Glenmore Guide: Pot Sizes

Need to knows:

  • Most pot sizes are measured by volume, in litres, starting at C1
  • Pots with less than 1 litre of soil are measured by diameter, in cm
  • The smallest plants come in trays and are known as ‘plugs’
  • Pot size doesn’t guarantee plant size
  • For feature trees, start at C15
  • Always check the growth rate and mature height and spread of a plant
  • Choose decorative pots with a diameter 1-3cm greater than a plant’s current pot
Navigating pot sizes poses problems for amateur and pro gardeners alike, but once you’ve understood the fundamentals, you’ll be able to select the right size with confidence.

How are pot sizes measured?

Here at Glenmore we believe in making things clear and simple – but there are a couple of complications with pot sizes. The first is that, depending on the size of the pot, there are different ways to measure them.

Most container-grown plants are priced by the volume of their pot, measured in litres, from C1 upwards. The ‘C’ stands for container, and the number signifies how many litres of soil it contains. Containers can be very large, but the average gardener is unlikely to purchase many plants above C40.

For volumes under 1 litre, pots are measured by their diameter at the top. Sizes can range from 5cm to 11cm. In reality, however, you will usually see 9cm pots, because most smaller volumes can be planted as ‘plugs’ in seed trays. Seed tray cells vary from 4cm to 8cm in diameter. So a P6 is a ‘plug’ in a 6cm diameter tray or pot.

Plugs themselves are often measured by their height in cm. Mini plugs measure from 4-6cm from base to tip and usually need 8-10 weeks of further indoor growth before planting out. Standard or medium plugs are 6-8cm in height. They will be ready for planting after 6 to 8 weeks. Jumbo plugs (8-12cm) are also called garden-ready plugs and, provided there is no longer a risk of frost, can go straight into hanging baskets, beds and outdoor pots.

How do pot sizes relate to plant size?

Unfortunately, the size of the pot doesn’t necessarily tell you the size of the plant. That’s because there are simply so many variables – different species of plants grow at different rates and have varying habits, root systems and mature sizes. What’s more, plants at various stages of maturity could be in the same pot. So a particular species of shrub in a C3 container (a pot with 3 litres of soil) will cost the same, whatever height the individual plant is.

 Container Volume Diameter in cm's Pot Height
P9 0,7 Litre Ø9 8.5cm
C2 2 Litre Ø17 13cm
C3 3 Litre Ø19 15.0cm
C5 5 Litre Ø22 18.0cm
C7,5 7.5 Litre Ø26 21cm
C15 15 Litre Ø29 27cm
C25 25 Litre Ø38 28cm
C35 35 Litre Ø43 34cm
C50 50 Litre Ø50 38cm
C70 70 Litre 55 44cm

If you know the size of plant you want, but aren’t sure which pot size to plump for, simply contact one of our friendly team (insert contact link), who will be happy to assist you.

What are the benefits of different pot sizes?

Depending on your budget, and the effect or purpose you have in mind for particular plants, the various pot sizes offer their own benefits.

Smaller pots are, as you might expect, more economic, because you aren’t paying to transport greater weights of soil. For some plant species, smaller sizes might also equate to greater ease and higher success rates in potting on or planting out.

However, for instant impact in your outdoor space, you’re likely to want to invest in a few ‘feature’ plants, and they will more often than not be in larger pots. For example, for an ornamental tree, you would need to start at around 15L (and you could expect a tree of at least 120cm height).

Before making your choices, it’s worth looking into the plant species to find out its final height and spread, and how quickly it will get there.

When do plants need repotting?

If you are planning to keep your plants in containers for a patio or balcony, you should not need to change the pot of a newly purchased plant for some time. However, as plants grow they will need more space for their root system if they are to remain at peak health. Here are some ways to know when to size up.

If a plant looks too big for its pot, or is beginning to overbalance, this is a good indicator that a larger container is needed. Check for roots growing out of drainage holes – a sure sign that it's feeling a bit squashed. If the soil looks to be in poor quality, is disintegrating, or is not absorbing water, it’s also time to give your plant a new environment. Finally, if it’s been years since you changed your plant’s container, it will probably thank you for the switch.

What size pot should I choose for repotting?

If you are simply replacing a plastic nursery pot with a more decorative one, you will probably need one the same size as its current container (just be sure to allow for good drainage with some gravel and good quality compost or soil).

If you’re upsizing, choose a pot 2-4 inches (approx. 5-10 cm) greater in diameter than the current container, depending on the plant’s growth rate. For particularly slow growers, you may only need a pot 1-2 inches larger.

What size decorative pot do I need?

If you intend to keep your plant in a plastic pot, but put that into a decorative pot or planter, you need to choose carefully to show your plant off effectively.

The most important consideration is size. You’ll want a decorative planter with an opening about 1cm greater than that of the nursery or ‘grow’ pot. For much larger plants (in pots with a diameter over 24cm), the planter’s opening can be up to 3cm larger, and for smaller ones (up to 14cm), go slightly smaller.

You’ll also want to check that the heights of the original and decorative pots are compatible. Otherwise you risk either the grow pot peeking out over the top, ruining the effect, or the plant being lost inside the planter. As a rule of thumb, go for a decorative pot 1-3cm taller than the grow pot.

Finally, consider your plant’s shape and appearance. If it’s very foliage-heavy, a taller and larger pot can help to keep the plant in proportion, whereas a less bushy species needs a shorter, slimmer pot in order to shine.

If you have any further questions, do get in touch and the team at Glenmore will be happy to offer advice.

Happy planting!