Flowers: It certainly has been a cold spring! However, pansies, violas, sweet alyssum, diascia, nemesia, snapdragons and stock plants can (usually) be set outside by about the third week of April. They will need to be covered if a heavy frost or freeze below 29 degrees is predicted, however, diascia, nemesia, snapdragons and stock may not handle those kinds of temperatures without dropping their flowers even if covered. Pansies and violas can tolerate a light frost typically. Even pansies and violas though, are not likely to tolerate a heavy frost without damage. If daytime temperatures are not above 38 degrees, I’d suggest waiting to plant any of these cold-tolerant annuals until it is at least in the low 40s. Planting them as early as possible allows them to get better established so they can tolerate the heat of summer better and last a little longer when it finally does get warm, but it still needs to be warm enough to plant in the first place.
Vegetables: Rhubarb can also be planted in mid-April if temperatures are in the upper 30s (or preferably, higher) and the ground is not frozen to a depth of 12 inches. This perennial crop is grown for its leafstalks (called petioles); the leaves are toxic. Some popular cultivars include: “Valentine,” “Victoria,” “Canada Red” and “Crimson Red,” which have red petioles. Rhubarb is generally purchased as bare-root crowns or as potted plants. It grows best in fertile, well-drained soils with good organic matter content. It should not be allowed to dry out before planting. Give rhubarb about a square yard of space and loosen the soil to a depth of about 10 inches before planting.
Do not eat any rhubarb petioles that have been exposed to a hard frost or freeze — the oxalic acid from the leaves can migrate from the leaves into the leafstalk. Besides causing severe stomach upset, if the frozen stalk is consumed the oxalic acid can crystallize in your kidneys and cause permanent organ damage.