What you see in a plant’s foliage is almost always the result of what is going on in the soil. When there are issues with soil quality, there will most certainly be issues with the root system. Small or poorly established root systems will mean a smaller plant, or often much worse. Because we do not see what is going on in the soil, we often overlook this aspect of the plant. Personally, I like to think of the root system as the brains of the plant. Everything starts there.

How can we help create larger root systems? With container plants, especially annuals, there are several growth phases where a clearer understanding of proper watering and nutrient adjustments will benefit various aspects of the plant. At the end of an annual’s growing season, the plant is harvested and then removed. Without further root development, the early phases of growth are critical. Let’s take a look at how to optimize root expansion, and review its ultimate effect on plant yield and vigor.

Two Types of Roots

Plants will primarily have one of two types of root systems: taproot or fibrous. Carrots and beets are examples of taproot plants, while the roots of basil and thyme are fibrous. There are many different types of roots within these two groups.

Taproots have lateral shoots to take up nutrients for the plant. In most edible fibrous plants (annuals, biennials and perennials), the smaller, more fibrous roots are responsible for nutrient and water uptake, as well as respiration—the uptake of oxygen. Knowing what type of root system your plants have is important, especially when you go to remove the plants from the soil.

Roots During Transplanting

If you’re growing plants from small starts rather than seeds, it is important to minimize shock and avoid traumatizing the roots. This means tearing open the root system with one’s hands during planting should be avoided. If the roots look like they are overcrowded and circling, this needs to be dealt with. The least traumatic way to do this is to take a sharp knife and cut several vertical slits in the root ball in 3-4 different spots. This will stop the circling and encourage the roots to go out in new directions with the least amount of trauma. Remember, trauma kills some or most of the roots, setting the plant back and reducing vigor.

During the sensitive transplant phase, the soil should be kept moist, not soaking wet. When soil is too wet, roots can’t breathe. The plant’s root system takes up oxygen and uses this to convert sugars into energy. The plant wants a good combination of moisture and air for optimal vigor and health.

Depending on the soil and temperature, watering during this phase should be done daily or every other day. Use a good-quality moisture meter to check. The lower portion of the soil should never dry out completely, but the top should dry towards 30% of the saturation level. The longer the time interval between waterings, the drier the surface soil, compelling the plant to create deeper roots to seek out moisture at the lower depths of the pot.

Avoid using strong nutrients during the transplant stage. Typically, the nutrients already in the soil are enough. Do not over-fertilize! Fertilizer is stimulating and a tender new transplant doesn’t need much stimulation. Wait until the plant has settled into its new container.

Roots During Phase I

Phase I of plant growth takes place during the first week following transplanting. This is the phase when a plant is most vulnerable to transplant shock. The roots are generally small and sparse as the tiny, fibrous roots have not yet begun to develop. It doesn’t take too many adverse conditions at this point to cause plant death. When stress occurs during this time, plant failure may not happen right away, but will show up later. Many problems can be attributed to stress during phase I.

Roots During Phase II

Phase II generally begins 7-10 days after the plant has established itself and is ready to begin growing. The root system is still small, so now is the best time to focus on developing a large, full root system. There are a number of nutrients that will help the roots expand, and deep watering practices are also important during this root development stage. For example, if we keep the soil too moist during this time, where is the incentive for the plant’s roots to expand?

They can get all the water and nutrients they need right there near the surface. Instead, make sure the soil is adequately saturated throughout the container from top to bottom, then allow enough time between watering so the surface dries and the roots expand to reach the lower levels of the pot to get water.

Depending on evaporation and transpiration, the soil near the bottom of a container can easily stay wet for as long as a 5-7 days. Slow-drip watering can accomplish deep penetration of the soil. Rapid application of water will leave portions of the root ball dry, which leads to the roots not developing deeper in the container.

The root development phase is often overlooked as the gardener goes directly into focusing on foliar growth. Large roots are important in terms of higher production. Soil types and temperatures can have a major effect on both evaporation and transpiration. Check the soil’s moisture levels to determine what your plant needs. Daily watering may be necessary, so always check.

Roots During Phases III and IV

During phases III (vegetative growth) and IV (fruit development), the plant will need to take up more nutrients and water. Before these phases begin, you should have helped the plant develop a large enough root system to provide these nutrients. This is accomplished by careful examination of the water-holding characteristics of the soil you are using and the rate of plant transpiration, and varying the watering schedule accordingly to produce maximum root systems while maintaining plant health and vigor.

Soil Saturation

Soil saturation is that point at which a particular soil will hold no more water or moisture. Different soil types have differing saturation levels, usually from 20-50% (the ratio of the volume of water to the volume of soil). When watering a container plant, if watered evenly throughout the soil structure and slowly, this will be the point where water begins to exit the bottom of the container. When a soil becomes too dry, the water is increasingly attached to the soil. The wilting point of a plant is reached when its roots can no longer acquire the moisture from the soil.

Enhancing a Plant's Root System

When water and nutrients aren’t enough, consider adding mycorrhizal fungi to the mix. Mycorrhizal fungi have a symbiotic relationship with roots and will make them appear and function as though the root system was more dense and extensive. The greater the existence of these beneficial fungi, the more productive the root system. One thing to note when purchasing mycorrhizae is that freshness counts. High temperatures kill many of these organisms. They need to be properly handled to give the roots of your plants the benefits you are looking for.

Taking Visual Clues

As you practice examining roots after harvesting an annual or biennial, you will become familiar with how a well-developed root system looks. You may even want to take photos. Gently wash soil or other media away to get a good look. When you get plants with under-developed root systems, try to note this and discover what might have been different that caused the problem.

Don’t make too many changes during one cycle, and if you do, make sure to note what they are and write down the results. Where possible, making notes during the ongoing harvest will help. This is often subjective, but still useful. The ultimate root system, when cleaned and photographed, can be more objective. Make sure you note what you change and use the resulting harvest and roots as a guide for your next grow.

Getting continuous, maximized yields from your crops will require a somewhat scientific approach. It doesn’t matter what type of media you are growing in, analyzing how the roots of your plants have done during their growth cycle is key to evaluating the quality and type of nutrients, water and watering schedules you have used throughout the various plant phases. Consider all of these things in addition to the soil or other media used to determine what changes you will make in your next cycle.