This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
When we come to inquire whether a Raspberry or Blackberry is hardy, it simply comes to asking whether any grower's plants are healthy. The Raspberry and Blackberry are naturally natives of very cold countries, and if the plants have their natural vital powers, are able to stand any degree of temperature they are likely to meet with in most parts of the United States. It is only when these vital powers become enfeebled that they succumb, and when thus enfeebled, even a white frost may be enough to render them "not hardy." Unfortunately this fact has not been made prominent, and hence little is known of those enfeebling causes. Yet it has been long enough known that kinds perfectly hardy in the severest weather come in time to be very tender; and this is especially true of those kinds related to the Rubus Idseus of Europe. Sometimes the vital power is weakened by the attacks of fungi on the leaves, at others on the roots, and then insects on the roots may be as bad as fungi.