Grains include cereals, legumes, and achenes. A cereal is the seed of a grassy plant: barley, corn (maize), millet, oats, rice, rye, sorghum, teff, triticale, wheat. Legumes include beans, peas, chickpeas, soybeans, and lentils. An achene is a pseudo-cereal; it’s a type of grain that resembles a cereal in its nutritional content and culinary uses. The most common achenes are: amaranth, buckwheat, and quinoa.
Achenes are superfoods. They have a similar protein, fat, and carbohydrate content to grains: lots of carbs, a fair amount of protein, and a little fat. But whereas most grains are deficient in the essential amino acid lysine, achenes are a complete protein. Buckwheat has about the same amount of protein as wheat. (The two plants are unrelated; buckwheat is not a type of wheat and contains no gluten.) Amaranth and quinoa are both higher in protein than wheat, rice, or corn.
Achenes are also higher in magnesium than cereal grains. A 2006 study published in the journal Epidemiology  found that high levels of magnesium from food decreased all-cause mortality by 40% and overall cancer mortality by 50%. (All-cause mortality is all causes of death put together as a set.) U.S. RDA for magnesium is 420 mg/day for men and 320 mg/day for women. The study also found that high levels of copper were harmful, increasing mortality rates. Cereals, cereal bran, and achenes are high in magnesium and low in copper. Achenes like amaranth, buckwheat, and quinoa are particularly good sources of magnesium.
As a good source of carbs and protein, achenes are a staple food. You can live off of achenes as your main source of energy (carbs), as long as you have a supplemental source of protein, such as legumes, some dietary fat (maybe from stored vegetable oil), and a modest-sized garden for a variety of fruits and vegetables.
From a prepping and survival point of view, achenes are the ideal type of grain to grow. Cereals typically have hulls that must be removed to eat the grain. If you grow your own wheat, rice, barley or other cereal, you will need a machine for hulling. Corn is an exception, but corn is low in total protein and very low in lysine; you should not use corn as your main staple food.
By comparison, achenes have no hulls to remove. To harvest the achene, you thresh the grain, i.e. separate the seeds from the rest of the plant. Then remove any bits of leaves and other debris. Wash the seeds, and it’s ready to cook. Amaranth, buckwheat, and quinoa can all be cooked by boiling in water, much like rice. Growing achenes in your survival garden means less work to produce a staple food.
Quinoa is unique among grains and achenes. It has a bitter saponin coating that must be washed off prior to cooking and eating. Rinse the quinoa seeds in water until the soapy saponin foam is gone. Then boil it in an excess of water, strain with a fine metal strainer, and give it a final rinse. Then it is ready to eat. The saponin coating protects the seeds from birds and insects as it is growing in the garden.
Hempseed is an unusual achene. It is low in carbs, but high in protein and fat. The fat composition is ideal. Hempseed and the oil pressed from hempseed has a good balance of the two essential fatty acids: omega-6 linoleic acid (LA) and omega-3 alpha-linolenic acid (ALA). It also has a healthy omega-6 fat called gamma-linolenic acid (GLA) and a healthy omega-3 fat called stearidonic acid (SDA). You could live off of hempseed oil as your only source of dietary fat.
The protein in hempseed has all the essential fatty acids in sufficient amounts. And the seeds are very high in magnesium, higher than any cereal or achene. However, hempseed can’t be your main protein source. The seeds are too high in fat and too high in magnesium to consume large quantities daily. Hempseed is best used as a supplemental source of protein.
Unfortunately, your survival garden cannot include hempseed, as it is illegal to grow in the United States. Many other nations either prohibit or restrict the growing of hemp (due to its similarity to the marijuana plant). Where growing hemp is permitted, under strict regulations, the restrictions might be acceptable for a commercial farm, but they are excessively burdensome for a backyard garden. That situation may change in the future. But for now, hempseed and hemp oil are good foods to buy at the local health food store, but not good for a survival garden.
 USDA National Nutrient Database for Standard Reference, Release 26.
 Leone et al., Zinc, Copper, and Magnesium and Risks for All-Cause, Cancer, and Cardiovascular Mortality; Epidemiology. May 2006, Volume 17, Issue 3, p. 308-314.