E-mail- The Right Size Container for Plant Growth

hey Doc - I’ve been stocking my flowering room with plants which have grown vegetatively for a minimum of 2 weeks. on the strongest plants selected for flowering the roots are typically matted up pretty good in the bottom of the 4 inch square pots I veg them in (from rooted cuttings). I do this because in a few different gardening books and stuff it's been mentioned that this root bound plant will more thoroughly fill out its new larger container's area of medium with more roots. do you think i might be better off using 1/2 gallon bags instead of the 4 inch square pots?


Thanks for the E-mail. I have written about potting up plants and container. The answer depends, if the roots are really matted up, meaning they are growing in circles and all twisted, then I disagree. The roots have wasted time and energy growing in a circle and you will have to cut and remove them to get good growth. I think the idea might be to get the plants to completely fill out the pot, and transplant it just before they grow in circles and around each other. I personally have found that bigger is better. When roots grow up against something hard, like a pot bottom, or a rock, they send a message to the plant that they can’t keep growing. If most or all the plant roots send this message, the plant will slow growth, and be a smaller plant with a lower yield. There is no such thing as too big a container. Well, sort of, the size constraint is not for the plant, but the space you have. If you can get enough plants in the area with ½ gallon pots/bags, I’d use that vs. the smaller pots.

Please read my previous post on containers and plant growth

Good Growing,
Dr. E.R. Myers

Containers - Know which is best for your plants

A while back I wrote about containers and suggested some minimum container sizes for plants that are good to grow indoors – Containers LINK. I would like to build on that and lead into the topic of how to set up a grow room which I will be talking about soon.  Most indoor growers will use several sizes of containers. Typically, plants are germinated or placed into small containers after germination and then transplanted to progressively larger containers until flowering. Starting many plants in small containers allows the grower to pick the best plants and get rid of the weak ones. Good growers cull inferior plants at each transplant. Starting many plants also allows growers to look for traits and attempt to incorporate these traits into a breeding program.

Seeds can be germinated in starter plugs and grown for 1- 3 weeks. If you are not growing in new potting soil, you should add nutrients. Young plants need all nutrients (N-P-K and micronutrients) but for seedlings, like cuttings you should give them a fertilizer with a high phosphorus percentage to promote root growth. No matter what nutrient regime you use, your plant growth will be inhibited if you do not transplant them into a larger container. I like the starter plugs because like rockwool they are easy to move and I think really minimizes any transplant shock. If I don’t use a starter plug I like to start seeds on paper towels and put a seed in a plastic cup with a potting soil mix. I like plastic cups because they are cheap, easy to get and are light and easy to move around. A six ounce cup will hold a plant for 1-2 weeks maximum. I use the 16 or 20 oz. cups for 3-4 weeks respectively. A half gallon container  is good for plants for upto six weeks. For minimum sizes for different plants please see my previous container post .  A good rule of thumb I have heard is that you should give a plant ½ gallon of medium to support 1-2 feet of growth. This rule depends on your use of fertilizer and proper watering. Obviously, plant growth habits differ so if you have a plant that has branching you should grow your plants in larger containers. As you can see in the picture at the bottom of this post, plants grown in containers that are too small are stunted and I can tell you will have lower yields.

While transplanting or ‘potting up’ is important to save space and enable a grower to pick the best plants from many, plants do better if they are NOT transplanted often. You can count on losing 3-4 days of growth after each transplant. However, if you are growing many different plant types, have multiple grow areas, or move your plants around you will need to do some transplanting to be a good grower.

As I have mentioned, my favorite containers are plastic pots. Plastic pots are inert, reusable, lightweight and convenient to use. Metal containers may react with the medium, nutrients or plants. Plastic bags are also convenient with their square bottoms. Fiber containers are also popular and I have used many types myself. They are not too expensive, can be used for more than one growing cycle and are easy to dispose of in a compost pile or in an outdoor garden as organic matter.
I will end with one last piece of advice concerning containers…. When you are transplanting into larger containers, some growers put sand or gravel at the bottom to increase drainage but also to balance the plants so that they do not easily tip over.

Good Growing,
Dr. E.R. Myers

In the picture below, notice the two white meter sticks between the plants.   I have planted 1, 2 and 3 corn plants in identical sized small plastic containers. I assume this means that the plant have 100% 50% and 33% of growth medium from left to right. You can see the size difference with your own eyes. For this little experiment, all plants germinated the exact same day, were grown under the same 600 watt HPS and given the same fertilizer regime. These plants are also a hybrid from the same batch so they are genetically the same. The only difference is due to the lack of container space for the roots. Notice too that the single plant on the left is a darker green. This is a sign of plant health and vigor. I have noticed that plants that are in larger containers and growing rapidly have a greater disease resistance than plants that are stunted in small containers. You may hear people tell you that you can grow plants in small containers if you provide all the right nutrients and water but if you have small containers and your plants roots are not growing this means that overall above ground growth and yield will be lower. Growing roots send out hormones telling the plant that things are ok and that is should continue to grow. If a root is blocked inside a small container it will tell the rest of the plant (via hormones or other molecular signals) not to grow since water and nutrient uptake may be limited.