassicas, is a magnet for garden pests. Feeding damage from cabbage worms, cabbage loopers, and cross-striped cabbage worms can stress the plants and lead to a bolted crop. Not to mention, if they do manage to head up, the caterpillars will tunnel into your broccoli heads!

Thankfully, it’s quite easy to keep these pests from ruining your crop without fighting a constant battle. Since broccoli doesn’t need insect pollinators to produce a harvest, you can simply cover your plants with insect netting so the offending butterflies and moths can’t lay their eggs on them.

Occasionally, a particularly determined cabbage butterfly or the odd Fall Armyworm might find its way under your netting, so it’s still a good idea to check the plants every now and then and make sure no shenanigans are underway.

We like to use standard rebar stakes and ¾ inch PVC conduit to make hoop tunnels for row covers such as insect netting. Read this post for step by step details on how we install them: How to Make DIY Hoop Tunnels to Protect Your Garden From Frost, Heat, and Pests.

In lieu of netting, Dipel dust, or the powdered form of Bt (Bacillus Thuringiensis) is pretty effective at protecting broccoli from caterpillars. However, it needs to be reapplied frequently, after every rain. Bt spray won’t work well for broccoli, since the leaves’ unique waxy surface causes moisture to bead up and run right off.

Other Broccoli Pests

Cutworms, slugs, and snails may also want to munch on your broccoli plants, which is mainly a problem when they’re small seedlings.

Cutworms are nocturnal caterpillars that hide out beneath the soil and come out at night to gnaw on seedling stems. Sometimes, they’ll chomp a small seedling completely in half. If you have trouble with cutworms, collaring seedlings with slices of a TP or paper towel roll works perfectly to deter them.

Slugs and snails love nothing more than a buffet of young brassica plants. In many cases, you can prevent major slug damage by checking the plants each morning and hand removing any slugs. If your slug numbers are through the roof, you may need to take further action. Read this post for more help dealing with slugs in your garden: Controlling Slugs in the Garden: 5 Natural Methods That Actually Work.   

Finally, cabbage root flies can cause trouble for a broccoli patch in some gardens. These flies lay eggs near the base of your plants, and their larva burrow under the soil and eat up the roots. Usually, you won’t know this is going on until your plants start to wilt and look puny.

While there’s not much you can do for a plant that’s already been assaulted by cabbage root fly larvae, you can prevent damage moving forward by blocking the adult flies’ access to the soil near the plants. Plant collars made with cardboard or small paper plates work well to shield the soil around the stem base.

Shielding Broccoli Plants From the Weather

You can’t control the weather, but you can definitely lower the chances of your broccoli wigging out and bolting due to unforeseen weather extremes. Both shade cloth and frost fabric can help keep your broccoli comfortable and stress-free during unfavorable temperatures.

When to Use Frost Fabric

To protect broccoli from cold, you can either use garden-specific frost fabric, old sheets, or even plastic sheeting or a tarp in a pinch. Some instances when you may want to cover broccoli plants before a possible frost include:

-When seedlings are small and newly transplanted. Broccoli seedlings that haven’t had plenty of time to establish are more likely to experience stress or damage from frost. If they’ve been in the ground less than two weeks, covering them is the safest bet.

-When the cold comes on suddenly. Cold-hardy plants actually change their internal chemistry to handle low temperatures. If temperatures fall sharply after a warm spell, the plants may not have time to prepare well.

-When daytime highs are staying chilly. If your frost covering is breathable, like a sheet or garden frost fabric, you can leave it on during the day so the sun will raise the temperature underneath. That way, your seedlings are less likely to decide they’ve already overwintered. Just keep a check on them and ventilate if needed so things don’t get TOO warm.    

–When plants are developing heads. Established broccoli plants aren’t bothered by frost, but the heads are more tender and may get burnt. You don’t want brown patches on your broccoli heads!

When to Use Shade Cloth

You can buy shade cloth for the garden in bulk rolls, or smaller, individual shade tarps. Shade cloth comes in various thickness levels; we’ve found the 40% works best for shading vegetable crops like broccoli while still letting essential sunlight through.

Shade cloth can lower both the soil temperature and the air temperature around your plants substantially. For broccoli, our shade cloth goes up any time temperatures climb above 70 during sunny weather. Doing this has HUGELY reduced the likelihood of our broccoli bolting in both the spring and fall.

Shade cloth/frost fabric, insect netting, and materials to build hoop tunnels require a small investment, but they’re well worth it, if you want reliable broccoli harvests each year. They’re quite helpful for other brassica crops, too, like cabbage, mustard greens, etc. If you remove and store them properly while they’re not in use, they’ll last you many seasons.

Broccoli Diseases

Broccoli plants aren’t too prone to disease problems, but there are a few bacteria and fungi than can affect them.

Fusarium yellows, a type of Fusarium wilt fungus that affects brassicas, causes wilting, yellowing, and plant loss in broccoli. It’s usually problematic when the soil temperatures are too warm, in the 75 – 85 degree range. If you have trouble with this fungus, you can grow resistant broccoli varieties like Belstar.

Two other major broccoli diseases, Clubroot and Black Rot, are almost always seed-borne. The best way to prevent those is to buy disease-free seeds from a reputable seed supplier.

Downy Mildew is a more common disease that attacks broccoli in home gardens. It causes irregular “bleached” spots on the leaves, which often contain little black speckles (spores). This mildew shows up during prolonged periods of cool and wet weather. Dryer, warmer days will slow it down.

I see a little bit of Downy Mildew almost every year in our spring brassicas, and we’ve never lost our harvest because of it. If you notice it, you can wait until the plants are dry and prune out the affected leaves.   

When to Harvest Broccoli Heads

For best flavor, you’ll want to cut your broccoli heads while the buds are still tight and the florets are packed closely together. If the beads start swelling or the florets begin to separate a bit, cut that head STAT. It won’t get any bigger at this point, and the plant is preparing to bloom. While still technically edible, broccoli will develop a bitter taste once those yellow flower petals start to peek out.

You’re going to need some SHARP, heavy duty pruning shears to cut your broccoli heads. Scissors or wimpy shears won’t make the job easy, since broccoli stems are basically tough little tree trunks. I like to harvest the small, tender leaves around the head, too. They make a delicious addition to a pot of steamed broccoli.

Once you’ve cut the heads, watch the plants for new side shoots (baby florets). While some are more prolific than others, most broccoli varieties will produce at least some secondary shoots after the main harvest.    

Storing and Preserving Broccoli

Fresh broccoli will keep for a week or so in the fridge. To keep it for longer, you can cut the head into bite-sized florets, blanch them, and freeze. Freezing is the best preservation method to maintain broccoli’s flavor and texture.

What to Do When Your Broccoli Bolts Anyway

Growing perfect broccoli heads may take a little trial and error, in every garden. Though we grow broccoli in both shoulder seasons, we’ve found fall-planted broccoli is more reliable and produces bigger, better-quality heads in our area. The seedlings can tolerate heat better than larger plants, and broccoli loves maturing into cooler weather. Try a fall crop this year, if you haven’t had good luck in the spring.

Even if your broccoli bolts, it’s not wasted space. Pollinators adore broccoli flowers. Instead of removing the plants, pull the netting back and let them have at it. It’s also an opportunity to harvest and save your own broccoli seeds for next time.