Tips on Growing Broccoli

I keep a seed inventory spreadsheet that goes back to 2003. On that 2003 list? Waltham 29 Broccoli. Broccoli and I go back a long time. It was one of the first vegetables I learned to love. I even ate it back in high school, when the list of vegetables I would eat was very short. I preferred it smothered in packaged cheese sauce back then. We mostly eat it in a stir fry now, but I’ve also made broccoli soup a time or two, and it often just shows up as a sauteed veggie side with whatever dinner we’re having. It’s a kitchen workhorse, and I freeze some of my crop every year.

Every year I start broccoli in my greenhouse, and ever year I have a few extra seedlings to sell. And every year most of them languish and don’t end up in a garden. I never understood this, until this year when I joined several online beginning gardening groups. I’ve come to realize that a lot of people have a hard time growing broccoli. They try it once, have a bad experience, and so never grow it again. So I thought I’d write up a few tips.

Broccoli is in the brassica family, which I wrote about extensively in this piece on the entire brassica plant family. I always welcome those first baby leaves in the spring, with their distinctive mustard family look. They tend to germinate quickly, often within 3 or 4 days. It’s a cool season crop, meaning it can take a light frost without damage, and it’s not a big fan of the high heat of summer. This means you can plant your broccoli out in your garden up to 4 weeks before your last frost date, when your soil temperatures are at about 50 degrees. The part of broccoli we choose to eat (the whole plant is edible) is the unopened flower buds. Flowering is triggered by a mixture of factors from day length to the age of the plant to stress, but is primarily dictated by soil temperatures (see further discussion below).

Broccoli can be direct seeded in the garden, but the seeds are small, making some thinning inevitable, and it takes a while for the seedlings to take off. Because I have slugs in my garden that will chomp down tiny seedlings, I start my broccoli in my greenhouse at about the same time I start my tomatoes, and then transplant out about 4 weeks later. For our area, that means I plant seeds at the end of the second week in March, and plant out mid April.

Tip #1. If you are buying broccoli from already started plants at a big box store, know this. Brassica family plants HATE to be root bound (when the roots reach the bottom of the pot and then just go around in a circle, forming a big white root mass). It stunts them, and they never recover. One year, in desperation after loosing a lot of my seedlings to a bad compost issue, I bought some root bound seedlings locally to try and make up for lost time. The plants barely doubled in size before forming a head, which was quite small. Along side them, the surviving plants I had started myself got quite large before they started forming large heads. So if you are buying your broccoli as seedlings, buy them EARLY in the season before they become root bound. Four to five weeks is about the maximum time they should be in a 3″ pot. Even less in a 4 or 6 pack.

Tip #2. Variety matters. There are a lot of different varieties of broccoli out there, but for some reason, the big box stores always seem to just carry Waltham 29 seed. This old open pollinated variety is named for Waltham, MA, where researchers at the University of Massachusetts developed it in the early 1950’s. It has a long maturity date of around 85 days from transplant. There are broccoli varieties with shorter maturity dates of 60 days or less. Some varieties form small heads of 4 inches or so. Some form large heads of 8 inches or more. Some are compact plants well suited for small spaces, raised beds and square foot gardening. Some are very large plants that need a couple of feet of room per plant (Waltham is one of them). Some give you one main crop and then are done. Some give you lots of side shoots after you’ve harvested the main head. Some are more suited for spring (look for heat tolerance and a fast maturity date). Some more suited for fall (Johnny’s has a nice visual chart). While Waltham 29 worked well for me in high elevation cool soil temperature Colorado, it is definitely not well suited for our quickly warming spring soils in SE Washington. This is one instance where I’m a fan of the “improved” hybrid varieties.

My two current favorite varieties for both spring and fall planting are Bay Meadows (good heat tolerance – seed may no longer be available) and Arcadia (tolerant of cold stress). Both have large heads and good side shoots. They mature for me within a few days of each other with very similar sized large heads. This year’s crop was started from seed on March 7th, potted up to 3″ pots a few weeks later, transplanted out on April 23rd, and harvested starting the third week in June. That’s about 100 days from seed to harvest. (Note that “date to maturity” on seed packages are from time of transplant, not from seed). While you might be able to get a late summer harvest of broccoli depending on your climate and variety, but it won’t taste nearly as good as it would if grown in the spring and fall. It’s really NOT a summer crop.

Tip #3. Broccoli takes a lot of nutrition, especially nitrogen. And when you think about it, this makes sense. You’re taking a tiny round seed, and in about 3 months, growing a plant that is up to two feet tall and wide, and harvesting several pounds from it. That takes a LOT out of the soil. So fertilizing well before you plant, and then maybe side dressing during the season is a good idea. (At the end of the season, when we take our old plants out, we have to use loppers to cut the main stalk they are so thick!). We compost our chicken manure, and find that our brassica beds really benefit from this nutrition.

Tip #4. Give the plants enough room! Depending on the variety, broccoli can need as little as 18″ or as much as 24″ per plant. If you pack them too close together, they will compete with each other for nutrition and sunlight, and your plants will be stunted and not very strong. But pack them in JUST close enough, and they will shade out weeds and you’ll have very little weed pressure as the plants mature. It might take some time to get this just right for the specific variety you are growing. Keep good notes, and if you have room, try a couple of different spacings and compare harvests.

Note its common even in the best of conditions for small broccoli plants to be “leggy”, meaning having long thin stems. This is pretty normal, and they recover from it fairly quickly once planted out. You can plant them a little extra deep to help hold them up, or just let them flop a bit.

