Bulb sizes explained

24th Nov 2022 | Growing Advice

Bulbs come in lots of different sizes. The smallest would be Eranthis Hymalis (size 4) and the largest is amaryllis at round size 30. Eremurus is perhaps the biggest. They have a huge root system, although they are impossible to measure so we call the top size, size 1. Similarly for tubers like dahlias, these are graded by size 1 or size 2 and downwards.

What do flower bulb sizes mean?

The number is simply a measurement in cm around the circumference of the bulb, for example, a size 10 allium bulb is 10cm in circumference.

When a flower bulb is graded, they go onto a grill will holes of a certain size. No bulb is perfectly round, so there is a little bit of compensation in grading. For example, daffodil bulbs are measured in 8/10, 10/12,12/14 or 14 + this just means that the bulbs are between 8 and 10cm, 10 and 12cm and so on.

Miniature Daffodil bulbs will be sizes from 8cm to 12cm depending on the variety, our Lincolnshire daffodils will be size 12/14.

Bigger is better, but there is a limit to the size the bulbs will grow. If the bulbs are too small then they wont flower. If you consider a bulb is a system of storing up food made up of a starchy substance. Whether a corm, bulb or tuber, this will help the bulb to grow when the time is right. If there isn’t enough of this substance then the bulb may only produce leaves, but they can still be kept for future years.

Tulip bulbs are a perfect example. When they finish flowering they split into many smaller bulbs. Excluding species varieties, tulips won’t perform well if they are under a size 9. Which is why tulip flowers don’t always reappear year after year.

The vast majority of tulip bulbs are used for the cut flower industry. A particular size of bulb is required to ensure that the tulips have the right weight flowerheads as dictated by European supermarkets.

Most tulip bulbs available are size 10/11. These are perfectly good at flowering and will not make any difference to the performance of the bulb. These also work out much more cost effective than buying the ‘biggest’. Some of the multiheaded or fancy varieties need a larger bulb, around 11/12, to feed the enormous amount of the flower it will produce.

Allium bulbs also come in a huge range of sizes from size 5 to size 20. Looking at the bulb is a good indication of how big the flower will be.

Planting bulbs according to size

The rule of thumb is to plant bulbs two times the depth of the size of the bulb. It is important to plant big bulbs deeper as this can help anchor the plant when it is fully grown. Planting small bulbs too deeply can stress the bulbs out by making them put too much energy into breaking the surface. Planting them just a touch deeper is still ok, it can help protect from pests and frosts.