What To Know About Growing in Containers

I’m getting reeducated with growing plants in containers this season, but it was an act of necessity until I realized I’ve never really discussed the topic in this column. From just a simple pot to an extravagant garden trough cast of cement and stone, an expensive terracotta pot or a raised vegetable garden bed, they are all containers.

However, each of these containers is an artificial environment with special challenges that need to be considered for the plants to be successful for more than just a few days. As an example, an 8-inch hanging basket of something as simple as impatiens or petunias will outgrow that container in just weeks. Then what?

My situation is that I’ve received 20 new lily varieties and without the other garden lilies being in bloom it’s difficult to know where the new ones should be planted when considering colors, height, scent and time of bloom. The solution seemed to be to grow the new lilies in pots and allow them to grow to size, flower, and make notes where the new bulbs would fit best in the garden. Then in the fall when the potted lilies have gone dormant simply pop them out of their pots and plant them where they will truly do well and look great with no further transplanting.

Ah, but could I grow the bulbs in pots and maintain them all summer with little to no risk? Would the roots tolerate the constrictions? How often would the pots dry out? And while the lilies were setting roots and not yet shoots would the bulbs simply rot in the pots with no way to get the moisture out of the soil? Just a few of my issues, but there were more.

As the sun hits the dark colored pots this causes the soil to warm rapidly. Would this cook the roots? Would the warm soil affect the flowering? And the rabbits, deer and woodchucks? Easy pickings for the foliage and flowerbud feasts for rabbits and groundhogs? How to deter them? Isolated issues? Not really. I remember coming into work on a late spring morning only to find five or 10 large terracotta pots of annuals and tropicals browsed down to tiny nubs.

Let’s look at the issues that you may have to deal with and how to mitigate most of them. First off, small pots with large plants that will only get larger during the summer means repotting at some point. Take that 8-inch hanging basket of hanging geraniums for an example. Once you get them home from the store it will only be days before you find yourself watering them several times on sunny day as the roots fill the pot. Obviously, the solution is a larger pot, but if you go from 8- to 10-inch diameter, you’re probably going to have to repot yet again in July.

The possible solutions are bumping up to a 12-inch basket, but then you’ve got an excess amount of soil that will hold water and may result in rotting in the root zone. You could trim the root ball and put up to only a 10-inch pot, but that will mean going to a 12-inch pot later on. Not hard to do, but if you’ve got lots of pots and not lots of time then what? The more fortunate among us have gardeners to do this. The even more fortunate simply replace the baskets with new ones as our local garden centers are more than happy to supply potential replacements all summer long.

Another possibility are wire baskets lined with sheets of sphagnum moss or coir. These baskets are available in much larger sizes and present some interesting opportunities. They also drain better, remain cooler and are much better for the plants, but you need to have the time to line the baskets (though some now come prelined) and as these baskets get heavier and heavier with their size then mounting them and holding them up becomes more of a challenge.

Getting back to the plastic baskets, what about color? A dark basket, let's say green, will absorb more heat, resulting in faster plant growth, and the roots will much prefer the darkness in the soil than the brightness provided by a white pot.

Watering will often be the biggest challenge in container gardening. Plants in containers can dry very quickly when exposed to sun and wind, and this is a big problem for deck plantings, around pools or along the bright beachfront deck. These pots may need watering several times a day, more for hangers than others. Oceanfront plantings can dry very quickly, but when the wind changes and becomes moisture-saturated there can be little evaporation from the pots, presenting yet another challenge, overwatering.

The best solution for this seems to be hanging these baskets under some kind of shelter that provides bright light but at the same time shields some of the direct sunlight. Think in terms of a pergola type structure or even some simple latticework that provides dappled shade but plenty of light.

Thick terracotta pots provide some insulation to the root zone, cutting the effects of the sun hitting the pots, but an added layer of protection in a mixed planting is to allow hanging or pendulous plants to droop over the edge of the pot, providing some shade along the rim and below. Keep clear of glazed pots though, especially dark-colored ones. I remember one shiny glazed blue pot that looked just amazing. However, the dark color heated the soil quickly so the plant selections for this pot had to be thoughtful ones.

Always make sure that whatever container you are planting, be it a pot, hanging basket or raised garden, has adequate drainage. No matter how good (or bad) a waterer you or your gardener is, if excess water has no place to drain out of the pot you will likely end up with mush. This is especially true after several days of rain. Make sure there are drainage holes on the bottom of the container and that they don’t get blocked or clogged.

For a raised garden, if your raised bed is atop native soil or some kind of paving, make sure there’s gravel or other porous material at the bottom that will allow water to leave the raised bed if moisture builds up. Don’t assume that a 1-foot raised bed put on top of your dirt area will simply drain into the dirt. Depends on the dirt. Gravel on the bottom will solve this issue even if it’s just an inch of bluestone gravel or pea gravel at the bottom.

The soil you use in any and all of your containers is also critical. It needs to drain well and at the same time be able to absorb enough moisture to allow the roots to absorb the water and nutrients. As always, my first pick would be ProMix BX. You can buy this by the bale or bag and it was the suggestion to use in my lily pots but it’s a pretty universal mix that works for most plantings and pots. Never use water-absorbing gels as these always cause problems, retaining too much moisture.

This is a peat-based mix, and if you allow it to dry out too often it will lose its ability to absorb water andwill shed it instead. If you see this happen you can simply add about a quarter of an ounce of liquid dish soap to a gallon of water and this will rewet the soil. It only needs to be done once though so don’t over do it. At most, in a very dry summer where you’ve allowed the soil to repeatedly dry out, you may have to do this once in July and again in late August.

But there’s no nutrients in products like ProMix, and I don’t like the pre-charged (with fertilizer) soils as you never know when that initial change leaches out and the nutrients are gone. Avoid chemical fertilizers in granular or liquid forms in your planters, and this includes products that may have “Miracle” in their name. There’s a good chance that on a hot day with hot soil the chemical fertilizers will burn the plant roots, and in no time, you’ve got crispy brown leaves instead of green ones.

Instead, use organic liquids like Neptune. Follow the label directions or simply add a small amount each time you water. For example, if it calls for one ounce per gallon of water every two weeks then cut it to a quarter ounce at each watering, or less. Granular organics like Plant-tone can be used in your initial planting mix. Then add 1 tablespoon of the fertilizer per 3 inches of pot diameter, and work it into the top of the soil every four weeks or so. Remember, organics are naturally timed released.

One other thing to consider when planting early in the season or later in the season: Pots and raised beds cool faster in early spring and early fall than those areas where plants are directly in the soil. Plants in planters and raised beds may therefore be susceptible to early and late freezes more so than similar plants that are direct planted.

Container plantings are great as are raised gardens. This is especially true if you are getting older and find it more and more challenging to bend over or work on the ground. These are great solutions as long as you and your gardener(s) know the challenges and how to manage them. Keep growing.