Four Secrets to Growing Large Broccoli Heads - Abundant Mini Gardens

Four Secrets to Growing Large Broccoli Heads

Very large head of broccoli

Are you frustrated that you can't grow large heads of broccoli? You're not alone! Many gardeners have this problem. Read below to discover my four secrets for harvesting huge heads:

1) Choose the right variety

Some varieties of broccoli have been bred for larger-size heads. Some varieties produce a large number of harvestable side-shoots after the main head has been cut. A few varieties do both. Do some research, and experiment with different varieties, to discover one that will produce well for you.

2) Never let the seedlings become root bound

I rarely see broccoli seedlings sold in the tiny 4-packs in commercial garden centers produce a decent size head. Broccoli is extremely sensitive to becoming root bound. Once their roots have become cramped in a container, they rarely ever produce good heads later – even if given excellent growing conditions. I don't buy commercial broccoli seedlings any more, as they usually don't produce well for me.

I usually start my broccoli seedlings (and other plants in the cabbage family) in large pots – about 3” across and 4” deep. I then transplant them as soon as they have four adult leaves. Their roots have not yet started circling inside the containers. But it's critical that they be planted as soon as they are ready, and not be held in pots beyond the ideal planting time.

One year, I decided to hold onto a few broccoli seedlings, in case some of the transplants in my garden died young and needed replacing. Meanwhile, I repotted those young broccoli seedlings into larger 1-quart pots while I waited to see if they would be needed as replacements. Even though they were only held in these larger pots for two weeks, they still became slightly root-bound and they never grew well after they were planted in the garden.

Stunted broccoli plant

The broccoli plant in the middle of these 3 plants was pot-bound when planted in the garden. It never caught up to the size of its neighbors, and only produced a tiny 1″ head.

These late transplants ended up growing only 1/4 of the size of the first transplants, and they only produced tiny 1″ heads. They also had much more damage from pests than my large, healthy plants.

3) Give the plants excellent growing conditions

Broccoli plants are heavy feeders, and need moist, nutrient-rich soil. If your soil has less than optimal fertility, then I recommend that you side-dress your plants with a slow-release organic fertilizer every 4 weeks – starting about 2 weeks after transplanting. Or you can provide a liquid fertilizer every couple of weeks or so.

If you'd like to see the difference that proper soil fertility can make, this article from a different website has an awesome photo, showing the difference in broccoli heads with regular soil, and well-fertilized soil:

Broccoli also strongly prefers cool growing conditions – generally below 80 F. They usually make good fall crops, when they can mature in cooler weather. But if you want to try harvesting them in somewhat warmer weather, select a variety that can tolerate that. Just don't expect to harvest a good crop during hot weather.

Here's one resource for recommended varieties for different seasons from Johnny's Selected Seeds: Broccoli Planting Program / Comparison Chart

For the best broccoli production, give them the conditions they need to grow quickly and vigorously. Any set-backs (even temporary) – whether it's from dry soil, poor fertility, or becoming root-bound when young – can stunt them for life.

4) Give the plants enough space

To grow to their maximum size, broccoli plants need plenty of elbow room in the garden. Healthy, vigorous plants can easily grow three feet across – from one side of the plant to the other. If you want to mix several types of plants in one garden bed, keep other plants at least 18″ away from each of your broccoli plants, and be aware that large broccoli plants will shade nearby smaller plants.

I decided to conduct a spacing experiment with our spring broccoli in 2014. Most of my bed was planted at 18″ spacing for each plant. Part of the bed was spaced at 12″, per recommendations from Square Foot Gardening and Johnny's Selected Seeds.

I grew the variety “Arcadia”. It had been bred to produce large heads, and numerous side shoots. It tolerates cold very well, but doesn't like the heat. So it isn't the best variety for our spring crop, as the weather often gets pretty hot in mid-June when we harvest our spring broccoli – but it did well enough for us.

Measuring a broccoli plant 3 feet across.

Broccoli plants, given good growing conditions, can grow quite large. My plants averaged 3 feet across.

The plants grew very large. Most of them reached 3 feet across, so their leaves intermingled and overlapped each other. They seemed to tolerate short-term temps in the high 80's/low 90's fairly decently, though they would have done better in cooler temps.

Some of the plants spaced 12″ apart lost the competition for light and became very stunted, and only produced tiny 1″ heads.

The plants spaced 18″ apart didn't seem to stunt each other. However, unless I test wider spacing, I won't know how much more vigorously the plants could have grown with even more room.


The broccoli heads from the 18″ spacing averaged 1.3 pounds each! Beautiful heads, as you can see.

Very large head of broccoli

This is a single head of broccoli, weighing 1.5 pounds, and over 8″ across. Produced from a variety bred to grow large heads, given 18″ spacing and excellent growing conditions.

