Written by: Dottie Baltz
The following is my advice on growing vegetables in containers for outdoor use. Be advised that techniques can differ based on your geographical location, the type of plants being grown and whether they are indoor or outdoor containers.
Choosing your Container
Choose larger containers whenever possible so you don't have to water them as often. The only exception would be for shallow rooted plants like lettuce or spinach. You can grow them easily in a shallow container to save space and to save soil. I try and stay away from unsealed clay pots as they absorb a lot of water. I still love the look of clay though, so you can soak them overnight in water before use and that will help retain moisture. You can also seal the outside of new clay pots with a specially formulated clay pot sealer or by brushing on clear polyurethane. Painting the pots with acrylic paints will also help to seal the pots, though you will want to seal the paint also to preserve your paint job. Herbs prefer to be on the dry side, so if you have clay pots you would like to use, consider reserving them for herbs.
Containers come in a variety of materials, shapes and sizes such as clay, plastic, concrete, ceramic, wood and resin. I prefer plastic and resin as they are lightweight, relatively inexpensive, last for many years and come in hundreds of shapes, sizes, colors and designs. Whatever you choose, make sure there are adequate drainage holes. Many resin containers do not have holes drilled in them so you can use them for other things, such as a water garden. They are very easy to drill, however, with a standard ¼" - ½" drill bit. They also sell containers that are self watering which can come in handy on hot days or while on vacation.
When using especially large pots, take into consideration what they will weigh when full and if they will need to be moved once you set them in place. If there is any chance you will have to move them, I'd suggest putting them on a plant dolly before filling the container with soil and plants. Plant dollies are usually flat and made of weather resistant wood or metal and have four caster wheels on them. They come in different sizes depending on your pot and are usually not visible when under the container. Make sure the weight rating on the dolly can handle the weight of the container when full of soil, plants and water.
Remember that most anything that can hold soil can be used as a container for plants, providing it has proper drainage holes, so use your imagination and have fun with it. Wooden boxes, 5-gallon buckets, totes, and pails all make good containers.
Choosing Your Growing Medium
Soil for containers should be lightweight and airy to allow the roots of your plants to get the oxygen and moisture they need to be healthy. Soil from your garden is too heavy and should not be used.
I tend to buy prepackaged organic potting soil in large bags at my local garden center or discount store because it's easier for me, though I know many people who like to mix their own potting soil. I like to add 1 part compost and 1 part composted manure to 4 parts potting soil to my containers as I feel it helps to retain moisture better than potting soil alone without smothering the roots. The compost and manure also add important nutrients and organisms to the soil that plants need to survive.
If you want to make your own potting soil, I suggest 1 part compost, 1 part peat moss, 1 part course sand, 1 part perlite or vermiculite and enough bone meal, blood meal or another organic fertilizer, following the package instructions for the amount of soil mix you are making. If you don't have perlite or vermiculite, substitute peat moss in it's place. Leaf Mold is also a good addition to any soil mix. 1 part leaf mold added to the above ingredients is fine. You don't have to be exact with the measurements and you may need to adjust the mix based on the types of vegetables or herbs you are growing. I have a whole page dedicated to making your own soil mixes and fertilizers that you can check out here.
There are many vegetables suitable for growing in containers. In fact, there are many varieties that will grow smaller or varieties that are especially suited for container gardening or "patio" gardening. They will usually have this information on the seed packet. Cucumbers and pole beans get to be a little tricky to grow in containers because they like to vine so much, but they can be trellised and some come in a bush variety so they take up less space.
Keep in mind your container size when choosing vegetables. Lettuce, spinach, green onions, parsley and most herbs only need 6"-12" of soil to grow well. Tomatoes and Cucumbers grow better in a 5-gallon bucket and most everything else grows well in sizes in between.
How to Plant
You can choose to start your vegetables from seed or buy transplants. For the most part, I start mine from seed as some veggies do not have a long growing season or they prefer not to be transplanted. Lettuce, spinach, radish, onions, peas, beats, carrots and beans are all very easy to start from seed. Most herbs are also very easy to start from seed.
Tomatoes, cucumbers and peppers have a little longer growing season and can be difficult to start from seed, so I like to buy transplants to add to my containers.
Consider companion planting with your vegetables and herbs to enhance flavor and to deter pests. Check out my article on companion planting for more information. You may also want to practice succession planting, meaning, plant one container and then plant another one of the same thing a week or two later so that they are not all ready to harvest all at the same time.
I prefer to pre-moisten my potting soil before adding it to my containers, especially if the mix is heavy in peat moss. Peat moss can be difficult to hydrate so it could take some time. One way is to add warm water to the potting mix as the peat moss absorbs warm water faster than cold water. Either way stirring the water into the potting mix will help it to absorb faster. Add water until the soil begins to stick together when you squeeze it in your hand.
Depending on your growing zone, you can plant cool season crops about 4-6 weeks before your last frost date and then plant your warm season crops when night-time temperatures are consistently in the mid 50s.
Put your potting soil in a wheel barrow or some other large flat container so it is easy to mix. Add water as directed above. Fill the containers approximately halfway with potting mix then add your plants if you are using transplants, then continue filling with soil. Keep the root ball of the plant at the same depth it was growing in the original container. Check the roots and see if they are root bound and wrapping around each other. If so, gently loosen the roots before planting. If sowing seeds, fill the containers all the way, leaving 1" or so of space at the top to allow for watering later, and plant your seeds according to the package instructions.
Most vegetables and herbs require full sun to grow and fruit well. Full sun is generally defined as six hours or more a day of direct sun. Most plants benefit from a few hours of shade during the hottest part of the day. But in general, give them as much sun as you can.
Watering and Fertilizing
Watering may need to be done everyday, especially when the heat of summer kicks in. I check my containers every morning and again in the later afternoon by either sticking my finger into the soil to see if it's dry or by simply lifting the pot to feel how heavy it is. A very lightweight container indicates that it needs watering. Always water the plant until water comes out of the bottom of the container. If the container has a saucer on the bottom, allow the water to drain into the saucer and then dump out the water. Never allow your plants to sit in water. If you are using self watering containers, fill the reservoir with water as needed. In extremely hot conditions, I have also had to water them from the top. You can do what's best for you and most come with instructions from the manufacturer.
If it has rained, don't assume that your plants don't need watering. It's still important to check the pots because for some reason, when it rains, the pots never seem to get watered as much as you think they will.
If you have used the proper amount of organic fertilizer in your mix and have included compost and manure you should not have to fertilize very often. Herbs prefer a leaner soil anyway and many plants won't form fruit and vegetables if there is too much nitrogen present, so I wouldn't fertilize too much during the growing season as it's just not necessary.
About halfway through the growing season, I will fertilize with compost tea about twice a month. If you have been getting a lot of rain, you may need to fertilize more often as the nutrients may be washed out of the soil more quickly.