Because all gardening is local


Evaluating squash pollination

A critical, but often overlooked or misunderstood part of growing squash is pollination. Some folks have a vague notion that bees are good – that bees have to visit the squash blossoms – and they assume bees will come.

In these days of pollinator decline, this is not a valid assumption. To grow squash sucessfully, one must ensure good pollination. And pollination is not just an on/off switch, where a bee just visits the flower and pollination is accomplished.

Well pollinated squash

A perfect squash resulting from almost every incipient seed fertilized.

Pollination is a progressive thing with any multi-seeded fruit. You can have many stages, from no pollination at all, to full pollination. This is because many grains of pollen must be delivered to the sticky stigma of the female flower – two pollen grains for each incipient seed. And these pollen grains must be evenly spread across the surface of the stigma.

The perfect squash in the photo above results from almost every incipient seed being fertilized. The act of fertilization stimulates the development of the flesh of the fruit.

Female squash blossoms have an ovary – the incipient fruit – at the base of the flower. If no pollen grains are delivered, the ovary will simply shrivel and dry up. If you touch it, it will fall off.

But, if a few grains are delivered, the fruit may actually start to grow. This fools some people into thinking that it’s pollinated.

Squash fruit abortion due to inadequate pollination

Squash fruit abortion due to inadequate pollination

The fruit will stop growing, shrivel up and often rot at the blossom end. This is not blossom end rot. Blossom end rot is a calcium metabolism problem; it shows up in fully-formed fruit; and it is much more common in tomatoes than squash.

There is a fungus involved here, but it is opportunistic; no amount of fungicide will prevent this fruit from aborting.

Poorly pollinated zucchini

Poorly pollinated zucchini: the fruit starts to grow, then shrivels and aborts.

Zucchini (courgette) and other squashes all will exhibit the same kind of response to inadequate pollination.

Let’s suppose a few more incipient seeds are fertilized, but still nowhere near the full quota of seeds. This is what you get:

Deformed squash fruit from inadequate pollination

Deformed squash fruit from inadequate pollination

The fruit may actually size up somewhat, but it still will be wrinkled and poor quality. It will also grow much slower than a well pollinated fruit.

Partly and fully pollinated squash

Partly and fully pollinated squash: the upper one did not have enough pollen for fertilization of all the seeds.

The poorly pollinated squash on the top is actually 2 days older than the well pollinated one on the bottom.

When I was in the pollination business, a farmer called me. He was just about in tears. He had planted 75 acres of yellow squash for market. During the winter, he had given me an order for bees to place on the fields, and he gave me the date he expected to need them. But he planted his squash much earlier than planned – taking a risk with frost – but he figured he’d win big if he had the first on the market.

Unfortunately he forgot to call and tell me he needed the bees earlier.  Now his harvest crew was on the first picking, and they were throwing away almost the entire picking. They looked just like the upper squash in the above photo. I went straight to his farm that morning, and bees were notably absent from his fields.

The bees were delivered that night, and the rest of his harvest was very good. The problem: The day of the first picking, there was very little squash on the market – and it was bringing $18 a box. If he’d had a normal picking, it would have brought a small fortune. But the price dropped rapidly, and by the fourth picking only brought him $4 per box. There was no 5th picking, as the market price of $2 a box would not pay the picking cost.

The lesson is clear: we need to pay much more attention to the bees. Pollination is a critical part of growing squash, whether for the garden or the farm. And commercial growers cannot assume wild bees will do the job!

Pollination failure can occasionally happen from a rainy day. In small plantings, you sometimes have crazy plants that have all female and no male flowers. And extreme heat can kill squash pollen. So there are other possible reasons for pollination failure. But persistent pollination problems usually stem from a lack of bees.

In an upcoming post, we’ll talk about the kinds of bees that do squash pollination.

4 Responses to “Evaluating squash pollination”

  1. June 25th, 2011 at 4:17 pm

    vickie says:

    this was so helpful, I have a wild bee hive in the yard but they are clearly still not enough, based on your pictures it is pretty clear I have pollination problems. Thanks for the great pics.

  2. July 18th, 2011 at 9:02 pm

    Ann says:

    Is pollination that complicated for all fruits and vegetables? I had trouble with my tomatoes last year, but this year they are doing great – I don’t think I’ve ever seen one look half-pollinated. My zucchini though, I’ve seen a couple that look like they might be missing a chromosome or two.

  3. August 26th, 2011 at 8:02 pm

    sarah roe says:

    YES! That is what i now think I have… have considered it blossom end rot but have been told that it is uncommon in Montana – I always support the bees by not spraying pesticides, even though the ladybugs are love ‘em and leave ‘em types, my aphids have eaten till their belly’s full. Now I wonder if some of my tomatoes are getting the brown blossom end, is it ‘the rot’ or lack of pollination? And, if it IS blossom end rot, is it ‘bad’ to spray calcium to deter it? Thanks, been so helpful… hope you get back to this lady farmer in Montana. Sarah Roe

  4. August 27th, 2011 at 11:24 pm

    Dave says:

    Sarah, it’s more likely that it’s blossom end rot with tomatoes. With them, inadequate pollination more often shows up as blossom drop or undersized fruit. Calcium may help, or it may clear up on its own. Do you have plenty of calcium in your soil? Acid soils tend to be very low in calcium. Blossom end rot in tomatoes can result from the plant not having enough calcium – or it can come from a failure within the plant to utilize calcium, caused mostly by uneven watering. I usually have a little BER, especially with some varieties. I generally ignore it, and it goes away – but I have calcium-rich soil. If the BER is not too bad, I cut it off and utilize the rest of the fruit.

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