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So far my Butternut squash plant was doing fine. The first two fruits turned black and fell off after blooming, which I think was because there was no male flower available. Now three more fruits bloomed and I supported pollination by hand and they are growing. However, the next small bumps, which I assume are to be female flowers, are turning yellow and then black, falling off even before blooming. What could be the cause of that ? I'm new to gardening and I'm wondering if I water incorrect or don't offer the right nourishments. The plant is located on a house flat roof in the city (8th floor) and it is my only Butternut squash plant. A Zucchini is right next to it. My neighbor has a Butternut plant too, which is located on the same roof.

There are some dead leaves beneath the still green ones in the center of the plant. Not sure if this is linked.

fallen off fruit fruit turning yellow dead leaves in the middle of the plant

Edit: The gravel that you see is what the plants are planted in. It is a mix of brick debris, compost, sand and soil that was provided by the building's maintainer. I don't get the impression that it gets very hot during the day. My neighbor's squash is planted in the same substrate and I didn't see their fruits falling off. I used a 2-2-2 NPK fertilizer at first but 12 days ago was the first time that I used a 3-2-5 NP fertilizer (it says tomatato fertilizer that can be used for squash too). I used the amount that is described on the fertilizer for the Zucchini and Butternut plants together since they are very close to each other. But I guess you are suggesting that I need another fertilizer that has more P?

My watering habits are irregular. I sometimes water in the morning, but more often in the evening. I watered a bit less lately because it was raining sometimes but maybe the leaves are diverting the water? I have just been on the roof and removed all the dead leaves and took some more pictures. I saw two new dry leaves, so maybe I didn't water enough?

Also maybe it is worth adding that this happened to some of my zucchinis lately after I successfully harvested two big normal zucchinis already : [rotten small zucchinis4] Could be because of no male flower available, but I don't know.

plant growing upwards dry leaf

Meera
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  • How hot does your roof get? Does that gravel on top of the soil get hot in full sun? Remove all dead or dying debris. The leaves look healthy, you must be fertilizing? Please tell us what your maintenance habits are...you've got flowering so I am assuming your fertilizer is in balance (less N than P or K). This male flower thing on squash always amazes me that the male flowers are first...or seems that way. Good for you to pollinate by hand. Tell us more information, anything, don't worry if it doesn't sound important to you? – stormy Jul 20 '18 at 07:43
  • Ok, I added more information to the question. – Meera Jul 20 '18 at 19:33

2 Answers2

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After a number of years growing butternut squash I have noted that in the very early stages of growth of the vines embryo fruits may form and then be abandoned; I have put this down to there not yet being enough volume of vines (area of leaves) for the plant to support fruit at that time, so it "changes its mind" and abandons the effort. In more technical terms it might be to do with plant hormone availability in sufficient quantities at that stage.

Once the vines do become established and there are clearly many leaves the fruit can make it past embryo stage but then fail due to ambient temperature. Hot weather is a particular issue; whether the bees are blinded by the intense sun or the heat makes other pollen and nectar sources more available or attractive I don't know.

Say a vine grows to 15 feet (which is not unusual) most of the good fruits will be in the 5-10 feet section. So don't expect too much while the mass of foliage is lacking, and plant several hills so that the vines intersect and overlap to maximize proximity of male and female flowers. Water well until established and then let them roar away.

Colin Beckingham
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  • I don't think that there were not enough vines because the plant was too young. If you look at the picture you will see several vines and at the time the embryonic fruits started to turn black instead of developing flowers there were already three partly developed fruits on the plant. Maybe there were not enough vines for more fruits, but why did the vines stop growing and started to grow dry and die ? Right now almost all vines died, no new ones are growing. – Meera Aug 27 '18 at 12:51
  • Perhaps water not consistently available at the roots? Squash of all kinds are happiest when their roots are in a compost pile and then vines can reach out into a sunny patch. – Colin Beckingham Aug 27 '18 at 12:57
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why blossoms of zucchini die and fall off

When the males do their thing they turn black and die and fall off the plant. I am still looking at fruits that started forming and then blackening, is that right?

stormy
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  • No, it is not about male blossoms falling off after blooming. The problem is that it looks like females fall off (with the small fruit) before blooming. – Meera Jul 21 '18 at 03:43
  • The female flowers got pollinated. They started to make fruit and then a fungus possibly, infected the baby fruit, the baby zucchinis. This might be the same thing that causes your female flowers to die before being able to produce zucchini. Females with small fruit are females pollinated. – stormy Jul 21 '18 at 08:45
  • I just added the Zucchini story because you said to include more information. It is not the Zucchini whose flowers die before blooming. I think it is not true that females with small fruits are pollinated. See here for example: 'Female squash blossoms have a small swollen embryonic fruit at their base, which will grow into a squash if the bee does what bees do. '  https://www.gardeningknowhow.com/edible/vegetables/squash/female-male-squash-blossoms.htm – Meera Jul 21 '18 at 12:46
  • Your link says the same. Something else is going on. How are you watering again? I like that your watering is irregular! It should be. You are watching what your plants need instead of forcing our schedule making control...on plants. Great. But do not water at night. Morning only. Watering at night is probably the largest maker of fungus amongus. Are you getting any healthy zucchinis? I love your cage for the butternut. You will soon experience powdery mildew. Being on a roof top and using vertical gardening should greatly reduce any problems. Great ventilation, aeration... – stormy Jul 22 '18 at 00:06
  • ...and I just found your fertilizer info and it is perfect formulations. No problems there...I am still looking into squash blossom rot? – stormy Jul 22 '18 at 01:16