Why Spring-Flowering Bulbs are Planted in Fall

‘Tis the season to plant bulbs for glorious spring color! Many spring-blooming bulbs require time to cool in the ground, which is why we plant them in fall before the winter comes. While there’s a decent window of when to plant bulbs, it’s important to not put them in the ground too early or too late.

When to Plant Spring-Flowering Bulbs

Many spring-flowering bulbs are native to montane areas. The bulbs remain dormant all summer, and then in autumn, the nights begin to cool, and there’s more rainfall, which breaks the dormancy, and the bulb sends out new roots. Root emergence typically happens when the soil temperature stays below 60ºF.

So, when planting bulbs in your yard, you should do so when the soil temperature reaches that point. You can use a soil thermometer to determine the temperature, though it’s OK to be a bit less precise. Instead, you can use the trees as a guide—when the majority of leaves on the trees have fallen, and the first frost occurs, it’s time to plant your bulbs.

Once you put the bulbs in the ground, they continue to root until the ground freezes. It generally takes about two weeks for bulbs to fully root.

Tips for Planting Bulbs

If you were to put the bulbs in the ground too late, well, you’d need a pickaxe, but also, the roots could freeze.

As the bulbs establish roots when the soil is the proper temperature, the bulb breaks down certain molecules to act as a sort of antifreeze, meaning the bulb can survive colder temperatures. That’s why plants need time to establish, and also answers the question of why you should avoid planting bulbs too late.

The opposite concern is planting bulbs too early. If the weather is still quite warm, then the soil will be warm too, for example, when the soil temperature is still in the 80’s. This can put stress on bulbs, and if the temperature, humidity, and moisture are right, fungus can grow and infect bulbs, or the bulbs can rot.

Some people suggest putting a handful of fertilizer at the bottom of a hole when planting bulbs, but we advise against this. As tender roots of bulbs emerge, fertilizer can burn them, and the roots can even catch fungus and rot.

If bulbs are planted in the fall and not watered, they might not initiate roots, and the bulbs would freeze. We’ve seen this happen in areas like Minneapolis, which tends to be dry in fall. So it’s very important to water after you plant bulbs.

What is Vernalization?

This process that bulbs go through is called vernalization. Vernalization is the cycle that promotes the bulb to flower after being exposed to cold temperatures for a prolonged period of time. Many bulbs need to go through this process to produce blooms in spring. Early bloomers take less time to develop than later bloomers, and knowing this, you can plan for staggered blooms in your garden.

If you live in a place that stays warm all winter, then you can always pre-chill bulbs yourself, which mimics the process that happens naturally in cooler parts of the country. The chilling period varies between bulbs, but you would plant the bulbs outside after the recommended chilling period indoors (like in a fridge) is complete.

Starting off strong right from when you plant the bulbs means they’ll end strong, resulting in colorful blooms in spring. Just like Goldilocks trying out the bears’ porridge, bulbs have a preference for soil temperature—not too hot, not too cold, but just right!