A hybrid plant is the result of cross pollinating two different plant varieties and growing the seed the cross produces. The plant that grows from that seed is considered a hybrid.
Why Hybridize Plants?
Commercial hybridizing is done to get some type of valued attribute of each initial variety into the offspring. Hybrids might be developed for disease resistance, size of plant, flower, or fruit, increased flowering, color, taste or any reason a plant might be considered special. Most modern plants currently on sale are hybrids.
How are Plants Hybridized?
For the initial crossing, pollen from one plant has to be transferred to the flower of another variety. Before doing so, the breeder has to decide which plant he wants to use as the female (the pistil) and which he wants to take pollen from (the stamen, male parts). The pistil is then pollinated manually, with the pollen. To prevent the plants from self-pollinating, all the stamens have to be removed from the plants that are going to pollinated. The fruits that form as a result of this cross pollination are harvested and the seeds are collected.
Getting to the desired result can take years of crossing. First time crosses are grown out the following year and the plants they produce are evaluated. If they meet expectations, the cross will be repeated and the seeds will be marketed the following year. But it can take many years before a hybrid with the desired traits is even created. If the resulting plants are disappointing, the breeder is back to square one.
Even when the breeder has a winner, the process continues. Seeds for popular commercial hybrids, like 'Sungold' and 'Early Girl' tomatoes, have to be crossed, harvested and saved every year. These are called F1 hybrids, for first generation, because they are the direct product of a cross. The breeder who first creates a hybrid owns the rights to it, which is why they can be more expensive than non-hybrids, or open pollinated, plants. Breeders guard the parentage of their hybrids closely.
Is there Anything Special about Growing Hybrids?
Success growing hybrid plants in the garden is pretty much on par with growing non-hybrids open pollinated plants (OP). While many of the most popular varieties of garden plants are hybrids because they have proven to be top performers in the garden, there are also many hybrid plants that were not well tested before being sold and proved to be disappointments. Often this is the result of testing in only one area, rather than under different growing conditions. Thankfully, these tend to disappear from the market as better plants are introduced.
What Happens When You Plant Seed from Hybrid Plants?
Because hybrids are a cross between varieties, the seed produced by hybrids will not grow true to seed. Seedlings grown from a hybrid could exhibit traits of one or both parent plants or be something totally surprising. Sometimes the seed is sterile and does not grow at all.
This doesn't mean you can't save and experiment. You could be totally delighted with the new plants. Just don't get attached to them, because you many never see plants like that again.
Once a Hybrid, Always a Hybrid?
It is possible for hybrid seed to be stabilized and become an open pollinated plant, so that it continually grows true to type. This takes a good amount of work, growing out several generations of seed, carefully selecting only those that are identical to the parents and discarding the rest. However many non-hybrid seeds are a result of this painstaking process.
Are Hybrid Plants Unnatural?
Most hybrid plants are intentional crosses, but hybridization can occur in nature. In fact, it happens quite often. Two nearby plants of different species can be cross pollinated by insects or the wind and the resulting seed simply falls on the soil and grows into a hybrid. Few of the flowers and vegetables we grow today are in their original wild form. In nature, hybrids are hit or miss. Commercial hybrids come about after a great deal of work and many attempts are discarded if they do not produce the desired results.
Whether the cross occurs by nature or man, do not confuse hybrids with genetically modified plants (GMO), which are created using techniques such as gene cloning. Hybrids are simply two plants that cross pollinated.
You might also be interested to know what the difference is between Heirloom, Hybrids and Open Pollinated Plants?