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Garden Help Desk: Shriveled squash may be sign of low pollination

By USU Extension - Special to the Daily Herald | Oct 14, 2023
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Squash is insect pollinated, and most of the pollination work is done by squash bees.
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When the tiny squash fruits at the base of squash blossoms don't get pollinated, the young squash will stop developing, shrivel and eventually dry or drop from the plant.
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Squash fruits begin as ovaries, AKA miniature squash, at the base of female squash blossoms. The ovary must be fertilized or the tiny fruit won't continue to develop.

What happened to my squash? This is my first time growing squash. They started out looking good, like always, but shriveled up before they got big enough to pick. I know squash likes warm weather, so did the cold weather ruin my plants?

This looks like a pollination problem. When a squash plant has plenty of flowers but doesn’t produce fruits, it usually means the flowers aren’t getting pollinated. Squash blossoms are pollinated by bees, and there are a couple of common reasons why bees may not be getting their job done.

Pesticides in the garden can cause poor pollination. If you sprayed an insecticide recently, you may not get good pollination until the chemical is no longer effective. The best way to avoid this problem in the future is to avoid pesticides or to choose products that are safer for beneficial insects and apply them carefully. It sounds counter-intuitive, but the fewer pesticides you use, the fewer insect problems you’re likely to have. There are many beneficial insects that help control your garden pests and most of the pesticides that kill the pests will also kill those good guys, including the bees that pollinate squash. If you have a pest problem that is getting out of control, avoid using broad-spectrum pesticides. If you’re giving your vegetable plants good care and attracting beneficial insects to your garden, you’re less likely to have serious pest problems.

I think you’ve probably hit the nail on the head with your question about the weather. Bees are fair-weather pollinators. They prefer to take the day off when the weather is cold, wet, overcast, windy or any combination of those conditions. Once the weather improves, they are usually out and busy foraging and pollinating again. I suspect you were already seeing a few new young squash fruits developing normally just a short time after skies cleared and temperatures improved. Enjoy these last few days of good garden weather!

Do you know which type of sawdust is best for my garden? I have some planters and just wanted to add sawdust in the fall and maybe draw out a little of the excessive nitrogen from the coffee grounds I had placed in them.

It’s unlikely you’ll need any kind of sawdust to deal with excess nitrogen.

Coffee grounds are considered relatively high in nitrogen, but the important word here is “relatively.” As a plant-based source of nitrogen, coffee grounds are considered rich, but the level is just 1%-2% nitrogen, about the same as grass clippings, and low compared to cottonseed meal (6%) or synthetic fertilizers, for example.

Another reason you probably don’t need to add sawdust to the soil is because coffee grounds aren’t composted and soil micro-organisms will be breaking them down and using soil nitrogen as they do that, just the way you were expecting your sawdust to work for you. Keep in mind that these soil organisms don’t “steal” or use up nitrogen, they actually just borrow the nitrogen while they decompose organic matter. Once decomposition is completed, the nitrogen is released back into the soil.

Is there a limit to the amount of coffee grounds you can add to your garden soil or compost?

You can work in about a 1/2-inch layer of coffee grounds into the upper 4 inches of your garden soil. Make sure the grounds are mixed into the soil uniformly.

For composting, keep the coffee grounds at not more than 20% of the materials you use. For example, add 1 part coffee grounds to 3 parts brown/carbon material (fall leaves for example), and 1 part green/nitrogen material such as fresh lawn clippings.

Don’t overdo it with coffee grounds in your garden soil or your compost. Some plants germinate a little better with coffee grounds in the soil or compost, but the germination of other plants can be inhibited.

Can you use coffee grounds as a mulch over the top of the soil in your flower beds, shrub beds and garden instead of mixing them into the soil? Coffee grounds have such a fine texture that they can pack together, making a dry barrier that is more difficult for air and water to move through, so it’s best to limit their use as a mulch. If you want to spread coffee grounds over your planting beds, apply only a thin layer, 1/2 inch at the most, and then add a thicker layer of compost or small bark nuggets over the grounds to help keep them moist and loose.


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