Fertilizing is very important for plants growing in soilless potting mixes. Indoor plants are typically fertilized at times that mirror their cycles of growth if they were grown outside. They grow and flower in the spring. They flourish in the summer. Their growth slows or they go dormant in the winter. Take care not to over-fertilize your plants. Plants should be fertilized only when they are actively growing. Most houseplants will not need to be fertilized more than once every 1 – 3 months, between March and September The type of plant, size of the pot and light conditions determine the correct amount and type of indoor plant fertilizer you should use and buy.