To harvest crops that aren't growable in your current in-game season, you can build a Greenhouse on your farm. You can build more than one Greenhouse and place them in any available area on your farm. The blueprint for the Greenhouse is available at Rebecca's Designs when you reach Summer of your second year. The blueprint costs 4800 G.
To construct a Greenhouse, you will need to combine 10 Iron, 10 Great Alpaca Yarn, 10 Coconuts, and 3 Moon Stone. Iron can be bought at Rebecca's shop for 510 G or found at the mine; Moon Stone is also found when mining. The Great Alpaca Yarn comes from the wool of a white Alpaca that has spent at least 50 hours at Neil's animal sanctuary, and then by placing the Great Alpaca Wool into the Cloth Maker inside the Maker Shed. You can also get Great Alpaca Wool by using the Nintendo Network and hoping another player sends their great-quality Alpacas to the wifi field.
The Coconut takes a little longer to collect. You can find it on the ground when you visit the Southern Island. Save your game before spending the money at the Travel Agency for the trip, and if you don't find any yellow Coconut on the ground under the palm trees, reload your save game and try another day. You can't grow a Coconut tree on your farm.
The Greenhouse CANNOT be destroyed by a bad-weather storm. If you want to move a Greenhouse from one area to another, you have to Edit your farm and put the Greenhouse back in your inventory; this will clear out any crops you have growing inside the Greenhouse.
Using the Greenhouse
Inside each Greenhouse will be four crop fields, a Watering Spot to fill your watering can, and a Sun Stand to set a seasonal sun. These sun stones will be for sale at the General Store for 500 G each the day after you buy the Greenhouse blueprint, and Hana sells an unlimited supply of them. Setting the season inside the Greenhouse is done by placing one of the stones into the Sun Stand box. To swap stones, have the new sun stone in your rucksack and press A while standing in front of the Sun Stand box. Any crop that can't grow in the new sun's season will be wilted the next day and not immediately after you swap stones. Crops that can survive in any season will continue to grow such as herbs, grains, and trees.
The four soil fields inside the Greenhouse are Regular Fields and not the high-level Soil Fields. Even though they are Regular Fields, you can't grow Big crops inside of the Greenhouse. You will only harvest normal-sized crops. Big crops and Giant crops are reserved for outdoor fields. The fields inside the Greenhouse cannot be swapped out or upgraded.
The indoor fields dry out at the same rate as outdoor fields, so you can water the crops inside of the Greenhouse twice per day. You can also use Fertilizer to increase the Star Rank of the indoor crops. For renewable crops, they will not produce forever while inside a Greenhouse; you will get about 10 to 15 harvests before the crop finally stops producing and needs to be replaced.