Various Pot Plant Sizes

How to Choose the Right Size Pot for Your Plant

Are you thinking of growing veggies and flowers in pots? Great idea! But what type of pots should you choose? You’ve a lot more leeway than the average production horticulturist who usually works with standard pot shapes and sizes, starts small to save space, and keeps growing on to larger and larger pots as the plant grows.

Ultimately, the pot sizes and shapes you use will be determined by a combination of factors: the type of plant you’re growing, practical considerations, and aesthetic ones. For the last of these, that’s up to you!

Why Starting Small and Moving to Big is Often Best

Have you grown seedlings in flats before? You can fit literally hundreds of plants into a few square metres of space. That makes caring for them really easy, and you’ll lose less water and plant food than you would if they were in bigger containers. Of course, they can’t stay there, so as soon as the roots have developed a nice, sturdy “plug,” it’s time to transplant.

When choosing the most practical size for your seedlings to be moved to, think about the growth tempo. In general, I think it’s worth going from plug or 10cm seedling pot into the “finished” size when it comes to annuals. They develop so fast!

As a broad rule of thumb, the “finished” size will be one third to one half of the final height and spread of a typical upright-growing plant. That should prevent it from getting top heavy and blowing over at the smallest gust of wind.

Tall Pots versus Wide Pots

When you water a plant, gravity takes the water down through the medium – but it’s not the only force that’s at work. The capillarity of your growing medium also has a role to play. As a result, pots have a layer of wetter soil at the point where capillarity cancels out gravity. This is known as the “perched water table.”

In practice, it means that tall, narrow pots will tend to dry out faster than shorter, wider ones. The perched water table is the same for both, but in the case of a tall pot, the perched water table is out of reach of the plant’s roots. In a broader pot, the perched water table is nearer the soil’s surface making for a higher level of moisture at root level.

Use this interesting fact when considering specific plants and what their water needs are. Tall pots will be better for plants that like dryer conditions, while broader, shallow pots are best for plants that like wetter conditions.

Know Your Plants

Some seedlings, especially those that are to produce root crops, are best sown where they are to grow. That doesn’t change when you’re growing in pots. With root crops, carrots for example, you obviously need enough depth for the roots to be well-formed. If the roots are shallow, for example, with radishes, depth is less important.

Some plants just don’t like to be transplanted at all. That means either sowing them directly into the pots where they’re to finish, or starting them in smaller pots and making sure the root balls will hold together well before attempting a transplant.

This brings us to transplant shock in general. It’s something you can mitigate to a large extent, but avoiding it altogether is certainly an argument in favour of sowing the seeds in the pots where you’ll finish them. You will use more space when they’re young, and they’ll take more water and liquid feeding per plant than they would in flats, but you get to skip transplanting altogether.

With most veggies being ready to harvest in three to four months, the potting soil doesn’t really get time to start losing structure, so the lack of fresh potting soil in a straightforward “sow and grow” approach needn’t be an issue.

General Pot Size Recommendations for Veggies

Not all pots are the same, but there are still some fairly standard sizes. Let’s try for a few generalisations that you can use as a starting point.

  • Carrots: Choose a container that’s at least 5cm deeper than the ultimate length of your carrots.
  • Cucumber: For most varieties, a 20cm-25cm pot is big enough.
  • Brinjals: They’ll need a bit more space. A 25cm to 30cm pot should do.
  • Green beans (Bush): Use a pot no smaller than a standard 20cm nursery pot.
  • Parsley: A 15cm to 20cm pot should give ample space.
  • Butter lettuce: A 20cm pot should be roomy enough.
  • Capsicums: Choose a 20 to 25cm pot and grow two plants per pot.
  • Radish: a 20cm pot with 3 plants or a window box of about the same depth.
  • Tomatoes: Choose a 30cm pot and dwarf tomato varieties.

For many urban farmers, growing veggies means growing in pots. Your most frequently-used sizes will be 20cm to 30cm pots, but most herbs will be fine in 20cm pots too. Play around with these basic guidelines and see what works for you! Every gardener’s conditions are different which makes general recommendations a place to start from while you work out what’s best for you.

Perennial Plants

Perennials, whether flowers, veggies, or any other type of plant, do need the progression from one pot size to the next. The basic idea is to let them develop enough to transplant easily without letting them get so root bound that they get stunted (unless you’re growing bonsai.) Apart from looking better in a size that suits their scale, transplanting gives them fresh, aerated potting soil and, when the medium contains slow-release nutrients, a fresh dose of food.

I test my plants by tapping the edge of the pot lightly and then trying to slip out the soil cake. If it tends to disintegrate, it isn’t ready yet! You want to see a network of roots holding the root ball together, and at this point you can move on to the next pot size up. It’s just a rule of thumb though. Some plants love being pot bound and may even stop flowering if they’re given too much space.

Wrong Pot? Your Plants Will “Tell” You

Troubleshooting your pot size choices is a matter of watching your plants. You might be in for a few surprises. For example, I planted Coleus in short, wide pots as well as standard 20cm nursery pots. The latter got much taller than their counterparts and did altogether better. The message? It pays to experiment – and remember, something as simple as changing your brand of potting soil might change everything!

Plants that get so root bound that the soil is just a mass of roots, or get too top-heavy and tend to fall over are obvious candidates for bigger pots – if not this year, then next year. On the other side of the coin, plants that never grow to match the size of their pots might be candidates for a smaller container next time around.

I love gardening. I even love the fact that it’s impossible to know everything there is to know about it! What works for me may not work for you, however, so use the information you see here as a suggestion, and adapt your methods based on the things your plants tell you!

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