How to Grow Giant Marrows: An Ultimate Guide

Tony O'Neill( Expert Gardener, Best Selling Author. )

Tony O’Neill, expert gardener and best-selling author of the famous “Simplify Vegetable Gardening,” “Composting Masterclass,” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill

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I typically grow four plants and use a space of approximately 35ft x 22ft or (10m x 6.7m). Soil needs to be free draining, and the area ideally should be sheltered to prevent damage to the plant’s large leaves.

Preparation starts in late September with the sowing of green manure.

I leave this to grow all through the winter. This does a few things which are quite important, but it is not necessary to do. Green manure is brilliant as the types I use, Field Beans and Hairy Vetch, are known as Legumes.

Legumes can take nitrogen from the air and store it in little capsules in their root system.
The Green manure protects the soil during the winter months from being battered by the rain and cold; This prevents it from compacting and destroying the soil structure.

Green manure germination in the giant marrow bed — Early start for green manure

3. By growing throughout the winter, you provide a food source for all the soil web life (Nematodes) and other bugs in the ground.

Green manure, when dug back into the ground, adds organic matter.

This is good as these microbes help to break down animal manure and pathogens in the soil. It helps to release locked-up fertilizers and so forth.

Full grown green manure — Green manure consisting of field beans and hairy vetch

In early March,

We cut down the manure right to the ground. We leave all this growth on top of the ground for a few days to start breaking down.

green manure cut and left to rot down — Strimmed green manure

After about a week, I dig the Green manure into the soil by hand. This does not have to be a fine tilth as this comes later.

double digging the giant marrow bed — Hand-turning the giant marrow bed

The next stage is to grow giant marrow.

It is to get a soil sample. I use Lancrop laboratories to do my testing. You could just as easily do a basic test with a home kit. Once you get the results of your soil test,

You can start looking at ways to adjust your soil for the optimum growing conditions for Giant Marrows. I add all amendments. Sometimes this might need more organic matter, so I add a lot of manure. This manure MUST be well-rotted if you add it at this time of year.

If you’re adding it in Autumn instead of green manure, then it does not need to be well-rotted.

Well rotted horse manure for giant marrows — A load of horse manure for a giant marrow bed

Now that the soil is nearly ready, we must look at the plants themselves. With any Giant vegetable, it is all about the genetics of the seed.

You need the right strain that has proven genetics to grow big. A normal marrow strain from a garden center or online company can never reach these sizes.

Time to set a giant marrow seed.

I set my seed to chit (to shoot) like you would potatoes at the end of April. I do this by selecting the seed I want to use, Filling a small seed tray with vermiculite or compost, and soaking it in lukewarm water.

I then set the seed on top of the soil and dusted it with some Mycorrhizal Fungi. Mycorrhizal fungi are tiny, harmless critters that attach themselves to plant roots and help plants use water and organic nutrients in the soil. They live on the roots of roughly 95% of all earth’s plant species.

Setting giant marrow seeds to chit — giant marrow seeds in a tray

This tray is now heated in a propagator to 21ºC (70ºF), and the seeds can chit. This takes around 7 to 10 days. They will then be ready to pot on into individual pots.

Chitted giant marrow seed — The root system of a giant marrow seed

I dust these seeds again with Mycorrhizal Fungi and plant them in small pots until they get going. We keep potting the marrows into larger pots, dusting with Mycorrhizal Fungi each time.

young Giant marrow plant — First true leaf of giant marrow plant

The fungi do a fantastic job of enabling the marrows root system to build. This provides it with the best start it could have. Each step of its life must be as stress-free as possible. The plant will not reach its full potential if it gets stressed at any stage.

Root system of a giant marrow — The giant marrow rootball

It is now early May.

We return to the ground where the marrows will be planted. We now rotovate this ground to a fine tilth. This incorporates all the feed and manure we have put onto the ground. By this stage, it has had time to break down.

Marrow bed finished after being rotovated — Rotovated marrow bed

We rake this bed flat. Taking out any real large stones that have risen to the surface. I then place four old bed frames in situ; this will be where each plant is planted. You can, of course, fashion all sorts of structures; this blocks the wind until the plant gets established and later is used to keep the sun from ripening the marrow.

completed preparation of the giant marrow bed — finished giant marrow bed

After the last frost at the beginning of June, I plant out my Marrows. We want to get them out into the soil to start acclimatizing them.

settling giant marrows into their growing spot — Planting out giant marrow plants

When planting giant marrows,

The Marrow gets a fine dusting of Mycorrhizal Fungi, then planted and watered in very well. We wrap the bed frames in fleece or plastic if there is adverse weather.

