Plant breeders seek to make better plants. They do this by controlling how plants interbreed or ‘cross’. By crossing selected plants with different but desirable features, they produce a plant that has the best features of both parents. This is then crossed further to produce a stable plant whose seeds produce plants that are true to type.

An example might be crossing a tomato with large orange fruits with one that has small very sweet red fruits in order to produce a tomato with small, very sweet orange fruits.  

There are several techniques used to cross plants (some of which are listed below), but producing F₁ hybrids is one of the commonest and most effective methods.

F₁ hybrids, which are largely annual and vegetable cultivars, are produced by crossing two stable seed lines (called inbred lines) that give rise to especially uniform progeny that possess good vigour, yield and other properties. Tomato ‘Cristal’ F₁ and sunflower ‘Harlequin’ F₁ are examples of F₁ hybrids. It will say on the seed packet if the variety is F₁

Pros and cons of buying F₁ seed

Pros:

• Greater uniformity of flowering, stature, yield and maturity period. Although uniformity is usually more valuable to commercial growers, hybrids usually offer worthwhile advantages to home gardeners over other forms of seeds
• Greater size and vigour of flowers or produce due to the phenomenon of hybrid vigour (heterosis). Hybrids are therefore more robust and better able to overcome adverse growing conditions
• Plant breeders benefit because they control the inbred lines. Therefore the hybrids they breed cannot be grown by other seed companies who lack the parent lines. Although this keeps costs of F₁ hybrid seed high, it is an incentive for breeders to produce new and better cultivars, to the benefit of all growers

Cons:

• Seeds saved from F₁ hybrid plants will not produce plants that are true to the parent type
• F₁ hybrid seed is expensive as it has to be recreated by crossing the parent inbred lines again. Self pollination of the parent inbred lines leads to poor quality plants called ‘selfs’. Preventing selfs is complicated and costly which also contributes to the expense of F₁ hybrid seed
• Due to the cost of maintaining the inbred lines, quite a lot of seed has to be sold for a hybrid to be commercially viable. For this reason hybrids are often only offered for a few years before coming off the market leaving gardeners to seek a replacement for favourite cultivars

The inbred lines are the critical part of F₁ hybrid production accounting for much of the cost and complexity of producing F₁ hybrid seed. Inbreeding is allowing closely related plants to set seed. This, after several generations, leads to a population of very similar plants. Inbred lines lack vigour and perform poorly and are difficult and expensive to maintain.

Perhaps surprisingly then, when two inbred lines are crossed, the first generation (F₁) is uniform and vigorous due to heterosis. Hybrid vigour is not fully understood but crosses between certain lines will produce especially vigorous offspring. Breeders of F₁ hybrids aim to use parent lines whose progeny show particularly strong hybrid vigour.

Some plants, lettuce for example, are intolerant of inbreeding and self-pollination. They suffer from ‘inbreeding depression’, causing a serious loss of vigour, fertility and stature. It is not possible to raise F₁ hybrids for species that are not readily inbred. Wild plants tend to be intolerant of inbreeding, but the process of domestication has led to populations that can tolerate inbreeding and therefore hybrids are feasible for many crops.

F₂ Hybrids
These offspring of F₁ hybrids are variable but for some crops this is desirable. F₂ hybrids are relatively inexpensive. Pansy ‘Joker Series’ AGM are a widely offered F₂ hybrid where the variation compared to F₁ hybrids is not considered to be a drawback and whose seed is significantly less expensive than F₁ hybrid seed.

Open-pollinated seeds
When a group of selected plants are grown together and allowed to freely pollinate each other, the seed is said to be open-pollinated. Before the advent of hybrids all seeds were open-pollinated. Open-pollinated seeds are the only option for plants that cannot be easily inbred – lettuces, peas and runner beans for example.

Open-pollinated seed is available for all crops offered as hybrids, but is usually significantly less uniform, vigorous and productive. Open-pollinated seed is relatively inexpensive and can often be readily collected by home gardeners. However, seeds harvested from garden plants will not always come true to their parent, particularly if there is a related plant nearby with which it could have hybridised. 

Other hybrids usually sold as plants rather than seed:

Specific hybrids
Theses are plants produced by crossing different species, for example Viburnum farreri and V. grandiflorum. The resulting offspring are indicated by an × symbol before the species name. This can occur in the wild but more are common in cultivation, as in Viburnum × bodnantense.

Generic hybrids
These are plants derived from crosses between two or more genera, such as Heuchera and Tiarella. The resulting offspring are indicated by an × symbol before the composite genus name, as in × Heucherella ‘Stoplight’.