Watering systems ease the burden of getting water to plants when they need it. This is especially true in greenhouses, since their roofs keep rain out--leaving all watering responsibility to the gardener. Choosing the right system for your needs involves assessing how much time you can commit to watering, how big an area you're trying to irrigate, and the maintenance you're willing or able to perform.
Garden hoses with perforations down the entire length are called soaker hoses. These can be laid along garden rows, looped around the base of orchard trees or snaked through a plant bed. One end of the hose is capped, forcing the perforations to drip at a relatively even pace. The disadvantage of soaker hoses is that they can rarely be automated. Soaker hoses can be buried under 1 inch or more of coarse mulch to reduce UV damage if you want to leave them in the garden once you've got the placement right.
Though similar to soaker hoses in that water is dispersed slowly for the soil and roots to absorb, drip irrigation is usually comprised of several emitters on a line dripping water onto a specific spot near the plant. These work particularly well for orchards, inside or outside of the greenhouse. If you have plants in individual containers, drip irrigation can deliver water to each container with little waste. You should be able to adjust the water flow for each emitter. Emitters, lines and valves can break or become clogged, making this system higher maintenance than the other systems.
For small gardens and/or small greenhouses, hand watering is a practical system. But for larger areas, hand watering should be a backup watering method, should cover plants not included in other watering systems or used on plants that need water more often than the rest of the plants on the system. Hand watering allows you to control the timing and amount of water directly, but is time consuming. It is impractical for slow, soak-style watering.
If the plants in your garden don't mind wet foliage regularly, sprinklers are an option. Sprinklers can cover a large area with each emitter, but water can be wasted from being blown away by wind, evaporation or run-off when the soil cannot absorb water as fast as it is applied. This waste can be reduced by using several emitters with low pressure, or by moving a single emitter under low pressure around the garden. Sprinklers aren't very practical for container plants.
Sprinklers and soaker hoses are better choices for the outdoor garden, while drip irrigation and hand watering is better for inside the greenhouse. While drip irrigation is the least wasteful watering system, it can also be the most expensive. Soaker hoses and sprinklers are comparable in price, though each has a range of prices based on quality and durability. Putting a timer or evaporation sensor on any irrigation system saves time and work, but plants should be monitored for water needs regularly, and the timers adjusted if needed.