Assessing A Plant’s Best Place To Put Down Roots

In last week’s column I suggested that now is the perfect time to look at your houseplants, and how to recognize the symptoms that indicate that a plant needs to be “bumped up” or repotted. The process is fairly easy and straightforward, but one of the major decisions you’ll have to make before repotting is what type of pot to use.

There wasn’t always a choice, though. Up until the 1960s virtually all pots were made of clay. This made the pots heavy, expensive and fragile and it also limited how far a grower could ship plants because of weight issues and breakage.Then there was the miracle of plastics. At the end of the 1960s methods of plastic injection and extrusion led to the introduction of inexpensive, lightweight plastic pots. Almost overnight the cost of plant pots dropped by nearly 80 percent and pot weight dropped by even more. This meant not only lower costs for plants but it also meant that growers hundreds to thousands of miles away could now ship potted plants at a lower cost than just shipping a clay pot without a plant. This was what led to the houseplant or indoor foliage boom of the late 1960s and early ‘70s, when plant stores were on nearly every corner and when yours truly made it through college selling potted plants.

But plastic pots brought to the market their own set of issues. Unlike the clay-based pots, the plastic pots’ costs were directly related to petroleum costs, and during periods of oil shortages or spikes in oil prices so went the price of plastic and pot prices.

Just as important was that both growers and consumers had to learn how to grow plants that were in plastic pots. Moisture doesn’t evaporate from plastic pots as it did from the porous clay pots, and the new lightweight peat-lite soils that were developed to be used in the plastic pots were also lightweight, so along with the lightweight plastic pots, plants were much less stable than in the days when heavy clay pots and heavier soils anchored the plants down. But most of us have learned and adapted, save those who still overwater. And yes, overwatering is probably the biggest issue that remains with growing plants in plastic pots.

I have my prejudices though. Yes, you can learn to grow just about any plant in a plastic pot. This learning involves using the right soils that drain well, and you also need to learn how to “read” a plant to attend to its watering needs. All fairly easily done if you pay attention.

Yet another, somewhat vexing issue has evolved with plastic pots. Their sizing. In the days when clay was king there were just a few kinds of pots. There were standard pots, where the diameter and depth had a set relationship (close to one-to-one), there were azalea pots, which were more shallow than deep, and there were bulb pots that were shallow and very wide. Oh, and everything was measured in inches.

Now, with plastic pots and various attempts at “standardization,” we have anything but. Plastic pots can be measured in inches or centimeters for depth and width, and the volume of a pot can be measured in pints, quarts, gallons or liters. It doesn’t make things easy and that’s especially true when you buy plants on the internet, where one grower in New York may be using a totally different potting system from one in Ohio. My prejudice, however, for houseplants and outside displays is for clay, and I always try to do my measuring in inches unless it’s nursery stock, when I look for gallons.

And just as there was a revolution in pots, there was a corresponding revolution in soils. When clay was king, greenhouses would use soils just like those in your garden for potting soils. They would add amendments like sand for drainage and compost for some added nutrients, and to avoid introducing pathogens to the soil that could damage or kill the plants, the soils would be sterilized with steam or heat from electricity to create “clean” dirt.

To keep everything on the light side, when plastic pots emerged on the scene a new type of potting soil was developed by Cornell University called peat-lite soils. These potting soils were (and are) peat moss-based with other amendments added such as perlite and vermiculite, depending on the intended blend—and because peat is innately sterile there was no need for heat treatment. These potting mixes were primarily manufactured in Canada, Michigan and Germany where the peat bogs are, compressed into plastic bales of several cubic feet and shipped to the greenhouse, where they could be stored or used immediately. It’s these peat-lite mixes that are still the basis for the potting soils that you find in the garden centers.

So here we are in 2016, when you can choose clay or plastic and you have a choice of several brands of potting mixes. Each brand of potting mix is a little different and you’ll find one you like and probably stick with it. Some come with a dose of fertilizer mixed in and some don’t. The bag will tell you what’s in the soil.

There are two downsides to these soils, though. The first is that a tiny insect known as the fungus gnat loves to live and reproduce in these peaty soils. The good news is that they are easy to control. If you should happen to buy a bag of potting soil and you see tiny black gnats fly out when you open it, then immediately return the soil, demand a refund and try another brand. The other downside is that when peat is allowed to repeatedly dry out it loses its ability to absorb water. For this reason, it’s a good idea to at least keep it ever so slightly moist between waterings.

Another consideration is that peat is somewhat inert, has little nutritive value and limited microbial activity. Some gardeners like to take a teaspoon to a tablespoon of garden soil or well-rotted compost and add it to their peat-lite potting mixes. This reintroduces soil microbes that are beneficial in stimulating root growth and the release of nutrients to the roots. Add just a little, though. And if you buy a potting soil that has no added fertilizer you can add your own and not have to worry about feeding your plants for months and months. For this you’ll want to check out a product like Osmocote, which is a fertilizer that’s micro-encapsulated so that the nutrients are released slowly over a period of several months. You can find products like this for a number of different plant types, and the package will also tell you about how long you can expect the fertilizer to last. They are not organic.

So those are some of the potting and pot basics. This is the time to get this work done, before you get too distracted by outdoor work. Your pot-bound plants will appreciate it and you’ll be amazed at how great they look when repotted. Keep growing.