Do Broccoli Seeds Need Light To Germinate?

By Paul Smart •  Updated: 08/21/22 •  6 min read

Do Broccoli Seeds Need Light To Germinate? Broccoli plants are one of the most commonly grown vegetables in the garden. They have the advantage over cauliflower of producing the main head and then side shoots which can produce additional harvests. So if you are planning to grow them in your garden one of the most common questions that is asked is do broccoli seeds need light to germinate?

Broccoli seeds do not need light to germinate and most sources recommend that the seeds be planted at a depth of approximately half an inch which is usually sufficient to exclude light from the seeds but not so deep that the plant can find its way to the surface.

When the seeds are planted they have sufficient energy within the seeds themselves to enable the plant to produce its first shoot and then get going, however, the amount of energy within the seed is limited which means that shortly after it sprouts it needs access to light in order to continue to develop.

This is because plants need light to photosynthesize which allows them to produce sugars that the plant can then use to continue to develop and grow. Most seedlings including broccoli will produce an initial seedling leaf which is normally a characteristic shape of the planting group that it comes from. In the case of brassicas the leaf has an appearance not too similar to clover leaf cut in half.  

All brassicas will produce this initial leaf before producing true leaves which are consistent with the shape and type of the mature plant. Once the plants get to this stage they are well and truly in need of plenty of light to continue to develop. If the location where you store the plants does not have enough light you will generally find that the seedlings become leggy.

What Is The Best Time To Sow Broccoli?

Broccoli is traditionally a cool weather vegetable that grows best in autumn and spring and in locations that have relatively mild winters it can also be grown throughout the winter because it is tolerant frosty conditions.

To ensure that the broccoli heads are produced in these cooler periods of the year most gardeners tend to plant in winter and also in mid to late summer which may seem counterintuitive. However, if you plant seeds in summer they will take approximately 4 to 6 weeks to the large enough to plant out into the garden which would mean that seedlings are being placed in the garden in early autumn. In some regions the weather is still relatively warm however the production of the flowerheads, which is what we eat, is still 4 weeks away which means that the plants are usually starting to produce at the time when the weather is really cooling down.

It is absolutely ideal for the broccoli to begin to mature at this time of year as the cold weather typically does not affect the heads very much unless the weather is absolutely extreme which means that they can sit in the garden for an extended period of time and then be picked as you need them.

For those that live in relatively cold climates broccoli can tolerate temperatures as low as 26°F (-3°C),  however, if you are getting below around 30°F (-1°C) is advisable to provide some sort of protection to the plants as that will ensure that the heads remain in good condition.

How To Grow Broccoli

As mentioned above the best time to sow broccoli seeds in most regions is in either midwinter or midsummer. These sowings can be done directly into the garden, however, we generally recommend that you plant in seed trays irrespective of whether it is in winter or summer. 

The main two reasons for this is that planting in seed trays allows the climatic conditions to be controlled easily. Secondly, it minimizes the amount of time that the plants spend in the garden which is highly beneficial in summer because there is often a large number of other crops that are being grown at the time.

To start the seeds off start by filling a seed tray with good quality seed raising mix and ensure that the mixture is firmed down into the individual cells creating solid plugs. This is advantageous because it makes it much easier to transplant the seedlings when they are ready without disturbing the roots.

Ideally, we recommend that you plant to 2 to 3 seeds per cell at a depth of approximately a half an inch. Once planted the seed should be placed in a relatively warm and sunny location that gets plenty of light. The seedlings will typically take between 7 and 10 days to appear depending upon the climatic conditions.

The seedlings will need to remain in the seed tray for a further 4 weeks or so before they get large enough to plant out into the garden. At this point and time they will typically be approximately 3 to 4 inches tall.

When planting the broccoli seedlings out into the garden we recommend spacing them approximately 10 inches apart in a location that receives 6 to 8 hours of sun per day at least. In terms of soil conditions, broccoli seedlings are relatively forgiving and will tolerate a relatively wide range of conditions, but like most vegetables, they do prefer rich moist and free draining soil.

Once the seedlings have been placed in the garden it is a good idea to apply a layer of mulch approximately 2 to 4 inches thick around the plants to help suppress weeds and retain moisture in the soil.

As the plants are susceptible to attack from slugs and snails it is also a good idea to apply snail bait around each seedling to minimize the chances of attack. If you are particularly concerned about snails you can also apply a cloche to the plants which will provide an additional physical barrier to protect the plants. However, cloches are generally not recommended when planting the seeds in early autumn when it is still relatively warm.

Harvesting Broccoli

Broccoli is a plant that can be harvested at any stage once the heads reach a size of approximately 4 inches wide however, some varieties will reach a width of 6 to 8 inches in the right conditions.  

However, once the heads reach a width of approximately 4 inches it is advisable to keep a very close eye on them as they can quickly open up and start to produce flowers which will spoil the quality of the heads.

If you see any signs of yellow flowers even just a hint of it the broccoli plants should be harvested immediately even if you have a gut of them as they can be frozen for use later on.

I hope you found this article useful and have great success growing your broccoli plants at home in your own garden, if you have any additional comments or questions please leave them in the section below.

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What Are The Different Types Of Broccoli?

How Many Broccoli Heads Grow On One Plant?

How Long Do Broccoli Side Shoots Take To Grow? And How Big Do They Get?

Is Broccoli A Flower? If Not What Is It?

Paul Smart