University of Vermont
Department of Plant and Soil Science

Summer News Article


Dr. Leonard Perry, Horticulture Professor
University of Vermont

Container gardens are increasingly popular for several reasons.  They’re an attractive way to add color to a patio, porch, or small space.  They satisfy the urge to garden if you have a green thumb but have limited space in which to plant flowers, or even some edible plants. Although  easy to plant and to get a more instant effect, they do need some care throughout the season to ensure healthy plants and continual bloom.
If the container plantings you made in spring are beginning to look a little leggy, or the foliage is yellowing, or they’re not blooming, don't despair.  These and other problems are generally readily fixed.   Here's a list of common problems--and the solutions--to help you get your container gardens back on the road to recovery:
Problem: You water regularly, but wilting still occurs.  The probable cause is poor drainage and aeration of soil. The solution is to repot using a lighter soil mix containing more organic matter. Or, if that's not a viable option, increase the number of drainage holes in the container.
Another possible cause is too many plants in the pot, taking up too much water.  You may need to repot,  removing a few plants. 
When repotting, add a water absorbing or soil moistening ingredient.  You can buy these at full service garden stores.  They are generally pellets that absorb many times their weight in water, releasing it and keeping soils moist.  They’re especially useful in clay pots or coir (coconut husk) lined baskets that dry out quickly.
Problem: Plants are tall and spindly. The probable cause is too little light, and perhaps too high nitrogen levels.  The solution is to  move the containers to a location that receives more sun or light a day. Stop fertilizing the plants, or decrease the amount.
Problem: Plants have stopped flowering.  There may be several possible causes.  An end to its flowering may be natural for this variety.  However, if you planted annuals, deadheading (removing spent flowers) often promotes branching and rebloom.
If the buds don't open, the cause could be disease and rots.  Dig up a plant; if roots are brown and not white, root rots are likely.  Dispose of infected plants and replant the container with fresh potting mix. Use one made for pots, and not garden soil which usually has disease organisms and doesn’t work well in containers.  If tops appear to have disease, this could be gray mold (botrytis).  Give plenty of air movement around plants to help dry leaves off, and don’t water late in the day so leaves will be dry over night.
Too much fertilizer can cause excess leaf growth at the expense of flowers, so fertilize less if the plant is lush but has no flowers. On the other hand, if leaf growth looks fine and not excessive, plants may need more fertilizer.  Many of the new “vegetative” varieties (those grown from cuttings and not seeds) need very high levels of fertility.
Problem:  Yellowed foliage, especially the lower leaves. The probable cause is too much water or too little fertilizer. If leaves yellow from the bottom of the plant first, and plants lack vigor, it often means excess water in the soil.  If leaves are just generally a bit yellow overall, this is likely from too little fertility.  The solution would be to water plants less often if the soil appears wet, and make sure the container has adequate drainage. In addition to watering less, fertilize more, especially if using low nutrition organic sources.
Problem:  Edges of leaves are brittle and dry. The probable cause is too much salt present in the soil, most likely from overfertilizing. (You might notice a whitish crust on the soil surface or pot edge, another sign of excess salt.) Moving containers quickly from shade to more sun also may be the cause. The solution, if from too much fertilizer, it to water generously--until water pours out of the drainage holes--to cleanse the soil and remove the salt. Avoid changing locations and light levels rapidly.
Problem: Leaf spots, powdery or rusty areas. Probable causes are low temperature, inadequate phosphate, or disease. Solutions are to move the container to a warmer location. Apply a fertilizer containing high phosphate (the middle number of the three in the analysis), such as a plant starter fertilizer. You also might try fungicides, although if the problem is serious you probably will need to toss the plant. To help identify if a disease, and if so which one, work with your local garden center or contact your state master gardener network to help identify the pest.  (For Vermont it is 
Problem: Foliage is riddled with small holes. The probable cause is insect pests. Again, work with your garden center or local master gardeners to identify the pest.  Solutions are to apply the least toxic insecticide that will do the job, following the instructions on the label carefully. If the pest problem isn’t serious, you and the plants may be able to tolerate some damage.

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