What makes onions split rather than grow?

Onions don't actually "split" in the sense of breaking apart. What you're likely observing is a condition called "bolting".

Here's why onions bolt:

* Stress: Onions are sensitive to temperature fluctuations, especially during the early growth stages. Extreme cold or heat can trigger bolting.

* Day Length: As the days get longer (spring and summer), onions sense this change and start to focus on flower and seed production, rather than bulb development.

* Variety: Some onion varieties are more prone to bolting than others.

* Age: As onions get older, they are more likely to bolt.

What happens when onions bolt:

* Flower stalk (bolt) forms: A tall, hollow stalk grows from the center of the onion plant.

* Bulb development is stunted: The onion bulb doesn't grow as large or may not develop at all.

* Seed production: The bolt eventually produces flowers, and then seeds.

How to prevent bolting:

* Choose the right variety: Select varieties known for their resistance to bolting.

* Plant at the right time: Plant onions early in the spring or fall when the days are shorter.

* Provide consistent moisture: Avoid letting the soil dry out completely.

* Use a shade cloth: Protect onions from extreme heat during the summer.

If you notice your onions bolting, you can still harvest the green tops for cooking, but the bulbs won't be usable for storage.

Important note: While it might seem like the bulb is "splitting", it's actually the plant's response to environmental signals.