Tip #5. Know your pest enemies. Broccoli is prone to several pests in our area, the primary ones being aphids and cabbage butterfly larvae. I find aphids are much worse when my plants are stressed from being too close together. Aphids LOVE all of those nooks and crannies in the heads, and can get heavy without being very obvious until you cut the head and rinse it and thousands of aphids are washed off. I spray with pyrethrins as a prevention when plants have been in the ground about a month, and then again as the heads are forming. Pyrethrins have a short term residual, and so if we don’t get a rain, it can help keep the aphids from getting established. Note pyrethrins are derived from a species of chrysanthemum and some formulations are approved for organic use – look for the OMRI seal – PyGanic is a good brand. I don’t like using horticultural oil, as it leaves a pretty obvious residue on the plant that doesn’t look good for sales. I’ve never been a fan of the dish soap spray, but if it works for you, go for it.

When its time for a second spray, on the forming heads, I include Bacillus thuringiensis in the mix. Bacillus thuringiensis, or BT for short, is a species of bacteria that lives in soil. It makes proteins that are toxic to caterpillar larvae when eaten. It is harmless to other insects and to humans. Cabbage butterflies, a pretty white butterfly with the black spots on its wings that flits around your garden in June, is a specialist on the brassica family. She lays her eggs on the plants, and when the eggs hatch, you get small green caterpillars that are VERY hard to see because they are the exact same color as the plant, eating your broccoli. Those caterpillars also love the nooks and crannies of the heads. It’s not the end of the world if you accidentally eat one. All they have been eating is broccoli, so that’s what they will taste like. But no one likes caterpillars in their food. The alternative preventative for these guys is to just keep your crop well covered with a row cover. But I find that 1) row covers are hard to secure well in our wind 2) tear easily and 3) prevent me from noticing the aphids who manage to sneak in anyway. So I don’t tend to use them.

For my own garden, slugs are also a big issue, especially in the spring. They will eat holes in the leaves, but also scrape the stems. Sometimes, if the plants are still small, this stem scraping can girdle the stem, preventing it from being able to transport nutrients, resulting in a plant that doesn’t die, but stops growing. There are many strategies for slugs (if you aren’t sure you have an issue, go out at night with a flashlight and look – that’s when they are active and you’ll see them right away). But because I’m planting 100 ft rows, hand picking or beer traps just aren’t adequate. So I use Sluggo slug bait. It’s approved for organic use, and contains iron phosphate, which is toxic to slugs but non-toxic to other insects and animals. It’s EXPENSIVE, so I only use it early in the season when the plants are small and vulnerable.

Tip #6. You can plant broccoli both in the spring and then again for a fall crop! Fall crops can be a bit tricky, because you need to start your seeds about 90 days before your first predicted frost. Our first frost is normally in mid to late October, which means I need to start my broccoli seeds in mid July, when its HOT. Brassicas don’t love super hot soil, so I usually start my seeds indoors. Once they are up and going, it’s tricky to find a place to put the seedlings outside, where they will get enough sun but not get fried by our hot summer days. I put mine where they will get full morning sun and some afternoon shade, and make sure they don’t dry out on hot days. And then I plant them into the garden about now (mid to late August).

As our day length gets shorter, the plants grow slower, and once we reach less than 10 hours of sunlight (late November) the plants will not make any further progress. So you want to aim for the heads to form in late October to early November in our area.

Interesting, days to maturity can vary, a LOT, depending on time of year (and as mentioned above, the date on the seed pack is from date of TRANSPLANT, not date of seeding).

Oregon State University did some interesting experiments with a program called CROPTIME, which is based on soil temperature/heat degree days. As an example, Arcadia broccoli matured in 86 days, 78 days, 72 days and 68 days from transplant when planted April 1, April 15, May 1 and May 15th respectively. That pattern reverses in the fall months. So Arcadia matured in 62 days, 64 days, 68 days and 79 days when planted out July 1st, July 15th, August 1st and August 15th respectively. All plants were transplanted when they had 2 to 4 true leaves (ie they were each started at different times about 4 weeks before transplanting out). These numbers would also vary based on YOUR location and how fast your soil is warming up or cooling down. Here’s that that looks like in chart form. Generally, the shorter the time to maturity, the smaller the head, which is yet another reason broccoli is better as a spring/fall crop.

Tip #7. Knowing exactly when to harvest can be tricky. Will it get bigger if you leave it for a few more days? Or will it start to open up in preparation for flowering? As SOON as I see the head is no longer tight, or that the individual buds are starting to expand, I pick the head. The more it’s spreading or budding, the tougher and stronger the flavor, so it doesn’t pay to wait.

Once you harvest your main head, many varieties will form “side shoots”, which are smaller heads that come out around where the main head had been. You need to harvest these a few times a week to keep them from starting to flower, but you can usually get a good sized second harvest after the main head has been removed. Eventually, these side shoots are too small to bother harvesting or the aphids win or you are simply sick of broccoli and you just give up and let the plant go to flower. Bees LOVE broccoli flowers, so I often leave them to bloom unless I need the space for a fall crop.

I find fresh broccoli will store for up to a month if kept in a bag in the crisper. But if you want to preserve some of your harvest for longer than that, you can freeze it. It is recommended that you blanch your broccoli first before freezing. Blanching (scalding vegetables in boiling water or steam for a short time) stops enzyme actions which can cause loss of flavor, color and texture. It also kills any bugs you may have missed when rinsing. Simply cut the head into the size florets you’d like to eat, and then put them in already boiling water for 3 minutes. Drain and submerge in ice water to stop the cooking, drain well again, so they don’t stick together in one large chunk (I often use a salad spinner), and then freeze. We use frozen broccoli for stir fries and for broccoli soup in the winter.

© 2021, where we’re miles away from perfecting the timing on every vegetable in the garden, but have got broccoli pretty dialed in, and find it a crop well worth growing. Want more content? Sign up for a monthly newsletter to your email inbox HERE.