The heads from the 12″ spacing averaged just 8.5 ounces each. Much smaller than the others.

Medium head of broccoli

This was the largest head from the 12″ spacing. Many of the other heads from plants spaced 12″ were smaller – even as small as 1″ from some very stunted plants.

However, the yield per square foot was very similar – 8.5 oz for 12″ spacing, and 8.3 oz for 18″ spacing.

But the 18″ spacing only needed 40% of the plants needed for 12″ spacing. A 100 square foot bed with 12″ spacing needs 100 plants; the 18″ spacing would only need 40 plants. That's a major savings in starting or buying young transplants. So, my choice is to go with the wider spacing in my garden.

Broccoli head ready to harvest

This head is ready to harvest. The florets will separate when gently pressed apart.

When to Harvest Broccoli

Have you always wondered when your broccoli head is ready to harvest? Pick your heads when the florets loosen up enough to be gently pressed apart. If they are too tight to spread when pressed, the head hasn't matured yet – it's certainly edible, but just hasn't reached maximum size yet.

Happy gardening!

~ Debra

  • W. Yavelak says:

    This is a very useful, thoughtful article. The attention to yield per number of plants was a great insight. Well done. Happy growing!

  • Debbie says:

    I love all the tips on gardening. I’ve been enjoying it for 10 years and I’m still learning. In South Florida we have iguanas and worms among a few problems. Any suggestions.

    • Debra says:

      Hi Debbie! I haven’t lived in Florida, so I’m not very familiar with iguanas. Here’s one link with suggestions for control: Iguanas in the Landscape A couple of options for controlling caterpillars in broccoli include covering the plants with row cover, or weekly spraying them with BT (an organic pesticide that only affects caterpillars).

  • Linda Casey says:

    I didn’t get ANY broccoli last season. I used the square foot gardening method and planted 12″ apart together with other brassicas like kale which did well. I only got one tiny cabbage.

    • Debra says:

      That’s frustrating, Linda! I’ve noticed that broccoli is extremely sensitive to getting root bound in their pots when they are young, and often won’t produce a decent head at all if that happens – even if they are given great growing conditions when they are planted in the garden. I try to give them larger containers to start in, and then plant them in the garden while they are still very young (maybe just 4 adult leaves).

    • W. Yavelak says:

      Also, with sq ft, if you are just starting, you likely purchased your compost for the “compost-vermiculite-peat moss mix” — and purchased compost is notoriously poor in nitrogen due to cost savings for companies waiting on compost to mature —- so it contains a lot of uncomposted bark of some sort. (The bark is more of a nitrogen ‘sink’ rather than source in the ‘soil.’) The book – sq ft gardening – goes into some depth about how to try to get a mix of many different composts —but it is difficult, esp if in a city. Some remedies for this are: Find the best compost you can find in your area that doesn’t look like mostly uncomposted bark (google “good aged compost” for a view of what it is supposed to look like, choose “images”). Or buy about four different composts you can find that look as good as you can find. Old *Old* composted chicken or turkey manure, or cow manure, as maybe 1/10th of your total compost amount can help with the Nitrogen amount. It must be old well composted or there will be live weed seeds, and it can be too ‘hot’ or acidic if not old and well composted. Then make your mix and send it for soil analysis at whatever local organization does it for you, or use a soil tester from the web, or get a cheap kit and just test it yourself to get a rough idea if you have enough nitrogen in your soil. A sign you didn’t have enough nitrogen is if your peppers did poorly as well – they are heavy nitrogen users as well. Peas, beans, and anything that can fix nitrogen from the air might have done better for you because they didn’t need as much nitrogen from the soil…. although even with these plants, if your mix was brand new and in a box rather than in contact with the actual ground, the microbes (Rhizobium) needed to fix nitrogen may not have been plentiful enough in your mix to work with the plants to make the nitrogen. So, then, if your Nitrogen is less than 40ppm ( a rough estimate of adequate supply) correct it with a fertilizer of your choice — if you test the potassium K, and phosphorous P levels, choose a fertilizer that either corrects deficiencies in these, or does NOT add them if you have enough K and P. Often, with the sq ft mix plan, you have plenty of K and P, and just need nitrogen — fish emulsion 5-1-1 (N,P,K) is great — or carefully measured plain old ammonium compounds 21-0-0 if you aren’t trying to stay ‘organic.’ Google state universities that do soil analysis — they are usually cheapest…. or try to do the home tests. For broccoli —- at about the 12 leaf stage, or half grown, they need a steady uninterrupted supply of nitrogen till harvest —- a side dressing of blood meal at the start of planting so that it soaks in and is available to the plant by the 12 leaf stage can work —or give a 5-1-1 fish emulsion diluted with water – watering starting at this point and every 2-3wks til harvest. Here is a good article on veggies and fertilizer :
      Happy growing!

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