Tomato Plant Types
Indeterminate, Semi-Determinate and Determinate
Part 1 of 2
What are the differences between plant types, and how do you know which type is best for your gardening needs? These are questions people ask themselves when ordering seeds or standing in the local greenhouse, trying to pick the plants they want to grow for the season. In this series, we will delve into this mysterious task so that when spring rolls around, you can make educated selections for your garden.
Let’s tackle the two main types that most people are familiar with: indeterminates and determinates. Most people know there’s one big difference, and that’s plant size—indeterminates are tall, and determinates are short. But there’s so much more to know beyond the sizes. For instance, there’s a gene that controls how the plants grow along the main compound stem called the self-pruning gene (SP_).
An indeterminate is controlled by the dominant gene (sp+). These types have the main stem and several side shoots that are identical to the main stem in composition and can even be pruned and trained into becoming a new leader stem. The blossom clusters form at roughly every three nodes, or stem sections, off the sides of the main stem and side shoots, always leaving stems and foliage to continue upward and outward growth. Indeterminates will continue to grow and produce a steady supply of fruit all season long until frost kills them. These types of plants are good for the gardener who enjoys a steady supply of different shapes and sizes of juicy, fresh eating tomatoes. Many types of cherries, heirlooms, open-pollinated, and even some hybrids are indeterminates.
A determinate plant is controlled by the recessive allele of the self-pruning gene (sp,sp). This type will generally only reach between two and four feet tall and have anywhere between six and 15 nodes before the terminal point. They have a smaller, more compact, and bushy type of habit. An easy way to tell them from other kinds of plant types is that they will form a terminal flower cluster and a terminal growth point, meaning that the branching habit stops at each flower cluster off the main stem, and the main stem itself ends in a terminal flower cluster; there is no continuation of vegetative growth. The blossom clusters typically form at the same time and also set fruit at the same time and ripen at the same time. This type of tomato is best suited for the avid home canner and commercial canneries because they set fruit all at once, allowing for a full harvest at one ripening stage. Determinates generally have an earlier maturity rate, which can lend the possibility of two larger harvests within one season for temperate, long-season climates. Some examples of determinates are Roma, San Marzano, and Fourth of July.
There’s also another, more complicated combination of the two that is known as semi-determinate. This type of plant growth is controlled by a major recessive gene (semi-determinate: sdt) off the recessive (sp- determinate). Much like the determinate varieties, these plants are bushy and flower at terminal points, the difference being they produce an additional six or more blossom clusters before reaching their mature height, topping out at around six feet tall, and will have a few more leaf sets per node. In northern climates, these are often mistaken for indeterminates because they can last a longer season before they stop at their terminal cluster. For this reason, many northern growers prefer semi-determinates. Some well-known varieties of semi-determinates are Celebrity, Glacier, and Bush Early Girl.
Please check back soon for the next article in this series!