'Necks' on African violets
African violets should be repotted about twice a year, or every 5-6 months. One mature, this simply means repotting the plant with some fresh soil, into the same size pot. Never use a pot larger than the plant's root system--for standards, this usually means about a 4" pot, for minis and semiminis, a pot no larger than 2 1/2". Over the course of time, your violet will have lost (or had removed) its older, lower, leaves, forming a "neck". Repotting is necessary to eliminate this.
Step 1: African violet with a "neck". A "neck" is the palm-tree like trunk that appears over time as the lower rows of leaves are removed. A well-grown violet should have its lowest row of leaves growing from the trunk at soil-level. When a neck exists, the lowest row of leaves are well above the soil level and pot rim. By repotting, this unsightly neck can be eliminated. This is easiest to do when done regularly, about every 5-6 months
Step 2: Cut-away bottom of root ball. Remove plant from its pot and remove the bottom of the root ball an amount equal to the size of the neck--i.e. if the neck is 1/2" long, remove 1/2" from the bottom of the root ball. This is why repotting is best done regularly, before the neck becomes too long. For example, in an extreme case, where a plant has a 2" neck, we would need to remove 2" from the bottom of the root ball. If the pot is only 2 1/4" deep, then nearly the entire root system needs to be removed! By repotting when the neck is still small, very little of the root system needs to be removed, and the plant will show few, if any, ill effects from repotting.
Step 3: Push plant back into same size pot. If this is a mature plant, a larger pot is not needed. Since a bottom portion of the root ball has been removed, the violet can now be pushed lower into the pot. The objective is to lower the plant enough so that the lowest row of leaves is even with the pot rim (i.e. no neck will be visible).
Step 4: Add fresh soil. The violet should now be lower in its pot, so that its lowest row of leaves is level with the pot rim. Add fresh soil, up to the pot rim, covering the neck. The neck will produce new roots into the added soil.
Step 5: The repotted violet. Once repotting is finished, no neck should be visible, and soil level and lower leaves should be even with the pot rim. Label the pot, and lightly water the plant. This is important--until the plant begins to develop new roots into the added soil, it will require a bit less water than before (it has a smaller root system). The more drastic the repotting, the more that this is the case.
Other tips. Improper pot size, poor soil, and too infrequent repotting are probably the most common causes of unhealthy violets amongst inexperienced growers. Though a miniature violet was used in this illustration, the same procedure is used for standard-size varieties. Most standards (unless grown for exhibition), are quite comfortable in a 4" pot--repotting does not mean continually putting violets into progressively larger pots! Use only a pot as large as the root system--adding soil only benefits the plant if it can develop a root system large enough to use that soil!
A very light, porous, soil-less potting mix is very highly recommended for most growers. If buying a commercially-available mix, judge a soil by its feel not its label! "African violet soils" are often the worst soil mixes for violets! Look for mixes with plenty of vermiculite and/or perlite (1/3 to 1/2 of mix), with a light, fluffy, consistency. Avoid thick, dark, heavy, soil mixes. Though an experienced grower can grow beautiful plants in almost anything, a light, soil-less mix is much more forgiving of over/underwatering and infrequent repotting and neglect.