How Deep Are a Zinnia's Roots?

Root Characteristics

• After zinnia seeds germinate, a slender seed root initially drives downward into the soil. A large taproot like that of a carrot doesn't form. Most roots are thin and branching, developing a matrix of threadlike roots outward from the plant stem in all directions. Most roots grow down about 6 inches, and only very tall varieties that are at a mature height may have a few roots reaching down to 10 inches to prevent winds from toppling plants. Depending on soil texture, some rare, errant roots may reach down to 14 to 18 inches.

Soil Types

• Zinnias need ample, even soil moisture to prevent wilting in the direct sun, especially during the hottest part of the afternoon. Porous, crumbly soil is easiest for the thin roots to grow through and downward to reach moisture. In sandy soils, roots grow deeper to 6 to 12 inches since the fast-draining sand particles do not hold as much moisture at the surface. By contrast, in a loamy soil rich in organic matter, moisture doesn't drain too quickly, so the top 6 inches remain adequately moist. Heavy and compacted clay soils do not allow zinnia roots to grow too deeply, perhaps 3 to 4 inches at most.

Soil Moisture Insight

• The fibrous zinnia plant roots do not tolerate soggy conditions, as the water prevents oxygen from reaching root hairs. Overly wet soil leads to root rot. It's best to alternate watering events to create temporarily wet and then moist conditions. This only works in well-drained soils. Alternating the amount of water causes the roots to grow more deeply to reach the higher moisture at deeper soil levels.

Relocating Zinnias

• It's easiest and healthiest to directly sow zinnia seeds where they are to grow for the rest of the season. However, nurseries sell zinnias in containers or seed tray cells that may be transplanted into the garden. Root-bound zinnias do not transplant well since the knots of constricting roots do not grow outward into the soil. To relocate an already growing zinnia, dig as much of the root ball of the plant as possible, trying to keep soil intact around the root system in the shovel. Immediately replant at the same depth in a new hole, and water to keep the soil moist. Some wilting will occur.