Plastic wrapping to protect giant marrows — Wrapped Giant marrows to protect from the wind.

As the plant grows and starts to vine, you want to maneuver it in the direction you wish it to grow. We then stake the vine to the ground using cut-off plastic coat hangers.

Giant marrow pegging — Pegging giant marrow vines

As the giant marrow plant gets bigger,

I bury the vines with soil; this helps the plant anchor itself into the ground and allows it to put down extra roots at each leaf node. The more roots, the more the plant can take up water and nutrients. We continued to bury vines, and I allowed the plants to reach 16ft in length and then stopped the tips from growing by removing the tip. I also did this with all side shoots.

Giant marrows in ready made beds — Giant marrows are starting to vine.

The marrow plants then start to flower. We are looking for a flower to appear around 10-12ft away from the main roots and preferably on the main stem.

Growing on side runners can still produce very large marrows. I have always tried to get them on the main stem, as this is the main artery for the plant.

Then should be able to produce more fluid and nutrients for the growing fruit. Once we locate a female flower in the right area, we can hand pollinate it.

The timing must be right. I pollinate mine around 48-50 days before the Malvern Autumn Show, where I hope to take a World Record one day.

Male and Female flowers can be shown by the female flower containing the fruit behind the bloom.

Differences between male and female giant marrow flowers — Male and Female Giant Marrow flowers

Giant marrow flower Prep

We cover or tie the female and male flowers up before they are due to open, this is to stop pollinating insects from cross-pollinating the flower. On the day they open, we take multiple male flowers and tear off all the petals leaving just the stamen section of the flower, we then use these and pollinate the female Pistil with the male stamen.

pollinating Giant marrow flowers — male and female giant marrow flower

Once we have finished pollinating the flower, we then tie the female flower back up again to stop cross-contamination.

tied giant marrow flower to prevent pollination — Female Giant Marrow flower

As the giant marrow forms

The flower will fall off, and successful pollination will occur. All we need to do is manage the plant for the next 48-50 days. To do this, we need to feed regularly; I do this using Maxicrop seaweed feed, Comfrey tea, and a sprinkling of Fishnure or 6x chicken pellets. Removing any side shoots regularly and maintaining the plant’s shape is important.

removal of side shoots — Make compost from giant marrow cuttings

and remove all flowers and unrequired marrows. I grow one marrow per plant. Later in the year, I will allow a second to form. This helps prevent the marrow from splitting

giant marrow rejects — , cutting off unwanted marrow.

I donate all the removed marrow to old-age complexes and homeless shelters. As well as keeping a few for use at home. Once a week, spray your plants with an anti-fungal spray or whole milk watered down 10/1. This will help prevent powdery mildew from forming.

This disease can wipe out your leaves, which are the plant’s solar panels. Without these, the marrow will not grow. The next step as the marrow grows is to lift it out of the soil before it gets too big.

As I have used here, you can use an insulation foam board that will also insulate against a cold floor or sand. This will help prevent the fruit from rotting.

protecting a giant marrow from rot — Giant Marrow sat on a sand bed

Giant marrow towards the end of summer

The plants get extremely big and take up a huge amount of water. 30-40 liters of water will be required for each plant.

summer growth — Giant Marrows in the Summer

Keep your marrow warm. Tucking it away on cold nights and covering it over in the daytime. This will prevent the sun from ripening it. If it gets ripe, it will harden and split when taking on water.

Sat on a bed of sand to insulate from the ground — Giant Marrow wrapped in fleece.

When it has finished growing, the marrow won’t grow any further. It is important to check growth with a tape measure over the next few days to confirm it has stopped.

Giant Marrow taping — Taping the marrow to calculate weight

We should now be very close to show day. Do not cut the marrow unless you think it could split by a sudden downpour. Let up on watering at this point too.

Try to leave the marrow on the plant as it will retain its weight. Cutting it early may cause it to lose some weight which could be all the difference on the show bench. If the marrow splits, Its game over

Showing the split that stopped this one growing — Game over

One step closer to your giant marrow.

If you have managed to keep your marrows growing without splitting, congratulations. You may now show it at your local show or one of the nationals. This is where the big boys play, such as Malvern.

Giant Marrow On The Show Bench — Showing At Malvern

So now you know what it takes to grow a giant marrow. It is a lot of work, fun, and sometimes heartache.

I hope this step-by-step growing guide will be of use to you. If you would like more information like this, why not visit us on our Youtube Channel or Facebook Page? Click the highlighted links.

I would love to see you there and chat. For now, happy gardening. I will speak to you soon 🙂