Potting Up – Which Pot Size is Correct for Potting On?

You have been growing seedlings in small pots and it’s time to move them to bigger pots. Or maybe you bought a small houseplant and it has grown too big for the pot and it needs more space. What size of pot should you use when potting up? Is it better to pot up one size at a time, or go to a big pot right away?

Is there a right answer or does the answer depend on your goals for the plant, the type of soil or the type of plant?

The answer is going to surprise many of you and it’ll show the value of listening to science.

Potting Up – Which Pot Size is Correct

I’m going to split this post into three part. Part one will look at common gardening advice, part two will look at what science says and part three will summarize things and give some guidance.

Potting Up – Gardening Advice

Online gardening advice is almost unanimous. You should pot on using a pot that is one size larger, or one to two inches bigger.

If you are growing seedlings you should move them from a small pot to a medium pot, and then a larger pot before moving them into the garden. Same goes with houseplant cuttings; always start in a small pot and move up slowly. Here are some explanations as to why this is important, according to gardening advice.

“A smaller amount of soil dries out more quickly and allows more oxygen to reach the roots, which they need to survive.”

“Tiny seeds and seedlings don’t necessarily want to be swimming in a huge sea of soil. It is easier to overwater, and their roots might struggle to develop. They do like to be hugged, just a little.”

“Moving from a seedling start mix into richer soil is a great growth-encouraging step – one we’d miss out on if we started in big pots.”

“If you put a very small plant into a large pot filled with potting medium, most of the medium will not be occupied by anything except bacteria and other life forms, some of which may not be desirable.”

“The extra potting medium may become ‘sour’ because it stays wet longer.”

“If you put a small plant in a big pot, you won’t get as much initial top-growth as a smaller pot, because it will develop it’s roots in the greater space at expense of foliage.”

From the Royal Horticultural Society, “an overpotted plant will show signs of stress because the large volume of new compost that was added when it was potted up is sitting wet for a long period, reducing aeration around the roots. Instead of the roots growing out into the new compost, they simply rot. Plants in the ground do not suffer the same fate as the soil is inherently better drained than compost in pots.”

I’ll review some of these claims below because they contain a number of myths.

Potting Up – What Science Says

The North Carolina Extension Gardener Handbook has this to say, “Choosing larger containers is beneficial, as they do not dry out as quickly and require less frequent watering. ”

Wow! That is the exact opposite of common gardening advice.

Growing in pots is big business and it’s a method used in a lot of research. Consequently there is a lot of data on this subject.

In trials using beans, plant growth increased with pot size, but smaller pots could produce equal growth if enough fertilizer was used, “showing that soil fertility was more critical than pot size in determining plant growth. ”

Growing tomato seedlings in spring using two different sized pots, found that both pots produced the same vegetative growth, but the larger pot produced a higher yield.

Germinated seed of Jatropha curcas, a species of flowering plant in the spurge family, grew best when planted in larger pots.

The Journal of Horticulture, in reviewing the growth of ornamental plants (houseplants) concluded that “research has confirmed that the larger pot size allowed the plant to grow taller whereas the smaller pot size restricted the height of the plants.”

And finally a meta study that looked at 65 studies that analyzed the effect of pot size on growth, found that “on average, a doubling of the pot size increased biomass production by 43%. Reduced growth in smaller pots is caused mainly by a reduction in photosynthesis.”

There is some science that suggest fruit trees grown in small pots may flower at an earlier age.

Many of the studies tested pot size along with fertilizer levels and/or water stress. Fertilizer and water are more important to growth, and can to some extent compensate for small pots.

The science is quite clear, larger pots produce more growth because plants are growing in a healthier condition.

Why are Bigger Pots Better?

Leaf and root growth is interdependent. The leaves produce the food that roots need to grow, and roots provide the water and nutrients leaves need.  Within certain limits, one will not grow without the other. A small pot restricts root growth, which in turn reduces top growth.

The idea that a big pot causes roots to grow at the expense of top growth is incorrect.

Plants with larger root systems at the time of transplant normally suffer less than transplants with root restrictions due to small pots. This is especially true for pot-bound plants.

Roots in smaller pots develop differently than those in large pots, tending to have fewer primary roots and smaller tap roots. The competition between crammed roots in a small pot results in lower oxygen levels which is the opposite expected by gardeners. Small pots also increase the chance of circulating roots and they increase root temperature because more are located between the soil and pot, an area that heats up quickly.

Above ground growth is also affected. Small pots result in less branching, as well as smaller and fewer leaves. Tomatoes will flower a few days sooner when transplanted from larger pots. Large pots can increase yield in some crops, but not all.

