The Kenwood Press
: 03/15/2014

Soil temp and planting in March and April

Steve Albert

Soil temperature much more than air temperature determines how quickly seeds germinate and transplants grow.

The second half of March and the beginning of April is the time to dig the soil and loosen it, allowing as much exposure as possible to the warmth of lengthening days. Mounding your planting beds and creating raised beds are other ways to ensure the quickest warm up of soil in spring.

To warm the soil even more quickly, spread clear plastic over vegetable, herb, and flower beds two to four weeks before you begin planting. Smooth down the plastic to eliminate as much air as possible from under the plastic; solar heat on clear plastic can raise the soil temperature as much as 10 to 15F.

How warm is your soil? A small, basic, and inexpensive outdoor thermometer can tell you. Simply dig a hole about six inches deep and place your thermometer at the bottom, put a landscape flag or plant marker in place and cover the hole. After about 15 minutes uncover the thermometer and read the temperature. A soil temperature of 60F or greater is ideal for transplants; temperatures approaching 70F are ideal for vegetable seeds. (Most vegetable seeds will germinate in lower temperatures, but it will take a few days longer than the number of days to germination on the back of seed packets.)

The sides of mounded or raised beds will soak up the sun’s warmth much quicker than flat planting beds. Even adding two or three inches of planting mix or aged compost to the top of a planting bed will do the trick; mounded beds six inches high or more is ideal. The south side of your bed – facing the sun – will warm quicker than the north side. Slanting a mounded bed to the south will raise the bed temperature.

If you are growing vegetables this year, make hills for vegetables such as corn that are planted in clusters. Make large mounds – about 10 inches high by 3 feet wide – for melons, sweet potatoes, cucumbers, and squashes. If you are growing tomatoes, dig the holes where you will be planting tomato transplants now; mound the soil on the north side of each hole so more sun can reach into the hole.

The growing season in the Sonoma Valley is about 270 days – that’s measured from the average last frost date in mid-May to the average first frost date in early December. You’ve got plenty of time to grow annual vegetables and flowers, so take the time to warm the soil in advance of sowing and planting.

Here are a few more garden tips for mid-March to mid-April:

  • Finish planting bare-root roses, trees, and berries as soon as possible. Try to get bare roots in the ground before they leaf out and break dormancy. 
  • Now is the ideal time to plant hedges. Hedges can serve as a living boundary, windbreak, or the ideal background for flowering plants and shrubs. Plant hedges now; if you plant later, the young growth already on the plants may wilt and die during the time it takes for roots to become established. 
  • Weeds are growing! Now is the time to get ahead of spring weeds. Don’t wait until they flower and scatter seed. Use a Dutch hoe around established plants to upend new weeds. You can let them wither and die in place. 
  • Check established shrubs and trees for winter damage – broken limbs or freeze damage. Now is the time to remove dead wood and damaged weak shoots. This applies to hydrangea, hypericum, caryopteris, fuschias, lavatera, lonicera, and salix. Pruning away damaged wood and tissue will promote lavish growth in just a month or two. 
  • It’s time to plant rhododendrons and azaleas – yes, there are native California varieties that can withstand drought. There are a host of species and hybrids, evergreen and deciduous that make great garden plants. Rhodys thrive in acid soil and afternoon shade. Prepare planting beds and holes for rhodys and azaleas now. Dig the ground deeply and add plenty of aged compost, peat, and leaf-mold. Soil that is well-drained and moisture retentive is ideal. 
  • Prepare pots and containers for spring and summer planting. Renew the soil in containers every year. Even if you can’t fully change the soil in a container, at least add new potting mix. Container grown plants rely on the soil for nutrients and nutrients are easily sapped in a growing season. If a plant has been in a container for several years and is performing poorly, it may be time to do some root pruning – removing brown and dry roots and allowing fresh feeder roots to take over. 
Some annual flowers for sowing in March and April: calendula, California poppy, cornflower, dahlia, delphinium, gaillardia, godetia, helianthus, helichrysum, iberis, ipomea, lobelia, mimulus, nemesia, phlox, nicotiana, petunia, rudbeckia, salvia, scabiosa, tagetes, verbena, and zinnia.