By Dr. Jeanette M. Endres, Master Gardener Trainee
When the nighttime temperatures fall to 40-45 degrees, it is time to bring in the houseplants. That may call for repotting some of the plants. The best time for repotting is in spring to summer when plants are actively growing. This is a good idea, but I do not always take the time in the beautiful spring weather to repot. When October comes and my ferns are pot bound from spring and summer growth, it’s time to downsize and repot. Some questions I ask myself: What soil/mix is best? Can I use garden soil? Is the bag marked “potting mix” the best? Should I go with potting mix with fertilizer? Can I make my own potting mix?
What do most house plants need? The mix should be porous for root aeration and drainage and able to retain moisture and nutrients. The mix should not be too open and loose so plants are unable to remain anchored in the pot or water runs through but not too solid so plants become waterlogged and roots rot. Most plants do best in slightly acidic mix. Soil mixes start out on the acidic side but become alkaline over time from water and fertilizer.
Garden soil used alone, either from your garden or commercial garden soil, is much cheaper than the soilless potting mix. However, never use garden soil by itself no matter how good it looks or how well things grow in the outside garden. Garden soil makes containers heavy which impedes drainage and aeration. The roots will not have the room to spread and sufficient moisture to penetrate the soil. Diseases, bacteria and weed seeds can easily come with garden soil. Use garden soil to repot only if it is modified or amended.
Commercial potting mix is soilless. You can purchase mixtures that contain no fertilizer or with fertilizer that reportedly last up to 7 months. Soilless potting mixes weigh less than garden soil, and the ingredients depend on the manufacture. Find mixes under a variety of trade names and in sizes ranging from a few quarts to large bales. Read labels since some soilless mixes are designed for specific plants, e.g. orchids or African violets.
Homemade potting mix is for the cost-conscious home gardeners and do-it-yourselfers who are looking for less expensive potting mixes. An acceptable soil-based mix contains one part sterilized garden soil, one part peat moss (by volume not weight) and one part perlite or coarse builders’ sand. Do not use fine beach sand or play sand. There are various methods to sterilize home garden soil. If using the oven, heat to an internal temperature of 180 degrees. Using compost, add two parts compost, two parts peat moss, one part perlite and one part vermiculite. A standard recipe for a homemade soilless mix published by the University of Illinois:
1.Pour 2 gallons of peat moss into a clean container
2.Add 2 gallons of perlite or vermiculite
3.Mix and moisten the mix before using.
4.In a separate container, mix 4 (level) Tablespoons limestone and slow release fertilizer (check label for quantity)
5.Add to the mixture with sufficient water before planting
Combine the ingredients well and wet the mixture.
Use a dilute solution of fertilizer (half strength) or a slow release complete fertilizer (Osmocote).
Purchasing the ingredients for soilless mixes may not result in a cheaper mix; however, it does offer the opportunity to be creative.
If creating your own potting mix is just too much work or too technical, consider a commercial potting mix either with or without fertilizer.
Repotting House Plants: The Soil