Seedling roots placed in larger pots will have good access to space, water, oxygen and nutrients. As they grow outward from the plant, they will not be hampered by a pot wall, allowing them to grow normally for a longer period of time. The inside pot wall is a dry place with few nutrients, especially once a lot of roots congregate there. It is not an ideal place for roots to grow.

Gardening Myths About Pot Size

The biggest myth is that potting media stays too wet and plant roots will rot. I am sure this belief is a result of the common advice given to all new gardeners, “don’t overwater your plants or the roots will rot.” Unfortunately many people interpret this to mean that pots should not receive a lot of water, but that is incorrect. What this advice is trying to say is, “don’t water too often.” Most potted plants can sit in water for several minutes, completely soaking both the media and roots, without any harm or rotting, provided the pot is allowed to drain afterwards. Even orchids can be submersed in water when it is time to water  them.

What kills plants is getting such treatment every day. They need to dry out between waterings.

A small plant in a big pot does have to be watered less often. Some water leaves by evaporation and some is absorbed by the roots. This causes more water to move from the perimeter of the media, to the center. This is a good thing, especially in dry conditions like a home in winter.

When potting soil is watered, the water soaks into the particles and fills both the small and large pores. Water in the larger pores quickly runs to the bottom of the pot. Within minutes, the media at the top is both moist and full of oxygen. It is true that the bottom inch or so does hold on to more water and does stay wetter, but most of the excess water runs out the bottom of the pot.

Root growth from a seedling happens in the top part of the pot, not in the bottom wet inch of media. That is why they don’t rot.

Heavy soil that contains a lot of clay can get saturated with water and push most of the air out of the soil and is one reason why it is not used in containers. But this does not happen in commercial potting media which is extremely porous and contains lots of large pores.

The above RHS comment said, “Plants in the ground do not suffer the same fate as the soil is inherently better drained than compost in pots.” Nothing can be further from the truth. Commercial potting mixes are extremely porous compared to most garden soil and it drains really well.

“Wet potting media gets sour.” Sour indicates acidity and there is no reason for potting media to become acidic because it is holding water. This is easy to check. Take a pot, fill it withy media and water it the same way you water your other plants. It will be fine.

“Seedlings like to be hugged”. Comparing plants to human characteristics is called anthropomorphism and it’s quite common. What is also true is that it encourages belief in more myths which is a good reason not to do it. As science has demonstrated, plants grow better without a hug.

Why Does Gardening Recommend a Small Pot and Science a Big Pot?

I have no idea. The following is just my speculation.

Watering plants has always been difficult for new gardeners and therefore experienced gardeners suggest using small pots. They are much less likely to be overwatered. So the suggestion is given to keep people from killing their plants and not to grow bigger plants. If you have a choice between a dead plant and a small plant – which is better?

Most gardeners love rules. It is easier to learn to pot up by one size rather than learn how to water correctly. I can teach the former in one sentence, but to teach the latter takes a 5 minute video.

People don’t think through a problem. When I first heard about the potting up rule, I asked myself, “Why should tomato seeds be started in small pots when direct planted peas are grown in a huge pot – the ground outside? Either the rule of small pots is wrong, or we should be starting peas in small pots as well! If roots rot in a large pot, why don’t they rot in the ground?

Gardeners, like everybody else, hates change. Grandpa said pot up slowly, so did dad, so why should I do something different? Don’t you just hate it when science proves grandpa wrong?

Which Pot Size is Correct for Potting Up?

Should you move one size up or go right to a big pot? The correct answer is ……. it depends.

Plants do better in a bigger pot. If your goal is to grow a big plant, go with the big pot. But there are exceptions.

People who overwater and kill plants should not use a big pot, at least not until they learn to water correctly.

People growing in rainy seasons can’t prevent overwaters plants outside. Using smaller pots makes sense here.

The goal might not be “big plants”. In most cases we don’t want indoor plants to grow too fast. Restricting their growth by giving them smaller pots might be the best option. Once such plants reach a desirable size- based on our wants, not that of the plant, it is a good idea to just report into the same size pot.

Science knows that plants grow best in big pots, but most nurseries start in smaller pots. Why? It’s a matter of space, water and fertilizer costs. It is not profitable to grow in bigger pots. The same can apply to the home gardener. Space on a window sill or under lights is at a premium. I have to balance space under my lights with pot size every spring to get my vegetable seedlings ready. Some gardeners have very small outdoor gardens and also have to balance space there.

A lot of people are interested in growing vegetable seedlings for the garden. Some even like to start seedlings in tiny pots like eggshells, ice cream cones or Jiffy Peat Pellets. Don’t do that – they’re too small. Start with the biggest pot you can accommodate.

Which size pot should you use? I hope this post has given you the facts. You can now apply these facts to your situation and use the appropriate pot size.

But whatever you decide, don’t blindly follow the gardening rule of moving up one pot size at a time.