Q. When do I know to harvest garlic, Walla Walla sweet onions, green onions and garlic?   A. Garlic should be harvested in our hot, dry climate when the tops have browned down by about 1/3. In other locations where it is not so hot they usually will harvest when the ½ of the tops have turned brown or when they fall over. We cannot wait that long in the hot desert. A common mistake is to wait too long to harvest garlic in the desert. This is elephant garlic in the foreground, not one of my favorites (but it is big and impresses nongardeners), but this is close to ready for harvesting. You can note the change in color of the foliage and dieback. Garlic that was in front of it was already harvested as it was an earlier garlic variety. The irrigation is drip tape with emitters spaced every four inches with the tape spaced one foot apart.             There is a thin, tight “wrapping” around the cloves when you harvest garlic correctly. For appearance purposes and storage it is important that this wrap be continuous and tight around the cloves. If you wait too long before the final harvest, this covering starts to rot away, leaving the cloves exposed.             Sweet onions like Walla Walla are harvested when the tops have fallen over. Do not step on the necks to ready them for harvesting unless you plan to eat them fresh and not planning them for storage. This is Contessa sweet onion. The top of the plant in the center has fallen over signalling the bulb is ready to harvest. The others are close but not ready yet. When the bulb is ready for harvest the neck will collapse near the bulb and the weight from the top will cause it to fall. This closes off the bulb and will not allow diseases to enter the interior of the bulb and helps for long term storage. The other thing to do for storage is cure the harvested bulbs. Leave the collapsed tops on the plants and put them off the ground and in the shade to cure for a few days. After the tops dry down, cut them off leaving an inch of the dried top remaining.             The drying down of the tops and tops falling over is a natural “sealing” process of the bulb. Stepping on the tops can allow disease organisms to enter the bulb because the neck has not dried down properly and may prevent any storage of the bulb.             Scallions are really nothing more than green or immature onions that have not yet begun to bulb, just barely started bulbing or cannot bulb. In the case of the Welsh onion, which is traditionally grown as a scallion, it cannot bulb up. White Lisbon onion is also grown as a scallion but this onion will bulb up if left in the ground too long. Harvest it before it starts to bulb.